Albumin Synthesis, Albuminuria and Hyperlipemia in Nephrotic Patients

Kidney International, Vol. 31(1987), pp. 1368—1376

Albumin synthesis, albuminuria and hyperlipemia in nephrotic patients

GEORGE A. KAYSEN, JOHN GAMBERTOGLIO, JAMES FELTS, and FLORENCE N. HUTCHISON

Renal Biochemistry Laboratory, Division of Nephrology, Department of Medicine, Veterans Administration Medical Center, Martinez; Department of Medicine, University of California at Davis; Departments of Pharmacy and Physiology, University of California at San Francisco, San Francisco, California, USA

Albumin synthesis, albuminuria and hyperlipemia in nephrotic pa. Hyperlipidemia is, in conjunction with albuminuria, and tients. Hyperlipemia is a common manifestation of the nephrotichypoalbuminemia, a characteristic of the nephrotic syndrome syndrome. Serum lipid concentrations have been observed by others to [1, 21. The mechanism for its occurrence is complex and be negatively correlated with serum protein concentration. Hyper- lipemia has been postulated to result from a coordinate increase in the involves a combination of reduced clearance of lipoproteins synthesis of both albumin and lipoproteins, as well as from theirfrom the circulation [2—6] and increased hepatic synthesis of decreased catabolism. Simultaneous measurements of serum lipid con- lipoproteins [3, 4, 7—9]. Most investigators have found a nega- centration and the rate of albumin synthesis have not been previously tive correlation between serum albumin concentration and reported. We measured the rate of albumin synthesis, urinary albumin serum cholesterol levels [3, 4, 10, 11], but serum triglyceride loss, serum albumin, protein, cholesterol and triglyceride concentration levels have been less clearly related to serum protein concen- in 13 nephrotic patients. Changes in the rate of albumin synthesis and in urinary albumin excretion were induced in eight patients by alteration in tration. dietary protein intake. The resultant changes in serum triglyceride and Plasma lipid concentrations can be reduced by infusion of cholesterol were analyzed by multiple regression analysis. The rate ofalbumin or other macromolecules capable of increasing serum albumin synthesis measured while patients were eating a low protein oncotic pressure [12, 13]. One interpretation of these data is diet was 12.61 1.20gIl .73 m2/day, well within normal limits, yet both that increased oncotic pressure leads to a reduction in the rate serum triglyceride and cholesterol concentrations were markedly ele- of albumin synthesis [13, 14]. It is postulated that the increased vated (265 65mgldl and 325 44mg/dl, respectively). Albumin rate of albumin synthesis that might occur in nephrosis might synthetic rate increased to 17.60 1.25gIl .73 m2/day when dietary protein intake was increased, while serum triglyceride and cholesterolcause a parallel increase in the rate of synthesis of all hepatic concentrations changed little; triglyceride concentration was 306 75 secretory proteins [7, 14, 15]. Reduction in the rate of albumin mg/dl and cholesterol 376 55mgldl. Serum cholesterol concentration, synthesis postulated to occur after the infusion of oncotically by multiple regression analysis, was dependent only upon the renal active macromolecules would then lead to the parallel reduction clearance of albumin P <0.0001,and changes in serum cholesterol in the rate of synthesis of lipoproteins. Albumin synthesis rates concentration was dependent only upon changes in the renal clearance were not measured during these studies, however. of albumin, P <0.001. Serum cholesterol concentration was completely independent of the rate of albumin synthesis. Serum triglyceride A potential relationship between the oncotic pressure of a concentration was also primarily dependent upon the renal clearance of putative intrahepatic interstitial pooi of albumin and the rate of albumin, r2 =0.663P <0.0001,with some contribution by the rate ofalbumin synthesis has been identified [16, 17], but no clear albumin synthesis, r2 =0.112,P =0.028.Changes in serum triglyceride relationship between the oncotic pressure of this pooi and concentration were also dependent upon changes in the rate of albumin serum oncotic pressure is discernable. It is difficult, therefore, synthesis, r2 =0.624,P <0.002.We could find no evidence for ato predict the rate of albumin synthesis from measurements of relationship between hypercholesterolemia and the rate of albumin synthesis in nephrosis. While serum triglyceride concentration showed serum oncotic pressure. some correlation with the rate of albumin synthesis, the very fact that Albumin homeostasis in nephrosis is complex. Multiple ob- the rate of albumin synthesis was only modestly increased, if at all, servers have been unable to demonstrate a correlation between while serum triglyceride concentration was grossly elevated, made it serum albumin concentration and the rate of albumin synthesis unlikely that there was a direct link between albumin synthesis and that [16—18], nor is there evidence that reduction in serum albumin of either triglycerides or of apolipoproteins. These data suggest that the renal loss of macromolecules, either albumin, or other substancesconcentration acts as a direct stimulus to albumin synthesis. cleared by the kidney in a parallel fashion with albumin, play a role in Albumin may even be synthesized at subnormal rates in some the deranged cholesterol metabolism in nephrosis, and perhaps also nephrotic patients [16]. In order to draw inferences between the contribute to derangements in triglyceride metabolism in these patients rate of albumin synthesis and plasma or serum lipid concentra- as well. tions in this syndrome, it is necessary to actually measure the rate of albumin synthesis, or steady state turnover. Since it is difficult to infer causality from linear correlation alone, it would Received for publication December 30, 1985 be useful to alter the rate of albumin synthesis in some way and and in revised form July 21 and December 8, 1986 determine the effect of that change on serum lipid concentra- © 1987 by the International Society of Nephrology tion. To ascertain the relationship, if any, between the rate of

1368 Hyperlipemia and albumin synthesis 1369

albumin synthesis and serum triglycerides and cholesterol con-sents the mean value obtained during the particular measure- centrations, albumin turnover at steady state was measured inment of albumin metabolism. thirteen patients with the nephrotic syndrome. When nephrotic Several patients were receiving medications: diuretics or patients [19] or rats [20] are fed a low protein diet, the rate ofantihypertensive agents to control blood pressure or edema, albumin synthesis is reduced. Therefore, paired studies weredigoxin, and one patient was receiving prednisone. The specific performed in eight of these patients to determine whetherdrugs that each patient was taking are referred to in Table 1. changes in the measured rate of albumin synthesis, urinaryDrug dosages were, wherever possible, not altered during the albumin excretion and serum albumin concentration that oc-course of study. curred subsequent to changes in the protein content of their diet might affect the serum concentration of cholesterol or triglyc- Measurement of albumin synthesis and catabolism erides. If serum was lactescent, it was centrifuged at 126000g for 10 minutes in a microultracentrifuge (Beckman, Palo Alto, Cali- Methods fornia, USA). The clear infranate was used for determination of Patients with proteinuria greater than 3.5g/24hr measured inradioactivity and for chemical measurements. One ml of each the renal clinic prior to admission were asked to participate inserum sample was counted in a Packard 3002 gamma counter this study. Informed consent was obtained from each patient.for three 10 minute periods and the results averaged. The All patients had known proteinura for at least six months priorcounting efficiency was 76%, the standard error of counting was to this study. Renal biopsy was performed prior to the study inless than 1%. One ml urine aliquots were precipitated with 10% nine of the patients. Diabetic renal disease was diagnosedtrichloracetic acid (TCA). The samples were centrifuged in a clinically in patients 2, 4 and 7 without renal biopsy. NoBeckman microfuge (Beckman) for five minutes, the supernate diagnosis was available for patient 13. The patients ranged indecanted and used to determine the fraction of nonprecipitable age from 20 to 78 years. Further clinical information is pre-1251. The pellet was dissolved in 1% sodium hydroxide and sented in Table 1. reprecipitated. The resulting pellet was dissolved in a total All patients were studied while on a fixed diet. Patients 1volume of 1 ml of 1% sodium hydroxide. The TCA supernatant through 3 were interviewed by a dietitian. A nutritional historyand the dissolved pellet were each counted for three 10 minute was obtained, and the patients were each placed on a constantperiods in a Packard 3002 gamma counter. Urine samples were diet containing a content of protein, carbohydrate and lipidanalyzed in triplicate. comparable to what they were eating prior to admission. Patient Total albumin turnover was measured by a pharmacokinetic 4 was on constant total parenteral nutrition prior to and duringmethod, integrating the plasma radioactivity curve [21, 22]. The the study. Patients 6 through 13 were studied in a paireddose of 1251albumininjected (D), divided by the integral of the fashion. Patients 6 through 11 were placed on a fixed dietplasma radioactivity curve (AUC) is a clearance term: containing 1.6 grams of protein and 35 kcal while patients 12 D and 13 were placed on an isocaloric diet containing only 0.8 g of protein per kilogram body weight. The diets were comparable in AUC lipid composition and in the content of sodium, calcium andThe product of this term and the serum albumin concentration phosphorus. Carbohydrate was substituted for protein in theis then total albumin loss by all routes [23]. Under steady state preparation of the low protein diet. Patient 5 received only theconditions, 1.6 g/kg protein diet. D All patients remained on the selected diet for a minimum of x serum albumin concentration = two days prior to study of albumin metabolism. The patientsAUC then received 10 drops of Lugol's solution twelve hours and one total albumin turnover. hour before injection of 125jalbuminto block thyroid uptake of 1251.1251albumin(Mallinkrodt, Inc. St. Louis, Missouri, USA)This value is also equal to the average rate of albumin synthesis 10 DCi, was injected into a peripheral vein. Blood samples (3during the period studied. Plasma volume is calculated by ml) were obtained from a peripheral vein at 5, 15, minutes andisostope dilution [23] using 1251 values. Plasma albumin mass at 1, 2, 5, and 10 hours after injection. Additional samples were(PAM) is equal to the product of plasma volume and serum drawn 24 hours after the injection and daily for two weeks.albumin concentration. Steady state volume of distribution Urine was collected daily. Following the first study of albumin(Vdss) was measured by the non-compartmental method of turnover, dietary composition was altered. Patients 6 throughBenet and Galeazzi [211. Total albumin mass (TAM) is the 11 were placed on the 0.8 gram protein per kilogram bodyproduct of the serum albumin concentration and the apparent weight diet, and patients 12 through 14 were given the 1.6 gramvolume of distribution of albumin. protein per kilogram body weight diet. The second diet was TAM =serumalbumin concentration x Vdss maintained for a five to fourteen day equilibration period. A second 10 Ci dose of 1251albuminwas administered, andExtravascular albumin mass (EVAM) is TAM minus PAM. albumin turnover was measured during the subsequent 14 days Albumin turnover measured by this method will equal the in an identical fashion as during the first dietary period. albumin synthesis rate if patients are in steady state, that is, if Creatinine clearance was measured in all patients but numberserum albumin concentration and urinary albumin excretion I. Each value represents the mean of 14 consecutive measure-rates remain constant. Since body weight, urinary albumin ments. In the nine patients in whom paired studies wereexcretion and serum albumin concentration did not vary signif- performed each value of creatinine clearance (Table 1) repre-icantly throughout these studies, steady state criteria were met. 1370 Kaysenet al

Table 1. Clinicalcharacteristics,parameters of albuminhomeostasis anddietary intakeof patients studied

Creatinine Calorie Protein Serum Weight Surface clearance cholesterol Patient Sex Age kg area m2 Diagnosis Medicationsa mI/mm Intake/kg mg/d! I. M 39 55.5 1.65 Membrano proliferative None NA 31.6 1.50 474 glomerulo nephritis 2. F 64 75.5 1.72 Diabetes mellitus 1,8 20.5 22.6 0.84 261 3. M 20 79 1.82 Focal glomerulo- None 103.6 39 1.44 207 nephritis 4. F 35 51 1.50 Diabetes mellitus 1,8 33.7 33 1.20 330 5. M 30 70.5 1.95 Focal glomerulo- 1 50.4 35 1.60 435 sclerosis 6. M 39 102 2.35 Focal glomerulo- 1,4,5,13 48.1 35 1.60 876 sclerosis 41.6 35 0.80 538 7. M 70 112 2.29 Diabetes mellitus 1,7,8,10,14 48.4 35 1.60 133 48.5 35 0.80 166 8. M 64 88 2.04 Membranous 1,6,9 72.5 35 1.60 328 nephropathy 68.8 35 0.80 236 9. M 49 68 1,85 Amyloidosis 3 26.8 35 1.60 390 20.1 35 0.80 390 10. M 74 81 2.07 Membrano proliferative 1,10,11 52.1 35 1.60 198 glomerulonephritis 49.4 35 0.80 168 11. F 78 60 1.61 Membranous 1,4 85.5 35 1.60 326 nephropathy 92.5 35 0.80 303 12. M 67 101 2.23 Membranous 1,4,15 46.6 35 1.60 378 nephropathy 48.4 35 0.80 390 13. M 69 60 1.61 Unknown (solitary 2,4,5 21.5 35 1.60 442 functioning kidney) 20.2 35 0.80 478 a Medications that patients were taking during the course of the study are indicated numerically:I, furosemide; 2,bumetanide; 3, hydrochlorothiazide; 4, metolazone; 5, amiloride; 6, prazosin; 7, hydralazine; 8, insulin; 9, tolbutamide; 10, digoxin; 11, isosorbide dinitrate; 12, coumadin; 13, prednisone; 14, allopurinol; 15, ferrous sulfate.

Albumin catabolic rate was measured by the method of Gitlin Renal albumin clearance during each experimental period andJaneway [241.Urinewas collecteduntil the excretion rate was calculated using the mean urinary albumin excretion per 24 of 1251 was negligible. The average specificradioactivityofhours and mean serum albumin concentration. urinary albumin was determined, and total urinary nonprotein bound 1251 was measured as well. Albumin catabolic rate is Measurement of bloodlipids then: Serumsamples were obtained during fasting once on day 10 of each dietary period, and were extracted with chloroform: Albumin cataboloic rate =Urinary nonprotein 1251 (total) methanol (2: 1). Salicic acid was added to the chloroform phase specifiè radioactivity of x to remove phospholipids [281. An aliquot containing 30 to 100 g (total time) urinary albumin of triglyceride was evaporated to dryness and redissolved in This method is based on the observation that the specificisopropanol for colorimetric determination according to the radioactivity of urinary albumin is equal to the specific radio-method of Fletcher [291. Total cholesterol was determined with activity of serum albumin at any given time, and the iodinea cholesterol oxidase kit (Sigma Kit No. 350-B). released from catabolism of i25J labeled albumin all appears in Statistics the urine as TCA soluble 1251.Themethod is dependent on a constant relationship between albuminuria and albumin cata- Data analysis were by multiple regression analysis using a bolic rate throughout the experimental period, but is unaffected National Institutes of Health computer research—resource de- by variations in albuminuria or by delay in iodine excretion. veloped specifically for use by biomedical scientists— PROPHET [30]. The program used generates a table of partial Measurementofserum and urine protein correlation coefficients and then analyzes these by a "stepwise Albumin was measured both in serum and urine by electro- regression procedure" [30]. Independent variables are added to immunodiffusion [251. Total protein was measured by theor removed from the equation depending upon their contribu- method of Bradford using an albumin standard [261. Values for tion to the overall fit of the model. The addition of any variable serum albumin and protein concentrations, and for urinarywill increase the overall r2 of the multiple correlation. The protein and albumin excretion rates represent the average of 14 program therefore only adds variables that increase the signif- daily consecutive measurements. Serum colloid osmotic pres- icance of the overall multiple regression at each step. Variables sure (COP) was calculated by the method of Nitta et al [27)for that do not increase the significance of the overall regression are serum with albumin/globulin ratios different than one. not added to the model. Following each interaction, the residual Hyperlipemia and albumin synthesis 1371

Table 1.Continued Extra Serum Serum Serum Albumin Albumin Total Plasma vascular Urinary Urinary Renal tn- albumin protein Serum albumin protein synthesis catabolic albumin albumin albumin glyc- concen- concen- oncotic excretion excretion albumin rate rate mass mass mass endes tration tration pressure clearance mg/dl gIdI g/dl Torr. g/l.73 m2124hr pJlmin gIl.73 ,n/24 hr g/!.73 in2 ND 2.71 4.75 13.8 5.10 9.59 124 15.05 11.2 112 51 61

406 3.55 6.32 20.7 2.10 3.02 41 7.83 6.5 I54 66 88 197 3.3 6.96 22.4 1.67 2.30 35 11.7 8.32 68 80.4 149

ND 2.19 3.47 9.56 3.68 5.40 101 16.4 12.7 140 70 70 344 1.32 3.99 8.75 17.1 23.9 1015 27 5.8 65.6 29.8 35.8

798 0.83 3.55 6.72 16.9 25.0 1916 24.2 6.8 47.9 23.8 24.1 554 0.81 3.25 6.10 9.5 14.8 1106 17.8 4.7 56.5 22.8 33.7 113 2.60 4.45 12.8 5.7 9.5 201 16.5 9.1 214 88 126 123 2.87 5.20 15.5 5.6 8.7 179 15.4 9.0 282 106 176 347 2.70 4.00 11.9 8.2 10.4 249 19.05 11.6 148 76 72 124 2.77 4.10 12.3 1.8 2.7 52 9.3 3.9 126 85 41 129 1.77 3.86 9.48 5.0 7.7 210 18.4 10.6 128 59 69 149 2.10 3.56 9.56 4.6 7.1 161 16.3 11.9 100 62 38 53 3.22 5.38 16.8 8.5 13.5 220 18.1 10.8 201 116 85 55 3.66 7.31 24.7 5.9 6.1 134 12.5 5.9 215 130 85 211 2.58 5.30 15.0 7.8 8.9 195 15.7 11.5 100 56 45 lOS 2.69 6.56 19.3 5.9 7.1 143 10.0 8.2 100 63 37 335 1.30 3.44 7.54 9.5 11.1 499 16.1 6.6 107 48 59 340 1.67 3.67 8.84 5.9 7.7 256 9.58 5.6 81 51 30 422 0.72 3.8 6.99 7.5 12.3 652 13.0 4.4 40 21 19 532 0.90 5.2 10.7 7.1 11.6 506 14.8 3.8 68 32 36 contribution of each of the variables to the fianal model arehistories were negative, no changes in statistical evaluation recomputed. Since several of the dependent variables arebased upon a known family history of deranged lipid metabo- dependent upon one another (serum protein concentration andlism was necessary. Diabetes mellitus might also potentially colloid osmotic pressure, for example), their partial correlationcause a change in serum triglyceride concentration indepen- coefficient will be reduced with each subsequent iteration. Thedently of the effect of the nephrotic syndrome. For this reason, initial tables of partial correlation coefficients are therefore notdata were analyzed once using all data points from all patients, presented in the results section, inasmuch as they are notand a second time omitting the diabetic patients to determine directly interpretable. The results of simple linear regressionwhether exclusion of the diabetic patients gave different statis- analysis also do not contribute to the development of a modeltical results. Results are presented as mean SEM. describing the relationship between the dependent variable and the multiple independent variables tested. The results of several Results linear regression analyses are presented, however, in graphic Patients clinical characteristics, laboratory values, parame- form to provide visual analysis of the relationships betweenters of albumin homeostasis and dietary caloric and protein several of the variables analyzed. No conclusions were drawn,intake appear in Table 1. however, from simple linear regression analysis. Measurement The rate of albumin synthesis measured while patients were of serum triglyceride was not performed in two patients (Nos. Iconsuming a low protein diet (<1 glkglday) was 12.61 1.20 and 4). Data from these patients were, therefore, not used in theg/l .73 m2/day, and when measured during consumption of a multivariant analysis to evaluate plasma triglycerides. Twohigher protein diet (>lg/kg/day) was 17.60 1.25 g/l.73 measurements of each parameter were available on patients 6m2/day. The rate of albumin catabolism was 6.61 0.89 g/l.73 through 13—one measurement on each diet. The set of data form2/day during consumption of the low protein diets and was each dietary period was evaluated for each patient as a separate9.12 0.78 g/l.73 m2/day during consumption of the higher entry. Two data points are therefore entered for each parameterprotein diets. The rate of albumin synthesis during consumption for each of these eight patients. Changes in the serum concen-of the low protein diet was within the normal range reported by tration of triglycerides and cholesterol that occurred afterus [231 (13.80 0.77) and the rate of albumin catabolism during variation in dietary composition were similarly correlated to theconsumption of either diet was below the normal range reported changes in the rate of albumin synthesis, urinary albuminby us and by other investigators [23, 16]. Both serum triglycer- excretion and serum albumin concentration. ide and cholesterol concentrations were markedly elevated, Family histories were obtained from each patient to deter-regardless of dietary protein content. Triglycerides were 265 mine whether there was any reason to suspect the presence of65 mg/dl for the nine measurements made on patients eating a underlying abnormalities in lipid metabolism in any patient thatdiet with a protein content of <1 g/kg/day, and cholesterol was might be unrelated to the nephrotic syndrome. Since these325 44 mg/dl. Triglycerides were 306 75 mg/dl when 1372 Kaysen et a!

27 A 900 25 23 800 21 0 700

19 .S E 600 0 17 a 500 . . a 15 a 0 400 13 . U . r =0.823 E 300 11 P 0.0001 • $ a 200 I 9 8.906 + .9492 * X (I) r=0.839S.. P = 0.0001 . 100 239.9+ .3013 X 2 4 6 8 10 12 14 16 18 0 Urinary albumin excretion, g/1.73 m2/day 0 500 1000 1500 'O00

Fig.1.Correlationbetween the rate of albumin synthesis and average Renalalbumin clearanceMI/mm urinary albumin excretion in nephroiic patients. measured during consumption of a diet with a protein content of >1 glkglday, and cholesterol was 376 55mg/dl. From these B data it was clear that neither serum cholesterol nor triglyceride 900 concentration were elevated as a result of an absolute increase 800 in the rate of albumin synthesis in these patients as a group. It was still possible however, that the relative rate of albumin 700 synthesis was responsible for variations of serum lipid concen- E 600 trations within the nephrotic patient population. 0 500 'a Urinary albumin excretion was dependent both upon the rate a a, 400 of albumin synthesis, with a partial correlation coefficient of 0 0.8226, P <0.001and upon renal albumin clearance, with a C) 300 0.0005, but E r = 0.763 . partial correlation coefficient of 0.8584 P < not upon 200 serum albumin concentration. The r value for the multiple a) P = 0.0001 U, 100 649.5 — 133.8 * X regression was 0.9231, r2 =0.8521.The relationship was unchanged by omission of the diabetic patients and was not 0 1 2 3.8 influenced by degree of renal insufficiency or by patient age. Thus, the rate of urinary albumin excretion is not itself an Serum albumin concentration, g/dI independent variable, but is instead a function of two indepen- Fig.2.Correlationbetween serum cholesterol concentration and the dent processes: 1.) the change in glomenilar permselectivityrenal clearance of albumin (A)andserum albumin concentration (B) in that is the primary process responsible for loss of macromol-nephrotic patients. ecules in the urine, and is most appropriately expressed as the renal clearance of albumin, or some other macromolecule; and 2.) the ability of the liver to increase the rate of albumintion on the rate of albumin synthesis, remained even when data synthesis sufficiently to stabilize serum—albumin concentrationobtained from the diabetic patients were removed from the in the presence of external loss; that is to re-establish a steadyanalysis. state in the presence of the increased renal clearance of It should be noted that serum cholesterol concentration macromolecules. correlated positively with the rate of albumin synthesis r = The rate of albumin synthesis was dependent only upon0.528,P <0.02,serum colloid osmotic pressure r =0.662,P < urinary albumin excretion with a partial correlation of 0.9492, P0.002,and with urinary albumin excretion r =0.741,P < <0.0001, and not upon either serum colloid osmotic pressure or 0.0005,as well as with the renal clearance of albumin r =0.892, serum albumin concentration. The linear relationship betweenP <0.0001(Fig. 2A), and with serum albumin concentration r the rate of albumin synthesis and urinary albumin excretion is=0.799P <0.0001(Fig. 2B) when analyzed by univariant shown in Figure 1. analysis, but the contribution of the first three parameters to Serum cholesterol concentration was dependent upon theserum cholesterol concentration were found to either be insig- renal clearance of albumin with a partial correlation coefficientnificant or not independent by multivariant analysis. of 0.8920, r2 =0.7957,P <0.0001 and upon serum albumin Changes in serum cholesterol concentration (Chol)result- concentration, with a partial correlation coefficient of 0.7988, r2ing from changes in dietary composition were dependent only = 0.6381,P =0.025.Residual r2 added to the regression byupon changes in renal albumin clearance (Albclear)by albumin synthesis rate was essentially zero. The dependence ofmultiple regression analysis, with a partial correlation coeffi- serum cholesterol concentration on the renal clearance ofcient of 0.9259, P <0.001),but not upon changes in serum albumin, and the independence of serum cholesterol concentra-albumin concentration (SAIb),changes in colloid osmotic Hyperlipemia and albumin synthesis 1373

A 120 700

600 40

'a a, 500

a' E Q 400 —60 >. S 0 I- E 300 —160 a'200 U) P=0.012 r = 0.814 50.08 + 20.06 X 100 P = 0.0001 ...... 141 +.342X =0.821 —260 —1 —13—11 —9 —7 —5 —3 —1 500 1000 1500 2000 XALB SYN, g/1.73 m2/day Renal albumin clearance, ,uI/min Fig.3. Correlation between serum triglyceride concentration and the renal clearance of albumin in nephrotic patients, B 120 pressure (COP),or changes in the rate of albumin synthesis ( AibSyn).These relationships remained unchanged whether 40 data obtained from all patients were analyzed or whether data .. from the three patients with diabetic nephropathy, or data from any other individual patient were removed from the array. E —60 Serum triglyceride concentration was dependent both upon0 the renal clearance of albumin, with a partial correlation coef- I- ficient of 0.8142, r2 =0.6629,P < 0.0001, and the rate of'I —160 r =0.729 albumin synthesis, with a partial correlation coefficient of P = 0.04 0.3340, r2 =0.1116,P 0.028, but not upon serum albumin 178.5 + 572.4 * X concentration, colloid osmotic pressure, or upon any other —260 parameter tested. This relationship was not changed by removal —0.05 0.05 0.15 0.25 0.35 0.45 of the data from the three patients with diabetic renal disease. The linear relationship between serum triglyceride concentra- Serum albumin, gidI tion and the renal clearance of albumin is shown in Figure 3. Fig.4. Correlation between changes in serum triglyceride concentra- When changes in serum triglycerides (Trig)that occurredtion ( Trig) and changes in albumin synthesis rate (iALBSYN) (A) subsequent to the alteration in dietary composition were ana-and changes in serum albumin concentration that resulted from alter- lyzed by multiple regression analysis,Trig was found to beation in dietary protein intake. dependent upon Li AlbSyn, with a partial correlation coefficient of 0.790, P < 0.002, (the linear relationship between these variables is demonstrated in Fig. 4A) and upon SAlb, P lipoprotein (VLDL) production. Any model of measurements performed on patients eating a high protein diet.hyperlipemia in nephrosis must also be consistent with the fact Albumin synthesis was within the normal range, or reduced forthat the rate of albumin synthesis is increased only modestly if patients consuming a low protein diet. Despite this, the serumat all in nephrotic patients as a group, while serum lipid concentration of both cholesterol and triglycerides was in-concentration remains markedly elevated. The absolute rate of creased in patients eating either diet. An increase in thealbumin synthesis then is probably not directly linked to the absolute rate of albumin synthesis is therefore not directlyincrease in serum triglyceride concentration; however within linked to the derangement in lipid metabolism, and the lack ofthe nephrotic patient population, serum triglyceride concentra- correlation between serum cholesterol concentration and thetion increases in direct proportion to the rate of albumin rate of albumin synthesis within this group of patients furthersynthesis. The mechanism for this apparent linkage remains excludes a direct relationship between hypercholesterolemiaobscure. It is clear however, that it is the renal loss of albumin and increased albumin synthesis in nephrotic patients. The rateand/or other macromolecules, and not an increase in albumin of albumin catabolism was reduced significantly compared tosynthesis which is central to the nephrotic syndrome. studies in normal populations [16, 19], thus reduction in the rate It is curious that a close correlation exists between the renal of albumin catabolism played a major role in adaption to theclearance of albumin and the concentration of serum lipids in urinary loss of the protein, and allowed these nephrotic patientsthese patients. A clue to the interaction between these param- to maintain albumin steady state, albeit at a reduced albumineters may be found by examining the interrelationship between mass, without a substantial increase in the rate of albuminthe urinary loss of glycoproteins and lipid metabolism. Since synthesis. proteins are not only filtered by the glomerulus, but are reab- Serum triglyceride concentration, like that of cholesterol,sorbed by the renal tubules, the renal clearance of proteins is was dependent partly upon the renal clearance of albumin (Fig.usually not useful as an indicator of changes in the glomerular 3), but unlike the case of cholesterol, serum triglyceride con-filtration of macromolecules [35]. Micropuncture studies in centration and variations in serum triglyceride concentration (nephrotic rats [36, 37], however, show that under conditions of Trig)were also dependent, at least in part, upon the relative ratenephrotic range proteinuria nearly all albumin filtered at the of albumin synthesis within the nephrotic patient populationglomerulus is excreted in the urine. Similarly [38], the fractional studied. renal clearance of albumin in nephrotic patients closely corre- McKenzie and Nestle [2] found that patients with the highestlates with changes in the glomerular permselectivity for, and the serum triglyceride—concentration had both the greatest urinaryrenal clearance of, other macromolecules. It is therefore prob- albumin—excretion and the most rapid triglyceride turnover.able that the glomerular changes in permselectivity that are Although these investigators measured neither the rate ofexpressed here as changes in the renal clearance of albumin albumin synthesis nor the renal clearance of albumin in theirmay lead to changes in the renal clearance of other macromol- patients, it is reasonable to assume that within their patientecules whose loss parallels the loss of albumin and influences population, albumin synthetic rate was greatest in their mostlipid metabolism. Results with nephrotic patients have shown albuminuric patients, as we have found, and that the rate ofthat urinary loss of a1-acid glycoprotein parallels urinary loss of both albumin synthesis and the serum triglyceride concentra-albumin [39]. This is undoubtedly true for other glycoproteins tion increased in direct proportion. Since they did not directlyas well. measure the rate of albumin turnover, it is not possible to Several workers have shown that as the nephrotic syndrome determine the quantitative relationship between the rate ofbecomes more severe, VLDL (and chylomicrons, CM) and albumin synthesis and the rate of triglyceride turnover in theirintermediate density lipoproteins, (IDL) increase and low den- patients. sity lipoproteins (LDL) decline [12]. Gherardi, Vecchia and It has been suggested that increases in the rate of albuminCalandra [40] have shown that the major very—low-density synthesis leads to an increase in the synthesis rates of otherlipoprotein in nephrotic rats closely resembled IDL. These hepatic secretory proteins [7, 14, 15]. The synthesis ofobservations taken together suggest a defect in the interaction lipoproteins and of albumin are both increased in the isolatedof VLDL (and chylomicrons) with lipoprotein lipase as well as perfused livers from nephrotic rats [32]. Early, et al [33]a defect in the hepatic metabolism of VLDL remnants and proposed that, because the rate of albumin synthesis waschylomicron remnants. This latter defect could interfere with increased in nephrosis, the synthesis of all proteins that arethe function of remnant particles (RM) as negative feedback secreted by the liver were also increased because they shared asignals to limit cholesterol and triglyceride synthesis by the common pathway for both synthesis and secretion. Most evi-liver [39, 41, 42] leading to an accelerated synthesis of VLDL dence however would support the hypothesis that thelipids and secretion of VLDL. hepatocyte does not regulate protein secretion [34], but that Other cofactors may be involved in these metabolic blocks. regulation of release of secreted proteins is at the level ofStaprans and Felts[6] demonstrated that nephrotic synthesis. The observation that, once synthesized, these pro-hyperlipemia in the rat is characterized by a decreased clear- Hyperlipemia and albumin synthesis 1375 ance of injected triglyceride. They found that the a1- acid drome. Biochem Biophys Res Comm 79:1272—1278, 1977 glycoprotein (aAG) fraction isolated from human nephrotic 7. MARSH JB, DRABKIN DK: Experimental reconstruction of meta- bolic patterns of lipid nephrosis. Key role of hepatic protein urine could stimulate the lipoprotein lipase reaction in vitro, synthesis in hyperlipidemia. Metabolism 9:946—955, 1960 and promptly corrected the triglyceride removal defect when 8. RADDING CM, STEINBERG D: Studies on the synthesis and secre- injected into nephrotic rats [6]. The active component in the tion of serum lipoproteins by rat liver slices, J Cliii Invest 39: urinary aAG fraction was identified as heparan sulfate [43, 44]. 1560—1568, 1960 Heparan sulfate is also present in human plasma [45]. It was 9. MARSH JB, SPARKS CE: Hepatic secretion of lipoproteins in the rat and the effect of experimental nephrosis. J Clin Invest 64:1229— further established that nephrotic patients excrete far less 1237, 1979 heparan sulfate than control subjects [44]. These findings sug- 10. NEWMARK SR, ANDERSON CF, DONADIDO JV, ELLEF50N RD: gest that some factor lost in urine of nephrotic patients influ- Lipoprotein profiles in adult nephrotics. Mayo Clinic Proc ences the synthesis of heparan sulfate which, in turn, may 50:359—364, 1975 11. APPEL GB, BLUM CB, CHIEN 5, KUNIS CL, APPEL AS: The influence the metabolism of CM, VLDL and possibly remnants hyperlipidemia of the nephrotic syndrome. N EngI J Med 312: leading to the observed altered lipoprotein pattern. Thus, these 1544—1548, 1985 data suggest that the loss of macromolecules, in parallel with12. BAXTER JH, GOODMAN HC, ALLEN JC: Effects of infusions of albumin, from the plasma into urine may be causally related to serum albumin on serum lipids and lipoproteins in nephrosis. J Clin Invest 40:490—498, 1961 the hyperlipemia of the nephrotic syndrome. 13. ALLEN JC, BAXTER JH, GOODMAN HC: Effects of dextran, Hyperlipemia in nephrosis is a complex phenomenon involv- polyvinyl—pyrrolidone and gamma globulin on the hyperlipidemia ing both increased synthesis of lipoproteins by the liver [3, 4, of experimental nephrosis. J Clin Invest 40:499—508, 1961 7—9] as well as a decrease in their catabolic rate [2—6, 4]. Its 14. SOOTHILL JA, KARK RM: The effects of infusions of salt—poor occurrence in the nephrotic syndrome may be the result of the human serum albumin on serum cholesterol cholinesterase, and albumin levels in healthy subjects and in patients ill with the urinary loss of macromolecules involved in the initial catabolic nephrotic syndrome. Clin Res Proc 4:140—141, 1956 reactions involved in the clearance of both chylomicrons and15. JONES AL, RUDERMAN NB, HERRERA MG: Electron microscopic hepatic VLDL. The renal clearance of these macromolecules and biochemical study of lipoprotein synthesis in the isolated may parallel the renal clearance of albumin. Although serum perfused rat liver. J Lipid Res 8:429—446, 1967 triglyceride concentration tends to parallel the rate of albumin16. JENSEN H, RossINo N, ANDERSON SB, JARNUM S: Albumin metabolism in the nephrotic syndrome in adults. Cliii Sci 33:445— synthesis in this nephrotic patient population, the observation 457, 1967 that the absolute rate of albumin synthesis is only slightly 17. ROTHSCHILD MA, ORATZ M, EVANS CD, SCHREIBER SS: Role of elevated makes a direct causal link between these processes hepatic interstitial albumin in regulating albumin synthesis. Am J Physiol 210:57—68, 1966 questionable. 18. ORATZ M: Oncotic pressure and albumin synthesis in Plasma Protein Turnover, edited by BlANCH! R, MARIANI G, MCFARLANE Acknowledgments AS. Baltimore, University Park Press, 1976, pp. 223—237 This research was funded in part by the United States Veterans 19. KAYSEN GA, GAMBERTOGLIO J, JIMINEZ I, JONES H, HUTCHISON Administration and United States Public Health Service Grants AM FN: Effect of dietary protein intake on albumin homeostasis in 25350, AM 21923 from the National Institute of Arthritis, Metabolism nephrotic patients. Kidney Int 29:572—577, 1986 and Digestive Diseases. Some of the studies were carried out in the20. KAYSEN GA, KIRKPATRICK WG, CousER WG: Albumin General Clinical Research Center (RR00083) San Francisco, California, homeostasis in the nephrotic rat: Nutritional considerations. Am J with the support by the Division of Research Resources, National Physiol 247:F192—F202, 1984 Institutes of Health, and some at the Veterans Medical Center,21. 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