Evidence for Suppression of Parathyroid Gland Activity by Hypermagnesemia

Evidence for suppression of parathyroid gland activity by hypermagnesemia

Shaul G. Massry, … , Jack W. Coburn, Charles R. Kleeman

J Clin Invest. 1970;49(9):1619-1629. https://doi.org/10.1172/JCI106379.

Research Article

The effect of hypermagnesemia, produced by MgCl2 infusion, on the activity of parathyroid glands, as assessed by changes in levels of serum calcium (SCa) and in the fraction of filtered phosphate excreted (CP/CCr), was studied in 11 intact and 4 thyroparathyroidectomized (T-PTX) dogs. To exclude the effect of diurnal variation in CP/CCr on the results, studies were initiated in both morning and afternoon hours and each study with MgCl2 infusion was paired with a control experiment in the same dog not receiving MgCl2. During MgCl2 infusion, serum phosphorus rose progressively. Despite this rise, the levels of CP/CCr fell in all experiments and were significantly different from values observed at the same time of the day in the paired control experiments. The concentrations of total SCa fell by 1.0-2.4 mg/100 ml with a proportional decrease in the levels of the diffusible and ionized fractions. The pattern of the fall in CP/CCr during MgCl2 resembled that observed after CaCl2 infusion (seven dogs) and that which acutely followed thyroparathyroidectomy (seven dogs). When parathyroid extract was given to dogs receiving MgCl2 infusion both CP/CCr and SCa rose, and MgCl2 infusion did not affect CP/CCr and SCa in T-PTX dogs. These results indicate that hypermagnesemia suppresses the activity of the parathyroid glands, probably, by inhibiting production and (or) release of the hormone, without interfering with […]

Find the latest version: https://jci.me/106379/pdf Evidence for Suppression of Parathyroid Gland Activity by Hypermagnesemia

SHAuL G. MASSRY, JACK W. COBURN, and CHARLEs R. KLEEMAN From Cedars-Sinai Medical Research Institute and the Departments of Medicine, Cedars-Sinai Medical Center, Veterans Administration Center, and the University of California at Los Angeles School of Medicine, Los Angeles, California 90048

A B S T R A C T The effect of hypermagnesemia, pro- of simultaneous modest hypocalcemia and hypermag- duced by MgCla infusion, on the activity of parathyroid nesemia suggests that a decrease in the level of serum glands, as assessed by changes in levels of serum cal- calcium is more potent than an increase in the concen- cium (Sca) and in the fraction of filtered phosphate tration of serum magnesium in the regulation of para- excreted (CP/Ccr), was studied in 11 intact and thyroid activity. 4 thyroparathyroidectomized (T-PTX) dogs. To exclude the effect of diurnal variation in CP/Ccr on the results, studies were initiated in both morning and after- INTRODUCTION noon hours and each study with MgCL infusion was Although it is well documented that the concentration paired with a control experiment in the same dog not of ionized calcium in the plasma is of primary impor- receiving MgCl2. During MgCL infusion, serum phos- tance in regulating parathyroid hormone secretion (1- phorus rose progressively. Despite this rise, the levels 3), evidence exists suggesting that the level of blood of CP/Ccr fell in all experiments and were significantly magnesium may also play a role in the regulation of the different from values observed at the same time of the activity of the parathyroid glands. In the rat, magnesium day in the paired control experiments. The concentra- deficiency and hypomagnesemia may stimulate the para- tions of total Sca fell by 1.0-2.4 mg/100 ml with a pro- thyroid glands (4-6). Gitelman, Kukolj, and Welt portional decrease in the levels of the diffusible and (7) reported data in nephrectomized rats compatible ionized fractions. The pattern of the fall in CP/Cc, with the concept that hypermagnesemia may suppress the during MgCl2 resembled that observed after CaClh in- activity of the parathyroid glands in the rat; their stud- fusion (seven dogs) and that which acutely followed ies, however, do not exclude the possibility that the hy- thyroparathyroidectomy (seven dogs). When parathy- pocalcemic effect of magnesium injections could have roid extract was given to dogs receiving MgCla infu- been produced by mechanism(s) other than the inhibi- sion both CP/CCr and Sc. rose, and MgClh infusion did tion of the parathyroid glands. In four experiments not affect CP/Ccr and Sc. in T-PTX dogs. These results utilizing the in vivo perfusion of isolated parathyroid indicate that hypermagnesemia suppresses the activity glands in goats and sheep with perfusate containing high of the parathyroid glands, probably, by inhibiting pro- concentration of magnesium, there was an inverse cor- duction and (or) release of the hormone, without interrelation between the level of parathyroid hormone in the fering with end-organ response. An increase in serum affluent blood and the concentration of magnesium in magnesium of 1.7-2.0 mg/100 ml was capable of pro- the perfusate in three experiments; this correlation was ducing the suppressive effect. Evaluation of the effect highly significant in only one study (8). The relation between magnesium metabolism and the This work was presented in part before the Western Sec- activity of parathyroid glands in the dog and man has tion of the American Federation for Clinical Research, not been delineated. Magnesium deficiency in the dog Carmel, Calif., 2 February 1968. may be associated with normal or low levels of serum Dr. Massry is an Established Investigator of the American the effect of acute Heart Association. calcium (9-12); hypermagnesemia Received for publication 18 February 1970 and in revised on the activity of the parathyroid glands has not been form 23 April 1970. studied in the species. In man, magnesium deficiency The Journal of Clinical Investigation Volume 49 1970 1619 may be accompanied by hypocal- and hypomagnesemia CONTROL INFUSION ; -itott MgCI2 cemia (13), which cannot be corrected by the adminis- I M tration of parathyroid extract, suggesting that the skele- I I 0 a 0 5' ton is rendered resistant to the action of parathormone. V- _ and U The repeated injections of magnesium salts (14) utE oral magnesium loading (15) to humans were associated 0)J------with a decrease in phosphate excretion; these results Di 911 were not attributed to inhibition of parathyroid glands. 0 Furthermore, the relative importance of simultaneous gm.UE a changes in the concentration of serum calcium and mag- %A E 9L nesium in the regulation of the activity of parathyroid .E _ 6. glands is not known. 0 The present study was undertaken to evaluate the effect of acute hypermagnesemia on the activity of the parathyroid glands in the dog as assessed by changes in the concentration of serum calcium and in the fraction T of filtered phosphate excreted. The influence of a simul- E4J0 UNO., ,.T-' -T. l-rlT-17 taneous hypermagnesemia and hypocalcemia on the ac- 10 tivity of these glands was also studied. Qz so 4: METHODS 52 experiments were carried out during pentobarbital anesthesia in 18 intact and 11 thyroparathyroidectomized (T-PTX) mongrel dogs weighing 16-25 kg. An oral water TIME (p.m.) load of 500 ml was given to the animals in each experiment FIGURE 2 Afternoon studies. The changes in mean levels of serum magnesium (Mg), calcium (Ca), and phosphorus ( P) and in the fraction of filtered phosphate excreted CONTROL INFUSION 0 without MgC12 (Cp/Cc,) in four intact dogs with and without the infusion a E 10 of MgCl2 (100,ug Mg'+/kg per min). The changes in CP/Cc, 0 are expressed as in Fig. 1. Ut £L~- :1-~ - ZuE I hr before the induction of anesthesia. Respiration was controlled by a Harvard respiratory pump, which was adjusted initially with a stroke volume of 10 ml/kg body weight and at a rate of 30 strokes/min. Blood pH was measured several times during each study with a Radiometer pH meter (London Co., Westlake, Ohio), and the stroke volume of the respirator was adjusted thereafter to maintain 150. blood pH within normal range; usually, little adjustment 0 was necessary. Glomerular filtration rate was measured by B.- .r1/ exogenous creatinine clearance. Urine was collected from w 100. a retention catheter, and the bladder was washed with air vO at the end of each period. Blood was obtained at the mid- V Z k.., point of each clearance period from an indwelling needle in 50. the femoral vein. The studies were ma I L I T . following performed. .0 _ Experiments zedth the infussion of magnesium chloride (15 dogs, 38 experiments). After three control clearance pe- O. T ---r- I" riods, each of 20 min duration, magnesium chloride was 10 11 12 1 2 3 4 5 added to 0.45%o saline, and this solution was infused at I ml/ a.m. TIME p.m. min for 6 hr to deliver 100 gug Mg'+/kg per min in each experiment unless noted otherwise; clearance collections of FIGURE 1 Morning studies. The changes in mean levels of 30 min duration were obtained throughout. To exclude the serum magnesium (Mg) and phosphorus (P) and in the effect of the diurnal variation in phosphate excretion on the fraction of filtered phosphate excreted (CP/CC,) in seven results, experiments were initiated in both morning (morn- intact dogs with and without the infusion of MgClz (100 ing studies) and afternoon (afternoon studies) hours, and Ag Mg'+/kg per min). In order to reduce the variability in each experiment with infusion of MgCl2 was paired with absolute fraction of CP/CCr from one experiment to another, a control study in which the same animal received only the changes in CP/Cc, are expressed as the per cent of the 0.4517o saline at 1 ml/min for 6 hr over the same period of mean of the three control clearances in each individual ex- the day during which the MgCl2 infusion was given. 5-10 periment. The brackets represent ±1 SE. days separated studies in the same dog. 1620 S. G. Massry, I. W. Coburn, and C. R. Kleeman Seven intact dogs, in which studies began in the morning kg per hr from 11: 30 a.m. to 2: 30 p.m. The calcium chlo- hours, received the infusion of MgCl2 between 10:30 a.m. ride infusion was replaced by 0.45% saline alone, and the and 4:30 p.m. Four of these dogs were restudied with a experiment continued for an additional 3 hr, with clearance second infusion of MgCls while they also received intra- collections obtained throughout the entire study. muscular injections of 100 U of parathyroid extract (sup- Experiments during and immediately following thyro- plied courtesy of Dr. A. S. Ridolfo, Eli Lilly & Co., Indian- parathyroidectomy (seven dogs, seven experiments). After apolis, Ind.) at 10:30 a.m., 12:30 p.m., and 2:30 p.m. three control clearance periods of 20-30 min each, thyropara- Four intact dogs, in which experiments were started in thyroidectomy was performed between 11:00 a.m. and 12:30 the afternoon hours, received the usual MgCls infusion be- p.m. Clearance studies were obtained during the surgical tween 3:00 p.m. and 9:00 p.m. These animals were restud- procedure and for 4 additional hr thereafter. ied with the amount of MgCl, reduced to deliver 40 Ag Blood and urine samples were analyzed for creatinine by Mg"|/kg per min from 3:00 p.m. to 6:00 p.m., and the ex- the Jaffe reaction (16) and for phosphate by the method periments were then continued until 9:00 p.m. with only of Fiske and Subbarow (17), and calcium and magnesium 0.45% saline infused. were measured by the Perkin Elmer atomic absorption The effect of MgC12 was also evaluated in four T-PTX spectrophotometer, model 303. Diffusible calcium and mag- dogs. Thyroparathyroidectomy was performed under pen- nesium and ionized calcium were measured in ultrafiltrates tobarbital anesthesia 2-3 days before the first study. The of serum prepared anaerobically in Lavietes chambers at complete removal of the parathyroid glands was confirmed room temperature (18). The concentration of ionized cal- by the appearance of hypocalcemia. The dogs received liberal cium was measured by a micromodification of the method of amounts of calcium and phosphate in their diet, and they Ettori and Scogan (19). Water content of serum was de- remained in good condition for at least 10 days. Both a con- termined by refractometry (Goldberg refractometer, Amer- trol experiment and a study with the infusion of MgCl2 ican Optical Corp., Scientific Instrument Div., Buffalo, were carried out in each dog between 3:00 p.m. and 9:00 N. Y.). p.m. Experiments with calcium chloride infusion (seven dogs, RESULTS seven experiments). After three control clearance periods of 20 min duration, CaCl2 was added to 0.45% saline, and In the anesthetized but otherwise intact dog, phosphate the solution was infused at 2 ml/min to deliver 5 mg Ca++/ excretion displays a distinct diurnal pattern (Figs. 1

TABLE I Detailed Protocal of Two Experiments, One without (Control) and One with MgC12 Infusion (i.v. Mg), Carried Out in Dog No. I

Sp C 0 Cce Sm1SCa Control Time Control i.v. Mg Control i.v. Mg Control i.v. Mg iv. Mg Control i.v. Mg m11min/ mg/100 ml ml/min mg/ 100 ml a.m. 8:00 Oral water load, 500 ml 8:30 Anesthesia with pentobarbital Priming dose of creatinine (500 mg) Infusion started: 7 mg creatinine/min delivered in 0.45% saline at 1 ml/min 9:30-9:50 44.8 41.9 1.25 1.68 8.9 9.3 3.9 16.1 18.9 9:50-10:10 48.4 39.6 1.25 1.68 8.9 9.3 3.9 16.1 18.9 10:10-10:30 49.0 44.5 1.38 1.68 9.0 9.3 4.0 7.8 13.9 10:30 In i.v. Mg experiment, MgCl2 added to infusate to deliver 100 ,gg mg/kg per min 10:30-11:00 40.6 41.1 2.10 4.2 5.2 6.1 11:00-11:30 43.6 42.8 1.38 3.49 9.1 9.0 4.3 2.5 3.0 11:30-12:00 45.9 43.0 4.55 - 4.2 4.9 0.6 p.m. 12:00-12:30 45.2 42.7 1.50 5.53 8.9 9.0 4.2 2.5 0.4 12:30-1:00 41.4 42.1 6.55 - 4.5 1.9 0.2 1:00-1:30 43.5 42.2 1.45 7.27 8.8 8.8 4.9 1.2 0.2 1:30-2:00 51.2 41.7 7.78 - 5.1 1.4 0.3 2:00-2:30 51.2 41.8 1.45 8.00 8.8 8.6 5.2 3.1 0.2 2:30-3:00 46.8 41.8 8.23 5.6 7.9 0.2 3:00-3:30 45.0 41.0 1.39 8.25 8.8 8.3 5.7 13.7 0.3 3:30-4:00 45.5 38.9 8.30 - 5.7 17.7 0.3 4:00-4:30 46.1 41.5 1.39 8.38 8.8 8.1 6.0 15.6 0.4 Abbreviations: Ccr = exogenous creatinine clearance; Sm, = serum magnesium; Sc. = serum calcium; Sp = serum phosphorus; Cp = phosphate clearance. Hypermagnesemia and Parathyroid Gland Activity 1621 TABLE I I Detailed Protocol of Two Experiments, One without (Control) and One with MgC12 (i.v. Mg), Carried out in Dog No. 8 between 2:00 p.m. and 9:00 p.m. C. CP X 100 Ccr Sfg Sca Sp CCr Time Control i.v. Mg Control i.v. Mg Control i.v. Mg Control i.v. Mg Control i.v. Mg m11min mg/100 ml ml/min mg/100 ml p.m. 12:30 Oral water load, 500 ml 1:00 Anesthesia with pentobarbital Priming dose of creatinine (500 mg) Infusion started: 7 mg creatinine/min delivered in 0.45% saline~at 1 ml/min 2:00-2:20 68.1 60.5 1.59 1.80 10.8 10.2 5.2 4.4 12.0 25.4 2:20-2:40 69.0 56.7 1.68 2.00 10.8 10.2 5.2 4.5 11.7 26.3 2:40-3:00 65.9 56.0 1.54 1.90 10.5 10.2 5.1 4.4 13.0 27.6 3:00 In i.v. Mg experiment, MgCl2 added to infusate to deliver 100 jug mg/kg per min 3:00-3:30 56.4 56.8 2.43 4.0 18.1 15.8 3:30-4:00 54.8 66.0 1.61 3.76 10.8 10.1 5.0 4.0 20.4 5.9 4:00-4:30 54.9 59.3 4.96 4.3 17.8 4.6 4:30-5:00 59.7 56.9 1.68 5.79 10.8 9.6 4.9 4.5 12.2 4.0 5:00-5:30 62.6 53.9 6.40 5.0 12.0 3.2 5:30-6:00 65.4 57.0 1.60 7.20 10.5 9.4 4.7 5.3 15.3 2.5 6:00-6:30 65.3 52.6 7.73 5.3 13.0 2.3 6:30-7:00 53.1 51.3 1.53 8.00 10.8 9.3 4.9 5.3 14.3 1.5 7:00-7:30 66.9 53.0 8.67 6.0 13.6 1.4 7:30-8:00 63.3 57.7 1.40 9.03 10.8 8.8 5.1 6.1 14.2 2.1 8:00-8:30 51.6 55.1 9.33 6.5 21.9 2.9 8:30-9:00 64.3 56.7 1.49 9.54 10.5 8.3 5.1 7.0 16.8 3.8 Abbreviations: CCr = exogenous creatinine clearance; Sm. = serum magnesium; Sc8 = serum calcium; Sp = serum phosphorus; Cp = phosphate clearance. and 2). The fraction of filtered phosphate excreted CP/Ccr then increases gradually to reach the same (CP/CCr) is usually high in the early morning hours, values observed during early morning hours between and it decreases to a nadir between 11 a.m. and 1 p.m.; 3:00 p.m. and 4:00 p.m. Fractional phosphate excretion either remains stable or increases slightly thereafter. CONTROL I INFUSION *. Without M9CI2 The possibility that prolonged anesthesia of 8 hr may 10 have affected the pattern of phosphate excretion could X9]-A---4--_4__ not be excluded. In contrast to the results in the in- ... T___4 tact dogs, the fraction of filtered phosphate excreted I" w84 fell in the afternoon hours in the T-PTX dogs. It is E possible that the diurnal excretory pattern of phosphate o 7 0 may be altered by thyroparathyroidectomy. N. 6. The detailed protocols of an experiment with mag- E nesium chloride infusion and its paired control study, * S. both initiated during the morning hours, are presented ° TOTAL in Table I, and representative protocols of similar stud- i4. * DIFFUSIBLE ies began in the afternoon hours are shown in Table II. U9 ' IONIZED The summaries of the morning and afternoon experi- 10 11 1-2 i i 3 4 5 ments are depicted in Figs. 1 and 2, respectively. The a.m. TIME p.m. concentrations in serum of total, diffusible, and ionized calcium in seven dogs receiving an infusion of MgC12 FIGURE 3 The effect Of MgCI2 infusion on the mean levels of total, diffusible, and ionized serum calcium in the serum from 10:30 a.m. to 4:30 p.m. are presented in Fig. 3. intact dogs shown in Fig. 1. The brackets indicate ±1 sE. The infusion of MgCLi caused a gradual increase in the 1622 S. G. Massry, J. W. Coburn, and C. R. Kleeman TABLE I I I Detailed Protocol of an Experiment with MgC12 and the Intramuscular Injection of Parathyroid Extract Carried Out in Intact Dogs between 9:30 a.m. and 4:30 p.m.

Time Ccr SMg Sca Sp CP X 100

ml/min mg/100 ml mg/100 ml mg/100 ml a.m. 8:00 Oral water load, 500 ml 8:30 Anesthesia with pentobarbital Primary dose of creatinine (500 mg) Infusion started: 7 mg creatinine/min delivered in 0.45% saline at 1 ml/mia 9:30-9:50 37.5 1.34 8.8 4.6 25.3 9:50-10:10 37.5 1.34 8.8 4.6 21.1 10:10-10:30 37.0 1.47 8.8 4.6 17.6 10:30 MgCl2 added to infusate to deliver 100 jug mg/kg per min Intramuscular injection of 200 U of parathyroid extract 10:30-11:00 39.1 1.97 5.2 24.3 11:00-11:30 39.2 2.10 8.7 5.1 30.6 11:30-12:00 43.2 4.59 5.2 31.3 p.m. 12:00-12:30 38.6 5.88 9.0 5.4 39.4 12:30 Intramuscular injection of 200 U of parathyroid extract 12:30-1:00 36.9 6.55 - 5.5 44.3 1:00-1:30 36.4 6.82 9.3 5.5 32.7 1:30-2:00 38.5 7.70 - 6.1 35.6 2:00-2:30 36.9 8.03 9.6 6.1 39.2 2:30 Intramuscular injection of 200 U of parathyroid extract 2:30-3:00 37.9 8:40 6.5 37.9 3:00-3:30 38.8 9.10 9.9 6.8 33.6 3:30-4:00 38.2 9.73 7.0 35.2 4:00-4:30 38.6 10.26 10.2 7.5 30.0

Abbreviations: Ccr = exogenous creatinine clearance; Sma = serum magnesium; Sca = serum calcium; Sp = serum phosphorus; Cp = phosphate clearance. serum concentration of total magnesium to levels of 8- long as the magnesium infusion was continued. In stud- 11 mg/100 ml at the end of the experiment; since there ies initiated in the morning hours, the fall in CP/CC' was no significant change in the per cent diffusibility during the first 2 hr of the MgCls coincided with the of serum magnesium, the increments in levels of dif decrease in CP/Cc7 observed as part of the normal fusible magnesium were proportional to changes of the diurnal variation; therefore, it was not possible to as- total concentration of this ion. The hypermagnesemia sess the level of serum magnesium at which a change in was associated with a decrease in the concentration of CP/Cc, had first occurred. However, the studies started total serum calcium of 1.0-2.4 mg/100 ml and a propor- in the afternoon hours would allow such an evaluation. tional fall in levels of the diffusible and ionized frac- When the concentration of serum magnesium increased tions of calcium. The decrement in the concentration of by 2.0-3.0 ml to levels of 3.5-4.5 mg/100 ml, there was serum calcium was gradual, and the mean value for the a significant fall in CP-Cow (P < 0.01). entire group was significantly lower than the control When parathyroid extract was administered to four level by the 5th hr of MgCl2 infusion (P = 0.02). animals receiving MgCls infusion, the concentration of The serum concentration of inorganic phosphorus in- serum calcium rose by 1.0-2.0 mg/100 ml (from 8.9 creased from 4.0 ±0.37 to 6.8 ±0.58 mg/100 ml (mean ±0.16 to 10.4 ±0.44 mg/100 ml). The levels of serum +SE) in the morning studies and from 4.9 ±0.20 to 8.1 phosphorus increased from 4.1 ±0.42 to 6.9 ±0.30 mg/ ±0.91 mg/100 ml in the afternoon studies. Despite this 100 ml, and the magnitude of this increase was similar increase in the level of serum phosphorus and the sub- to that observed in experiments with only MgCL in- sequent rise in filtered phosphate, the fraction of filtered fused. Fractional phosphate excretion increased to more phosphate excreted fell markedly and remained low as than 250% of values observed before PTE administra-

Hypermagnesemia and Parathyroid Gland Activity 1623 CONTROL MgCI2 INFUSION :W th PTE (P < 0.05) after serum magnesium had increased by 111 1.7-2.0 mg/100 ml to a mean level of 3.28 +0.09 mg/ 100 ml. Following the discontinuation of MgCl2 infusion, aE I there was a gradual fall in serum magnesium, serum to 9 "LL calcium remained low, but CP/Cc, rose abruptly and often U reached values greater than those seen before the infusion of MgCl2 (Table V). A summary of four such ex- studies demon- 7. periments is presented in Fig. 6. These strate that CP/Cc, may increase in the presence of relative hypocalcemia despite the same degree of hyper-

-a magnesemia which otherwise causes CP/Cc, to fall. 0 Detailed protocols of an acute experiment during I-. . z T-PTX and of that with the infusion of CaCl2 are presented in Tables VI and VII, respectively. The removal a fall in the &Z4~ of the parathyroid glands was followed by MI concentration of serum calcium by 1.0-2.0 mg/100 ml and by a marked decrease in CP/Ccr; the decrease in serum calcium level was significant (P < 0.05) by 2-3 hr after the removal of the parathyroid glands. The infusion of 10 11 12 1 2 3 4 5 CaCI2 with the subsequent hypercalcemia was also fol- a.m. p.m. lowed by marked fall in CP/Ccr. The pattern and the TIME magnitude of changes in CP/Ccr following either thyro- FIGURE 4 The mean values of serum calcium (Ca) and the or calcium infusion resembled changes in the fraction of filtered phosphate excreted (CP/ parathyroidectomy closely Ccr) in four intact dogs receiving MgCl2 infusion (100 Aig those observed during the infusion of MgCl2 (Fig. 7). Mg++/kg per min) with and without the administration of parathyroid extract (PTE). The changes in CP/Cc, are DISCUSSION expressed as in Fig. 1. Although certain data are available suggesting that hy- tion. The per cent change in CP/CCr noted in dogs re- permagnesemia may suppress the activity of the para- ceiving MgClh and PTE was significantly different thyroid glands in vivo, such an effect has not been es- from that observed in the same dogs receiving only tablished with certainty. Gitelman, Kukolj, and Welt MgCl2. A detailed protocol of an experiment with the (7) showed that the injection of magnesium chloride administration of MgCl2 and PTE is presented in Table to nephrectomized rats resulted in a small but sig- III. The changes in the concentration of serum calcium nificant decrease in the serum concentration of ionized and in CP/Ccr observed in four dogs receiving MgCL2 calcium when compared with nephrectomized rats re- infusion, with and without PTE administration, are ceiving only saline; the injection of magnesium chloride shown in Fig. 4. did not affect the level of ionized calcium in the blood Representative experiments in a T-PTX dog, with and when given to hypocalcemic parathyroidectomized and without MgCla infusion, are shown in Table IV. The nephrectomized rats. These data are consistent with the summary of changes in the concentrations of serum hypothesis that hypermagnesemia may suppress the ac- calcium and in CP/Ccr in all such studies is presented in tivity of the parathyroid glands but are not conclusive. Fig. 5. The infusion of MgCla caused a significant rise The possibility that hypermagnesemia may interfere with in the concentration of serum phosphorus (from 5.5 the action of parathyroid hormone on bone was not ex- +0.37 to 7.4 ±0.21 mg/100 ml) and a slight decrease in cluded. Buckle, Care, Cooper, and Gitelman (8) per- the levels of serum calcium. However, in these T-PTX formed in vivo perfusion of isolated parathyroid glands animals, the levels of serum calcium and the changes in of one sheep and three goats with a blood perfusate con- CPICa, were not different during MgCl2 infusion from taining magnesium in a concentration of 3.0-5.7 mg/100 observations made in control studies carried out with- ml. A decrease in the level of parathyroid hormone out MgCl2 infusions in the same dogs. (PTH) in the effluent blood occurred. However, only The infusion of 40 yig Mg^/kg per min caused a in two goats did the change in PTH concentration ex- smaller rise in the concentration of serum magnesium to ceed 1 SD (1.6 ng PTH/ml) for their immunoassay for 4.27 ±0.07 mg/100 ml after 3 hr of the infusion; by measuring parathyroid hormone; furthermore, in one of this time, serum calcium had fallen from 10.8 +0.14 to these goats, the correlation between the level of para- 9.8 ±0.26 mg/100 ml and CP/CCr had decreased mark- thyroid in the effluent blood and the magnesium concen- edly (Table V). This fall in CP/CCr was significant tration in the perfusate was not statistically significant. 1624 S. G. Massry, J. W. Coburn, and C. R. Kleeman TABLE IV Detailed Protocol of Two Experiments, One without (Control) and One with MgCl2 Infusion (i-v. Mg), Carried Out in the Same Thyroparathyroidectomized Dog between 2:00 p.m. and 9:00 p.m.

Ccr Smg Sca Sp CcrXl10 Time Control i.v. Mg Control i.v. Mg Control i.v. Mg Control i.v. Mg Control i.v. Mg p.m. 12:30 Oral water load, 500 ml 1:00 Anesthesia with pentobarbital Priming dose of creatinine (500 mg) Infusion I started: 7 mg creatinine/min delivered in 0.45 saline at 1 ml/min 2:00-2:20 69.0 59.8 1.50 1.47 7.4 7.6 5.8 4.9 10.3 15.7 2:20-2:40 69.0 66.8 1.54 1.50 7.4 7.6 5.7 4.9 9.5 14.9 2:40-3:00 70.0 67.2 1.53 1.54 7.4 7.5 5.4 4.9 8.5 12.3 3:00 In i.v. Mg experiment, MgC12 added to infusate to deliver 100 jg mg/kg per min 3:00-3:30 82.0 71.3 2.75 4.7 8.3 6.0 3:30-4:00 84.5 74.7 1.51 3.85 7.4 7.4 4.4 4.4 1.2 6.1 4:00-4:30 85.4 72.5 4.65 4.4 1.7 5.7 4:30-5:00 79.0 77.4 1.49 5.60 7.4 7.5 5.1 4.5 2.4 2.6 5:00-5:30 78.2 72.7 6.70 4.7 1.9 1.8 5:30-6:00 80.5 68.6 1.45 7.35 7.3 7.3 5.3 4.9 1.8 1.8 6:00-6:30 85.4 76.5 8.25 5.6 4.8 2.3 6:30-7:00 79.3 76.4 1.48 9.35 7.4 7.3 5.0 5.8 6.0 6.5 7:00-7:30 81.5 80.7 10.15 6.10 6.3 8.4 7:30-8:00 83.2 77.7 1.48 10.85 7.3 7.4 4.9 6.40 8.0 9.0 8:00-8:30 84.0 78.1 11.55 6.90 10.2 12.0 8:30-9:00 85.1 80.8 1.50 12.10 7.3 7.4 4.9 7.30 8.6 14.6

Abbreviations: Cc, = endogenous creatinine clearance; Smg = serum magnesium; SCa = serum calcium; Sp = serum phosphorus; Cp = phosphate clearance.

The results of the present study demonstrate that levels of serum calcium caused by MgCL in the rat acute hypermagnesemia is associated with a significant could be due to enhanced secretion of calcitonin. How- decrease in the concentration of both total and ionized serum calcium, a rise in the level of blood inorganic CONTROL lNFUSOmw 1 phosphorus, and a marked fall in the fraction of filtered a a phosphate excreted. Theoretically, several mechanisms 0 8- could be responsible for these changes. Magnesium in- L--I7r fusion is associated with an increase in urinary calcium excretion (20-22), and such losses might conceivably cause a fall levels. urinary cal- in serum calcium Total 0 cium excretion, which was measured in six dogs, varied ') z 100- from 25 to 90 mg over the entire 6 hr period of MgCl. a.z infusion. This amount of calcium is less than the theo- w retical loss necessary to cause the observed decrease in 50- serum calcium levels, calculated from the assumption that calcium is distributed in extracellular fluid; since this- is a gross underestimate of the distribution of 3 4 5 6 7 8 9 miscible calcium (23), it is apparent that the fall in TIME (p~m) the levels of serum calcium cannot be explained by the urinary losses of this ion. FIGURE 5 The changes in mean levels of serum calcium (Ca) and in the changes in the fraction of filtered phos- Stimulation of secretion of calcitonin can produce a phate excreted (Cp/Co7) in four thyroparathyroidectomized fall in serum calcium (24), and studies of Radde, Witter- dogs without and with MgCl, infused (100 ,ug Mg++/kg per man, and Pensuwan (25) suggest that the early fall in min). The changes in CG/Ca, are expressed as in Fig. 1. Hypermagnesemia and Parathyroid Gland Activity 1625 TABLE V Detailed Protocol of an Experiment in Which a Smaller Amount of MgCl2 was Infused into Intact Dog No. 8 for a Shorter Period of Time

Time Ccr SMg Sca Sp Ccr X 100 Mlmin mg/100 ml mg/100 ml mg/100 ml p.m. 12:30 Oral water load, 500 ml 1:00 Anesthesia with pentobarbital Priming dose of creatinine (500 mg) Infusion I started: 7 mg creatinine/min delivered in 0.45% saline at 1 ml/min 2:00-2:20 50.0 1.74 11.2 5.0 25.5 2:20-2:40 52.4 1.69 11.2 4.9 26.4 2:40-3:00 52.8 1.68 11.1 5.0 23.0 3:00 Infusion II started: 40 ug mg/kg per min delivered in 0.45% saline at 1 ml/min 3:00-3:30 54.2 2.04 5.1 15.3 3:30-4:00 57.4 2.87 10.5 4.7 9.8 4:00-4:30 48.8 3.16 4.7 5.5 4:30-5:00 58.1 3.72 10.3 4.9 1.5 5:00-5:30 46.2 4.08 5.4 1.4 5:30-6:00 58.1 4.13 10.0 5.6 2.6 6:00 Infusion II discontinued 6:00-6:30 60.6 3.88 5.7 5.9 6:30-7:00 60.0 3.45 9.8 6.0 13.6 7:00-7:30 64.1 3.44 6.4 17.0 7:30-8:00 66.2 3.30 9.6 6.5 28.4 8:00-8:30 64.2 3.11 6.3 36.2 8:30-9:00 67.0 2.76 9.6 6.4 40.6

Abbreviations: Cc, = exogenous creatinine clearance; Smg = serum magnesium; Sca serum calcium; Sp = serum phosphorus; Cp = phosphate clearance. ever, perfusion of the thyroid glands of goats with blood tion of phosphate. During MgCl2 infusion, glomerular containing a high concentration of magnesium failed to filtration rate remained stable and the levels of serum augment the secretion of calcitonin (26). In addition, phosphate increased; hence, filtered load of phosphate studies reported by Kleeman, Bernstein, and Chapman was elevated at a time when urinary excretion was suggest that the homeostatic regulation of the levels of falling, indicating an increase in the tubular reabsorption serum calcium in the dog is not affected by removal of of phosphate. the thyroid glands (27). Following the administration Several features of the present study favor the con- of calcitonin, a decrease in serum calcium level occurs clusion that the infusion of MgClh produces its effect much more rapidly than was observed in the present on serum calcium and phosphate excretion through in- study during the infusion of MgCl2; furthermore, calci- hibition of the activity of the parathyroid glands. The tonin does not cause a decrease in urinary excretion of effects of acute hypermagnesemia on the concentration phosphate (26). Although the present study does not of serum calcium and on phosphate excretion were simi- exclude the possibility that infusion of MgCh may stim- lar to those observed after parathyroidectomy; and the ulate secretion of calcitonin in the dog, it is unlikely changes in urinary phosphate which occurred with hy- that such an event could be responsible for the present percalcemia, which is known to inhibit activity of the results. parathyroid glands (1-3), closely resembled those noted The fall in fractional excretion of phosphate observed with hypermagnesemia. Furthermore, the elevation in during MgCL infusion occurred at a time of the day serum calcium and the marked increase in fractional when phosphate excretion is normally stable or rising. phosphate excretion which followed the administration A decrease in Cp/Cor could either be due to a reduction of parathyroid extract to hypermagnesemic dogs indi- in filtered phosphate or to enhanced tubular reabsorp- cate that hypermagnesemia, per se, does not interfere 1626 S. G. Massry, J. W. Coburn, and C. R. Kleeman TABLE CONTRL 47CI VI Detailed Protocol of an Experiment with Thyroparathyroidectomy Carried Out between 11 a.m. and 6:00 06- (T-PTX) p.m. CP Time Cce ScS Sp Ccr X 100 0~ ml/min mg/100 ml mg/100 ml W E a.m. 8:15 Oral water load of 500 ml 9:00 Anesthesia with pentobarbital Priming dose of creatinine (500 mg) 101 Infusion started to deliver 6 mg creatinine/min in 0.45% saline at 2 ml/min w 9 10:00-10:20 55.4 9.8 5.2 17.5 10:20-10:40 49.3 9.6 5.2 22.1 10:40-11:00 52.1 9.6 5.0 23.8 Thyroparathyroidectomy performed between 11:00 and 12:30 p.m. 11:00-11:30 44.4 9.7 4.7 23.0 I-- 11:30-12:00 46.7 9.6 4.8 14.6 p.m. 12:00-12:30 42.8 9.3 4.9 6.8 12:30-1:00 50.4 9.0 5.2 3.1 'z 1:00-1:30 53.7 8.8 5.0 3.4 1:30-2:00 52.7 8.9 5.4 5.7 2:00-2:30 54.3 8.6 5.8 0.7 50 2:30-3:00 52.9 8.5 5.9 0.3 3:00-3:30 48.9 8.5 5.8 0.9 3:30-4:00 53.2 8.2 5.9 0.6

Abbreviations: Ccr = exogenous creatinine clearance; Sca = serum cal- 3 4 5 6 7 8 9 cium; Sp = serum phosphorus; CP = phosphate clearance. TIME (p.m.) FIGURE 6 The mean levels of serum magnesium (Mg) and during calcium infusion, and this phenomenon has been calcium (Ca) and changes in the fraction of filtered phos- attributed to a shift of phosphate from soft tissues to phate excreted (CP/Ccr) with a lower rate of magnesium in- the extracellular space (28). It is possible that acute fusion (40 ttg Mg"+/kg per min) from 3 p.m. to 6 p.m. in hypermagnesemia exerts a similar effect. the same four dogs shown in Fig. 2. The changes in CP/Cca are expressed as in Fig. 1. The solid symbols (0) indicate observations with MgCl2 and the open symbols (0), TABLE VII those without. Detailed Protocol of an Experiment with Calcium Infusion into an Intact Dog Carried Out between 9:20 a.m. with the action of parathyroid hormone on its target to 5:00 p.m. organs, e.g., the skeleton and kidneys. Finally, hypermagnesemia failed to produce similar changes in serum -P X 100 calcium levels and phosphate excretion in thyropara- Time ccr Sc. Sp Cc. thyroidectomized animals; this provides evidence that mI/min mg/100 ml mg/100 ml these effects of hypermagnesemia are mediated through a.m. 7:45 Oral water load of 500 ml 8:30 Anesthesia with pentobarbital the parathyroid glands and that hypermagnesemia, per Priming dose of creatinine (500 mg) se, does not significantly influence either the movement Infusion I started to deliver 6 mg creatinine/min in 0.45% of calcium from bone or the excretion of phosphate by saline at rate of 2 ml/min 10:00-10:30 37.6 10.5 3.2 24.7 the kidneys. The mechanisms through which a rise in 10:30-11:00 39.6 10.8 3.2 22.5 the concentration of magnesium in the blood may in- 11:00-11:30 35.0 10.4 3.3 22.9 11:30 Infusion II started to deliver 5 mg Ca++/kg per hr in 0.45% hibit the activity of the parathyroid glands are not de- saline at 1 ml/min for 3 hr lineated in the present study; hypermagnesemia may 11:30-12:00 39.9 11.2 3.4 17.8 inhibit the formation and (or) release of parathyroid p.m. 12:00-12:30 38.1 12.0 3.6 11.8 12:30-1:00 31.5 12.8 3.8 4.4 hormone. 1:00-1:30 32.3 13.3 3.8 1.5 The rise in the concentration of serum phosphorus 1:30-2:00 33.8 13.7 4.4 1.2 2:00-2:30 33.7 14.6 4.8 0.5 during hypermagnesemia is probably unrelated to the 2:30 Infusion II discontinued. inhibition of the parathyroid glands or to the decrease 2:30-3:30 33.6 14.3 4.6 0.2 in the urinary excretion of phosphate, since hyperphos- 3:30-4:30 30.0 13.1 4.4 0.3 phatemia occurred during hypermagnesemia, both in 4:30-5:30 31.0 12.7 4.2 0.5 intact dogs receiving PTE and in the T-PTX animals. Abbreviations: Ccr = exogenous creatinine clearance; Sca = serum cal- An increase in the level of serum phosphorus occurs cium; Sp = serum phosphorus; CP = phosphate clearance. Hypermagnesemia and Parathyroid Gland Activity 1627 EFFECT OF CaC12 INFUSION OR T-PTX countered in man under certain physiological and patho- ON PHOSPHATE CLEARANCE logical states (e.g. following ingestion of oral magnesium salts and in uremia); therefore, it is possible that the relation between the levels of serum magnesium 12-11~~~~~~~~~~~~~~~~~~~~~ T it - and activity of the parathyroid glands has physiological VE significance. 4 0o McIntyre, Boss, and Throughton (29) suggested u o 10 CALCIUM INFUSION . that an interrelationship between magnesium and parathyroid

U,0 hormone is important in the regulation of serum mag- T5PTX 0 I nesium level. The ability of the kidneys to alter urinary 8 . ,I. N 1 19~~~~~~~~~~~~~~~~N excretion of magnesium with changes in the concentration of magnesium in blood is probably one of the ma- jor factors controlling the serum level of this ion (30), IC and evidence has accumulated suggesting that parathy- . Z roid hormone causes enhanced renal tubular reabsorption of magnesium (22, 29). With hypermagnesemia and Q S9z S consequent inhibition of the parathyroid glands, tubular reabsorption of magnesium may fall with enhanced urinary excretion; hence, serum magnesium level would 11l 1'2 1 2 3 4 S 6 return toward normal. a.rm. p.m. In the present study, a small decrease in the level of TIME serum calcium was associated with an increase in frac- FIGURE 7 The changes in mean serum calcium and in the fraction of filtered phosphate excreted (CP/Ccr) with cal- tional phosphate excretion and, presumably, increased cium infusion (seven dogs) and following thyroparathy- parathyroid activity, despite the presence of hypermag- roidectomy, T-PTX (seven dogs). The changes in CP/Ccr nesemia which was initially sufficient to decrease CP/Cc7. are expressed as in Fig. 1. The horizontal brackets en- These observations suggest that a decrease in the levels compass the period of time during which either calcium was infused or the parathyroid glands were surgically removed. of serum calcium is more potent than a rise in the level of magnesium in the regulation of parathyroid The results of the present study also permit an evalu- activity. ation of the magnitude of increment in the concentration of serum magnesium which is required to inhibit the REFERENCES activity of the parathyroid glands. Data from the after- 1. Sherwood, L. M., J. T. Potts, Jr., A. D. Care, G. P. noon experiments, in which 100 Ag Mg++/kg per min was Mayer, and G. D. Aurbach. 1966. Evaluation by radio- immunoassay of factors controlling the secretion of given, suggest that the activity of the parathyroid parathyroid hormone Nature (London). 209: 52. glands had already been suppressed as the concentra- 2. Care, A. D., L. M. Sherwood, J. T. Potts, Jr., and G. D. tion of serum magnesium increased by 2.0-3.5 mg/100 Aurbach. 1966. Perfusion of the isolated parathyroid ml to levels of 3.5-5.0 mg/100 ml. Since the rate of rise gland of the goat and sheep. Nature (London). 209: 55. 3. Arnaud, C. D., Jr., A. M. Tenenhouse, and H. Rasmus- in the level of serum magnesium was rapid in these sen. 1967. Parathyroid hormone. Annu. Rev. Physiol. experiments, it is possible that even a smaller incre- 29: 349. ment in the concentration of serum magnesium may 4. Whang, R., and L. G. Welt. 1963. Observations in experi- suppress the parathyroid glands. The results from the mental magnesium depletion. J. Clin. Invest. 42: 305. experiments in which a smaller amount of magnesium 5. Heaton, F. W. 1965. The parathtyroid glands and magnesium metabolism in the rat. Clin. Sci. 28: 543. was infused (40 Ag/kg per min), indeed demonstrate 6. Gitelman, H. J., S. Kukolj, and L. G. Welt. 1968. The that a significant fall in CP/Ccr became evident with a influence of the parathtyroid glands on the hypercalce- rise in the level of serum magnesium of only 1.7-2.0 mia of experimental magnesium depletion in the rat. mg/100 ml and at a serum concentration of 3.1-3.5 mg/ J. Clin. Invest. 47: 118. 7. Gitelman, H. J., S. Kukolj, and L. G. Welt. 1968. In- 100 ml. Although alterations in CP/Ccr may be a sensi- hibition of parathyroid gland activity by hypermagne- tive index of changes in the function of the parathyroid semia. Amer. J. Physiol. 215: 483. glands, the measurement of the levels of the parathy- 8. Buckle, R. M., A. D. Care, C. W. Cooper, and H. J. roid hormone in the blood will permit a more precise Gitelman. 1968. The influence of plasma magnesium con- evaluation of the minimal increment in the level of se- centration on parathyroid hormone secretion. J. Endo- crinol. 42: 529. rum magnesium which is necessary to suppress the ac- 9. Vitale, J. J., E. E. Hellerstein, M. Nakamura, and tivity of the glands. Increments in the concentration B. Lown. 1961. Effects of magnesium-deficient diet upon of serum magnesium of 1.7-2.0 mg/100 ml may be en- puppies. Circ. Res. 9: 387. 1628 S. G. Massry, J. W. Coburn, and C. R. Kleeman 10. Wener, J., K. Pintar, M. A. Simon, R. Motola, R. Fried- 20. Chesley, L. C., and I. Tepper. 1958. Some effects of mag- man, A. Mayman, and R. Schucher. 1964. The effects of nesium loading upon renal excretion of magnesium and prolonged hypomagnesemia on the cardiovascular system certain other electrolytes. J. Clin. Invest. 37: 1362. in young dogs. Amer. Heart J. 67: 221. 21. Samiy, A. H. E., J. L. Brown, D. L. Globus, R. H. 11. Bunce, G. E., K. J. Jenkins, and P. H. Phillips. 1962. Kessler, and D. D. Thompson. 1960. Interrelation between The mineral requirements of the dog. III. The mag- renal transport systems of magnesium and calcium. nesium requirement. J. Nutr. 76: 17. Amer. J. Physiol. 198: 599. 12. Chiemchaisri, Y., and P. H. Phillips. 1963. Effect of 22. Massry, S. G., J. W. Coburn, and C. R. Kleeman. 1969. dietary fluoride upon the magnesium calcinosis syndrome. Renal handling of magnesium in the dog. Amer. J. J. Nutr. 81: 307. Physiol. 216: 1460. 13. Estep, H., W. A. Shaw, C. Watlington, R. Hobe, W. 23. Bronner, F. 1964. Dynamics and function of calcium. In Holland, and S. G. Tucker. 1969. Hypocalcemia due to Mineral Metabolism. C. L. Comar and F. Bronner, edi- hypomagnesemia and reversible parathyroid hormone un- tors. Academic Press Inc., New York. 2A: 341. responsiveness J. Clin. Endocrinol. 29: 842. 24. Hirsch, F. F., and P. L. Munson. 1969. Thyrocalcitonin. 14. Ardill, B. L., J. A. Halliday, J. D. Morrison, H. C. Physiol. Res. 49: 548. Mulholland, and R. A. Womersley. 1962. Interrelations 25. Radde, I. C., E. R. Wittermann, and S. Pensuwan. 1968. of magnesium, phosphate, and calcium metabolism. Effect of thyroid and parathyroid on hypocalcemia oc- Clin. Sci. 23: 67. curring after a magnesium load. Endocrinology. 83: 1285. 15. Briscoe, A. M., and C. Ragan. 1966. Effect of mag- 26. Care, A. D., T. Duncan, and D. Webster. 1967. Thyro- nesium on calcium metabolism in man. Amer. J. Clin. calcitonin and its role in calcium homeostasis. J. Endo- Nutr. 19: 296. crinol. 37: 155. 16. Bonsnes, R. W., and H. H. Taussky. 1945. On the 27. Kleeman, C. R., D. Bernstein, and L. Chapman. 1967. colorimetric determination of creatinine by the Jaffe reac- The role of thyrocalcitonin (TCT) in the correction of tion. J. Biol. Chem. 158: 581. hypercalcemia in the dog. Clin. Res. 15: 124. 17. Fiske, C. H., and Y. Subbarow. 1925. The colorimetric 28. Chen, P. S., Jr., and W. F. Neuman. 1955. Renal reab- determination of phosphorus. J. Biol. Chem. 66: 375. sorption of calcium through its inhibition by various 18. Lavietes, P. H. 1937. Anaerobic ultrafiltration. J. Biol. chemical agents. Amer. J. Physiol. 180: 632. Chem. 120: 267. 29. MacIntyre, I., S. Boss, and V. A. Troughton. 1963. 19. Ettori, J., and S. M. Scoggan. 1961. A determination Parathyroid hormone and magnesium homeostasis. Nature of ionized calcium in serum by two-wavelength (London). 198: 1058. spectrophotometry of a metal indicator. Clin. Chim. 30. Walser, M. 1967. Magnesium metabolism. Ergeb. Physiol. Acta. 6: 861. Biol. Chem. Exp. Pharmakol. 59: 185.

Hypernagnesemia and Parathyroid Gland Activity 1629