Diuretic Effect of Bumetanide in Isolated Perfused Kidneys of Milan Hypertensive Rats

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Kidney International, Vol. 37 (1990), pp. 1084—1089

Diuretic effect of bumetanide in isolated perfused kidneys of Milan hypertensive rats

PATRICIA SALVATI, ROMANA G.FERRARIO,and GIUSEPPE BIANCHI

Farmitalia Carlo Erba Istituto Ricerche, Nerviano, Milano and Istituto Patologia Medica Universitâ di Sassari, Sassari, Italy

Diuretic effect of bumetanide in isolated perfused kidneys of Milan In genetic models of hypertension in the rat, Na,K,Cl hypertensive rats. The Milan hypertensive strain of rats (MHS) iscotransport has also been found to be either decreased, such as characterized in the prehypertensive phase (4 weeks of age) by a significantly faster bumetanide-sensitive cell membrane Na,K,Cl-in the spontaneously hypertensive rat (SHR) [9], or increased, cotransport as compared to matched normotensive controls (MNS). such as in the Milan hypertensive strain (MHS) [10]. The isolated kidney preparation, which allows the study of renal In fact MHS rats have a faster Na,K,Cl cotransport function under controlled in vitro conditions, was chosen to compare compared to normotensive controls, not only in erythrocytes, the natriuretic eff'ct of the loop diuretic, bumetanide, to that of two but also in vascular smooth muscle cells (VSMC) and thick other diuretics (amiloride and hydrochlorothiazide) acting on different parts of the nephron. Concentrations ranging from l0 to lO M were ascending limb tubular cells, as shown by previous in vitro tested in 4-week-old MHS and MNS. Our results showed that thestudies [11, 121. In addition, other abnormalities (such as lower natriuretic response to all diuretics was greater in MHS as compared to cell volume, lower intracellular Na content compared to MNS when evaluated as absolute Na excretion (UNa); this is likely controls, etc.) have also been found to be similar in MHS because of the faster basal glomerular filtration rate (GFR) in the hypertensive strain (874 126 in MHS vs. 556 33 sl min g' kerythrocytes and kidney tubular cells [13—15]. Moreover, young wt in MNS, P < 0.05). However, when calculated either as a difference prehypertensive MHS are characterized by a faster tubular Na from basal values (LUNa*), or per ml of glomerular filtration rate, the reabsorption linked to the subsequent development of hyper- response of MHS kidneys to amiloride and hydrochlorothiazide was tension [10]. similar in the two strains: LUNa after amiloride at (l0- M was +2.1 For all these reasons the Milan strain might be considered as 0.7 in MHS versus +1.2 0.2 mol .gg k wt in MNS; after hydrochlorothiazide l0- Mit was +1.7 0.9 in MHS versus +1.1 an interesting tool for correlating Na transport abnormalities 0.4 moI min' g' k wt in MNS, values not statistically different.and kidney function in hypertension. On the other hand, UNa* was disproportionally greater in MHS after It is in fact possible that loop diuretics, which selectively bumetanide (at l0 M +13.5 1.0 in MHS vs. +1.7 0.7 inhibit Na,K,Cl cotransport, might have a more pro- mol. min .gk wt in MNS, P < 0.001). GFR and renal perfusion flow rate were not modified by the three diuretics. Since isolatednounced natriuretic effect in this strain of rats as compared to kidneys showed a selectively higher natriuretic response to the loop their normotensive controls. In order to test this hypothesis, the diuretic, it is therefore possible to speculate that this class of diuretics activity of the loop diuretic, bumetanide, in comparison with might have a more pronounced natriuretic effect also in vivo in animal other diuretics acting on different parts of the nephron, such as models characterized by a faster Na ,K,Cl —cotransport.These data amiloride and hydrochiorothiazide, was studied using the iso- provide a working hypothesis for controlled studies in hypertensive patients with the same cotransport abnormality. lated perfused kidney. In this preparation, controlled extracellular ion and drug concentrations can be obtained in the absence of extrarenal nervous or humoral influences.

Several abnormalities in cell membrane ion transports have Methods been found in humans with essential hypertension, mainly using Kidney perfusion red blood cells as an experimental model [1—4]. One of these Kidneys from male MHS (28 1.0 days old, mean body abnormalities is observed in the Na ,K ,C1 cotransport, 1.3 g, N = and age-matched normotensive defined as the ouabain-resistant, bumetanide-sensitive Naweight 82.9 26) controls (mean body weight 83.10.9 g, N = 25) were isolated pathway [5]. Na,K,Cl outward cotransport has been re-and perfused according to the technique previously described ported to be either slower or faster in erythrocytes of essential [16]. hypertensives as compared to matched controls [6, 7], with a The mean kidney weights (kidney wt) were 0.360.05 and bimodal frequency distribution curve suggesting different sub-0.44 0.06 g (P < 0.01) for MHS and MNS, respectively. groups of patients [8]. Briefly, under Na pentobarbital anesthesia (45 mg/kg i.p.), the abdominal aorta rather than the renal artery itself was cannulated retrogradely (19 gauge needle) to permit perfusion of Received for publication November 28, 1988 kidneys from young animals. Collateral vessels between the and in revised form September 22, 1989 insertion point and the right renal artery were all ligated. The Accepted for publication November 13, 1989 right ureter was cannulated (0.5 cm segment of PE-lO con- © 1990 by the International Society of Nephrology nected to PE-50, Clay Adams, Parsippany, New Jersey, USA)

1084 Salvati et a!: Diuretic action in perfused kidneys 1085 and the kidney was then removed and transferred to a constant- 40 temperature (37.00.5°C) recirculating perfusion apparatus. The perfusion pressure distal to the tip of the cannula was 20 approximately matched to the mean arterial pressure determined previously in separate groups of conscious rats of the 0 same age: 92.5 and 100 mm Hg for young MNS and MHS, respectively. * These pressures were kept constant throughout the experi- 1000 ment by adjusting perfusion flow with a needle valve (Nupro Company, Ohio, USA) placed in parallel with the arterial line. Perfusion flow was measured by an in-line flow meter (A.H Thomas, Philadelphia, Pennsylvania, USA). Two in-line 8 pm - 500 filters (Millipore Corporation, Bedford, Massachusetts, USA) continuously filtered the perfusate during the experiment. 0•- The perfusion medium was a Krebs-Henseleit buffer solution (K-H) containing 6.7% wt/vol bovine serum albumin (fraction V, Miles Laboratories, Elkhart, Indiana, USA). The composition of the buffer (mM) was: NaCI 113, KCI 4.6 + U)100 Na2HPO4 1.8, NaH2PO4 0.6, MgSO4 1.17, NaHCO3 25 and CaCl2 1.00. The pH was maintained at 7.45 with a 5% CO2-95% z(ow 90 02 mixture. In addition the medium contained: glucose 7.5 m, a mixture 1 I I of amino acids (Solamin, Pierrel, Italy) as previously described 0 15 30 45 60 75 [17], vasopressin (40 ng/lOO ml), aldosterone (12.5 ng/lOO ml), Time,minutes corticosterone (1.6 g/l00 ml) and, Cr34VEDTA (2.0 mM) for Fig.1.Timecourse of overall function in isolated perfused kidneys estimation of renal clearance. Urinary and evaporative lossesfrom MHS (•,N 8) and MNS rats (0, N =8)after 30 mm of were replaced by an infusion of saline with the same composi-equilibration. Abbreviations are: RPF, renal perfusion flow; GFR, tion as K-H, except that Na concentration was 80 m and Kglomerular filtration rate; % Na reabsorption, fractional Na reabsorp- concentration was 25 mi. After 30 minutes of equilibration,tion. Data are mean sEM;*P<0.05;**P<0.01by unpaired t-test. three control samples of five minutes each (both urine and perfusate) were taken followed by three five-minute clearance periods for each concentration of diuretic. Either bumetanide, amiloride or hydrochlorothiazide were added to the medium cumulatively, (tenfold increasing concentrations from l0— tochemicals were from Sigma (Sigma Chemical Co, St. Louis, io- M), Further groups of MHS (N =8)and MNS (N =8) Missouri, USA). kidneys were vehicle-treated and used as time-dependent controls. Statistics All values are expressed as mean SEM and when appropri- Analyses and calculations ate are corrected per gram of wet kidney weight. Data were Urine flow rate (pVmin) was determined gravimetrically inanalyzed by unpaired analysis of variance. When the F-value preweighed vials. Na and K concentrations (mol/ml), wereindicated overall significance, specific comparisons were made measured by flame photometry with lithium as the internalusing the Duncan's test [18]. standard in both perfusate and urine samples (IL 243 Lexing- All comparison differences were considered significant when ton, Massachusetts, USA); Cr3/EDTA concentration was de-P < 0.05. termined by spectrophotometry (Yasco Uvidec 610, Tokyo, Results Japan). Functional parameters included: glomerular filtration rate Renal hemodynamics (GFR), urinary Na and K excretion (UNa; UK*), calculated with standard formulas; sodium excretion (UNa*) per ml of Renal perfusion flow (RPF) in control rats was similar in the glomerular filtration rate (Cl1®0) was calculated as following: two strains throughout the experimental period (Fig. 1). Renal vascular resistence (RVR) was 9.50.7 in MHS and 8.30.8 UNa(/1mol min g kidneywt) mm Hg. mm m1' in MNS at the beginning of perfusion (not UNa/ClI®O =______.-' statistically different). Addition of any of the three diuretics to GFR(pi. mm g kidneywt) the medium did not modify RPF or RVR in either strain. -l 1000 (l. mm -'g kidney wt) GFR of control MHS was significantly higher than GFR of MNS, as previously reported for rats of this age [161; at the beginning of perfusion GFR of MHS was in fact 873.8 125.7 Drugs versus 555.8 33.5 a1. min' .g'kidney wt in MNS (P < Bumetanide was from Sigma Tau (Italy). Amiloride hydro-0.05).This difference remained unchanged throughout the ex- chloride, hydrochlorothiazide and all other compounds andperiment both in control and diuretic-perfused kidneys. 1086 Salvati et a!: Diuretic action in perfused kidneys

Young MHS Young MNS Young MRS Young MNS

Amiloride 50 8 Amiloride 6 * * 40 30 4 4 ** ** 20 2 2 0 0 10 101

Bumetanide Bumetanide 18 120 16 110 •014 100 0 .5 50 C - 40 0 * a) a) E 30 a' 21 a, o 01 20 zC, Hydrochlorothjazjde 110 Hydrochiorothiazide 4T ** 50 40 o 01 30 30 * B 10-i 1O_6 io io B i07 106 iO iO 20 [MI I I I I I I I I I I I I 10 0 15 30 45 60 75 (rn/n)0 15 30 45 60 75 B io106iO iO2ofB i0106 io-io- Fig.2. Effectof three different diuretics on urinary sodium excretion I I I II I [Mi I I I II I (UN6c)ofisolated kidneys from MHS rats (closed symbols) and MNS 0 15 30 45 60 75 (rn/n)0 15 30 45 60 75 rats (open symbols) (N 6,with the exception of amiloride-treated MNSkidneys,N=5). Controls(•--•,MHS; MNS)received Fig. 3.Effect of three different diuretics on urinary volume (Up) of the vehicle only (N =8).Data are mean SEM; * P <0.05; ** P <0.01; isolated kidneys from MHS rats (closed symbols) and MNS rats (open '6 P < 0.001, from corresponding control values. symbols) (N=6, with the exception of amiloride-treated MNS kidneys, N=5).Controls (•--•,MHS;O--, MNS) received the solvent only (N =8).Data are mean SEM; * P < 0.05; *** P < 0.001, from corresponding control values.

Urine volume and urinary Na + excretion Table 1.Effectof the highest concentration (l0 M) of each diuretic Baseline urinary Na excretion (UNa+) was significantlyon urinary sodium excretion in isolated kidneys from both strains of higher in MHS as compared to MNS (2.300.43 vs. 0.57 rats 0.08 mol .min'.g'kidney wt, P <0.05, as well as urinary Rat UNa* excretion volume (U); Figs. 2 and 3). However, fractional Na reabsorp- Treatment strain j.unolmin'.g'kidney wt tion was similar in the two strains, as shown in Figure 1. In the Amiloride MNS+ 1.18 0.25 (+169.3 30.7%) control kidneys these parameters remained unchanged through- NS out the experimental period. MHS+2.09 0.73 (+77.3 25.7%) Amiloride (from l0— to iO M) had no effect on urinaryBumetanide MNS+ 1.67 0.67 (+239.3 95.7%) volume (U) whereas it significantly (P < 0.01) increased UNa+ P < 0.001 MHS vs. MNS in a concentration-dependent manner, reaching a plateau at MHS+ 13.5 1.01 (+437.2 32.5%) i0 M in both strains. Although the absolute value of UNa* wasHydrochlorothiazideMNS+ 1.14 0.38 (+ 163.1 55.1%) always greater in MHS than in MNS (P < 0.05),therewas no NS significant difference between MHS and MNS in their re- MHS+ 1.73 0.97 (+55.9 31.2%) sponses to amiloride (UNa from vehicle-treated groups) due Values show absolute (UN.) and % differences between diuretic- to the differing baseline values of UNa in the two strains (Tabletreated and matched vehicle-treated rats. Data are means SEM (N I). 6, with the exception of amiloride-treated MNS kidneys, N =5). Bumetanide (from l0— to iO M) caused a concentration related increase of both U,,, and UNa* with a threshold of 106 M in MHS and of l0- M in MNS. In contrast to the resultsexcretion was expressed per ml of glomerular filtration rate, U,,, obtained with amiloride, the bumetanide-induced increase inparalleled the functional behavior of UNa+ in both strains. UNa+ (UNa*) was greater in MHS at all concentrations (dataHydrochlorothiazide did not modify U,,, or UNa in concentra- referring to iO M are reported in Table I). This disproportion-tions ranging from l0— to l0— even at iO M the effect was ally greater response of MHS was independent from differencesmodest in both strains. Table 1 also shows the percent increase in basal tubular load and natriuresis as confirmed by datain UNa' after the maximal concentration (l0— M) of the three reported in Table 2, showing that it also remained when Nadiuretics; only after bumetanide was the percent increase in Salvati et al: Diuretic action in perfused kidneys 1087

Table 2. Effect of bumetanide (from i0- to 104M) on urinary Na Young MHS Young MNS excretion corrected for ml of glomerular filtration rate (UNa*/ClI) Amiloride in isolated kidneys from MHS and MNS rats 3 UN+/Cllj mo1.min'.g kidney wt Treatment EM] MHS MNS pa Control — 2.7 0.5 1.1 0.1 <0.01 ** .** *** N =16 Bumetanide l0 4.1 0.6 1.1 0.2 <0.01 N =12 7.3 2.1 1.4 0.2 <0.05 4 Bumetanide i0 20.5 1.8 3.30.7 <0.001 3 l0— 32.0 3.6 7.5 1.5 <0.001 2 Data are means 5EM. 6Levelof statistical significance of the difference between MHS and

MNS 6, x a, Hydrochlorothjazide 3 ** *** UNa+ greaterin MHS as compared to MNS, whereas the 2 contrary was true after amiloride and hydrochlorothiazide. 0 Urinary potassium excretion B iO' 10_a io— io B iO106 io io As previously observed for U,,, and UNa*, urinary K excre- I-I I I I I IMI I II tion (UK+) was significantly higher (P < 0.001) under basal 0 15 30 45 60 75 (mm) 0 15 30 45 60 75 conditions in isolated MHS kidneys as compared to MNS (1.7 0.19 in MHS vs. 0.60.07 mo1. min g1 kidney wt inFig. 4. Effectof three different diuretics on urinary potassium excretion(UK) ofisolated kidneys from MHS rats (closed symbols)and MNS, respectively; Fig. 4). MNS rats (open symbols) (N = 6, with the exception of amiloride- Because of its potassium-sparing effect, amiloride induced atreated MNS kidneys, N = 5). Controls(•--•,MHS; K--, MNS) significant decrease in UK* in both strains. Already at the firstreceived the solvent only (N =8).Data are mean sErf; *P< 0.05; ** < 0.01; *** coicentration (10—v M),UK*was almost abolished in both P P< 0.001, from corresponding control values. strains (0.2 0.03 in MHS vs. 0.2 0.01 tmol .ming kidney wt in MNS). Bumetanide significantly increased UK. starting at iO— M inaccelerated absolute tubular reabsorption in this strain shown in both strains, yet the effect was greater in MHS (+ 1.80.22 invivo [21]. The defective oxygen delivery capability of the MHS vs. +0.350.17 moI .min.gkidney wt in MNS,perfusion in vitro system [22, 23] may contribute to the failure P < 0.01). of MHS tubular reabsorption to cope with the increased load. The kaliuretic effect following hydrochlorothiazide adminis-These mechanisms, together with an increased load delivery to tration was similar in kidneys from MHS and MNS with aboth the thick ascending limb and the distal convoluted tubule threshold concentration of l05M in both strains (at iO- M:may cause a difference in the overall tubular response to +1.0 0.22 in MHS vs. +0.81 0.27 tmol .min'.g'diuretics between the two strains in a manner that is difficult to kidney wt in MNS, not statistically different). predict. For these reasons, we studied the effects of three different diuretics in order to rule out a possible aspecific Discussion difference between MHS and MNS, independent of the mech- The major limitations in using cell-free Krebs perfused iso-anism of action of the diuretics. Our results show that bumet- lated kidney preparations are related to the time dependentanide causes a selectively greater diuretic and natriuretic re- changes that occur during the perfusion. However, in thesponse in MHS than in MNS. control kidneys from both MNS and MHS rats under our Moreover, the threshold bumetanide concentration seems to experimental conditions urinary volume, electrolyte excretionbe lower in MHS than in MNS while with the other two as well as RPF and RVR remained remarkably stable through-diuretics no difference in their threshold concentrations was out the experiment. If we compare isolated kidney functionfound. If we express the natriuretic effect as percent increase with the in vivo values, a lower GFR than in intact consciousfrom control values, only the response to bumetanide was rats of the same strain is found as previously described [161.greater in MHS as compared to MNS. The contrary was true for Other parameters, such as urine flow and fractional Naboth amiloride and hydrochlorothiazide (Table 1). urinary excretion, are similar in the present experiments to the The difference in tubular load between strains cannot entirely in vivo data, likely because of the addition to the perfusionaccount for the greater natriuresis observed in MHS as shown medium of aldosterone, vasopressin and aminoacids whichby the data corrected per ml of glomerular filtration rate. On the were shown to improve kidney function in vitro [19, 20].other hand we have to take into account that the increased load However, under these experimental conditions, where sys-may cause per se an accelerated tubular Na reabsorption in temic, nervous or humoral feedback mechanisms are lost, theMHS. However, we do not favor this explanation as Nat, K, difference in GFR between MHS and MNS is greater than inC1 cotransport has also been found to be increased in MHS vivo; this leads to a greater tubular load in MHS compared toerythrocytes and cultured VSMC, where extracellular influences, MNS with a consequent increase in UNa* in spite of thesuch as load, are avoided [10, 121. Data in the literature have 1088 Salvati et al: Diuretic action in perfused kidneys shown that in isolated kidneys from normal rats, where tubularthe finding that bumetanide-sensitive sodium extrusion was load was artifically augmented by increasing perfusion pres-lower in SHR erythrocytes than it was in WKY, indicating sure, the percent increase in UNa+afterthe ioop diureticabnormally low outward Na ,K ,C1 cotransport [9]. furosemide, was greater under conditions of lower tubular load The relevance of these in vitro findings to the in vivo situation [24]. remains to be established. However, we can speculate that loop Another limitation in interpreting our results is that we don'tdiuretics might have a more pronounced natriuretic effect in know whether the bumetanide pharmacokinetics and tubularthat subgroup of hypertensive patients characterized by an secretion were similar in the two strains, and therefore weincreased Na,K,Cl cotransport in red blood cells. cannot conclude that the greater effect of bumetanide in MHS was actually due to its higher affinity for Na ,K ,C1 cotrans- Acknowledgments port in this strain. On the other hand our hypothesis is consis- The generous gifts of bumetanide (from Sigma Tau, Italy), and tent with data obtained by studying the effect of bumetanide onSolamin (from Pierre!, Italy) are acknowledged. Portions of this work Na ,K ,C1 cotransport in isolated vesicles prepared from thewere presented at the International Meeting on Diuretics, (Sorrento, thick ascending limb membranes of MHS and MNS. In fact Italy, May 26—30, 1986) and have appeared in abstract form (Diuretics: under those experimental conditions a greater bumetanide-Basic Pharmacological, and Clinical Aspect, edited by Andreucci VE and Dat Canton A, Martinus Nijhoff Publishing, 1987, p 265).The sensitive Na flux and also a greater affinity for bumetanideauthors thank Mrs. L. Contardi and Mr. F. Brambilla for their technical were observed in MHS [11]. These findings, together with theassistance, and Mrs. R. Marazzini for typing the manuscript. above reported data on erythrocytes and on VSMC, suggest that an intrinsic defect in membrane function might be present Reprintrequests to P. Salvati, M.D., Farmitalia Carlo Erba Istituto in MHS kidneys. Beside the inhibition of Na transport acrossRicerche, Via Giovanni XXIII, n.23- 20014 Nerviano, Milano, Italy. the luminal membrane of the ascending limb, bumetanide may also increase Na excretion through the depression of the References tubuloglomerular feedback (TGF) mechanism [25].However, 1. Bi..s.r'cm G: Ion transport across blood cell membrane in essential this additional effect of bumetanide may not be present in MHS hypertension. CurrOpinion Cardiol 1:634—639,1986 since the TGF is already depresseJ in young MHS compared to 2. POSTNOV Y, ORLov S, SHEVCHENKO A, ADLER AM: Altered matched MNS [26]. sodium permeability, calcium binding and Na, K-ATPase activity in the red blood cell membrane in essential hypertension, Pflugers Amiloride has been shown to inhibit Na-H countertrans- Archiv 371:263—269, 1977 port in the proximal tubule [27], however, this activity cannot 3. GARAY RP, MEYER P: A new test showing abnormal net Na and explain its modest natriuretic effect, due to a compensatory K fluxes in erythrocytes of essential hypertensive patients. Lancet increase in Na reabsorption in the ioop of Henle [28]. Hence i:349—353,1979 4. CANEssA M, ADRAGNA N, SOLOMON HS, CONNOLLY TM, TOSTE- an action of amiloride in the distal tubule is the likely explana- SON D: Increased sodium-lithium countertransport in red cells of tion of its observed effects on UNa* and UK* in kidneys isolated patients with essential hypertension. NEngI J Med 302:772—776, from MHS and MNS [29, 30]. 1980 Hydrochiorothiazide-induced natriuresis was negligible up to 5. LAUF PK, ADRAGNA NC, GARAY RP: Activation by N-ethylmale- l0 M. On the other hand kaliuresis became significant in both imide of a latent K-Cl flux in human red blood cells. AmJ Physiol 246:C385—C390,1984 strains at a concentration ten times lower than that which 6. TOSTESON DC, ADRAGNA N, BIzE I, SOLOMON H, CANESSA M: elicited natriuresis. This could be due either to the augmented Membranes, ions and hypertension. C/inSci 6l:5s—IOs,1981 Na delivery to the distal nephron or to a stimulating effect of 7. GARAY RP, DAGHER G, PERNELLET MG, DEVYNCK MA, MEYER P: hydrochiorothiazide on K secretion in that nephron segment Inherited defect in a (Na-K) cotransport system in erythrocytes from essential hypertensive patients. Nature(London) 284:281— [25]. 282, 1980 Inconclusion the isolated kidney proved to be a suitable 8. Cusi D, BARLASSINA C, FERRANDI M, BREVI R, VAZZOLA A, model for studying the mechanism of action of diuretics, as BlANcH! G: Erythrocyte membrane transport systems as possible renal function changes induced by amiloride, bumetanide and markers for essential hypertension. ClinSd 63:57s—59s,1982 hydrochlorothiazide were consistent with their known renal 9.DEMENDONCA M, KNORR A, GRICHOIS ML, BEN-ISHAY D, GARAY RP, MEYER P: Erythrocytic sqdium ion transport systems activities in humans. in primary and secondary hypertension of the rat. Kidneymt Moreover, the results described here show that the diuretic 21(suppl.II):S69—S75, 1982 effect of bumetanide appears at a lower concentration in MHS 10. BlANcH! G,FERRARIP, TRIz!o D, FERRANDI M, TORIELLI L, than in MNS. Also the maximal diuretic activity of bumetanide BARBER BR, POLL! F: Red blood cell abnormalities and spontane- was greater in MHS, while the effects of the other two diuretics ous hypertension in the rat. A genetically determined link. Hyper- tension 7:319—325,1985 were similar in the two strains. 11. FERRANDI M, SALARDI S, PARENT! P, FERRARI P, BIANCHI G, The most likely explanation for this greater effect of bumet- BRAw R, KARLISH SJD: Na, K, Cl co-transporter mediated Rb anide in the hypertensive strain is that a greater portion of Na fluxes in membrane vesicles from kidneys of normotensive and transport in MHS tubular cells is mediated by cotransport as hypertensive rats. BiochemBiophys Acta (inpress) 12. CANESSA M: Sodium transport in the pathophysiology of hyperten- suggested by previous in vitro studies [11, 13]. sion. Abstract book of International Symposium on "Genetic However, the greater natriuretic response to a ioop diuretic is factors in hypertension". (Abstract) 15,1989 not a common characteristic of all hypertensive rat strains; in13. FERRARI P, FERRANDI M, TORIELLI L, CANESSA M, BIANcHI G: fact the natriuretic response to furosemide was significantly Relationship between erythrocyte volume and sodium transport in diminished in isolated kidneys from spontaneously hyperten- the Milan hypertensive rat and age-dependent changes. JHyperten 5:199—206,1987 sive rats (SHR) as compared to kidneys from Wistar Kyoto14. HANOZET G, PARENTI P. SALVATI P: Presence of a potential- normotensive controls (WKY) [31]. This is relevant in view of sensitive Na transport across renal brush-border membrane yes- Salvati et a!: Diuretic action in perfused kidneys 1089

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