Furosemide and bumetanide-sensitive Na+ fluxes in erythrocytes from genetically hypertensive rats (SHR)

Red cell ion transport abnormalities in experimental hypertension

The original attractive hypothesis on the important role of elevated cell Na+ concentration in the pathogenesis of hypertension stimulated a search for generalized membrane defects and ion transport abnormalities in various easily accessible cells including erythrocytes. An attempt is made here to compare this hypothesis with the data on red cell ion transport alterations that were observed in experimental hypertension over the last 15 years. Several methodological (presence of extracellular Na+ in incubation media, kinetic approach to the evaluation of transport systems) and physiological problems (potassium depletion, age-dependent changes) are discussed in more detail because they can substantially modify the results obtained. Available data suggest a possible contribution of augmented Na+ leak to the development of both genetic and salt-dependent experimental hypertension. The role of alterations in the activity of the Na(+)-K+ pump or the Na(+)-K+ cotransport system still remains unclear.

Association of red blood cell sodium leak with blood pressure in recombinant inbred strains

Red blood cell Na+ content as well as ouabain-resistant Na+ and Rb+ (K+) transport (susceptible or resistant to inhibition by loop diuretics) were determined in spontaneously hypertensive rats (SHR) and normotensive Brown Norway (BN) rats the erythrocytes of which were incubated in either saline or Mg(2+)-sucrose medium. Elevated ouabain-resistant Na+ net uptake contrasted with slightly decreased red blood cell Na+ content in SHR compared with BN rats. Acceleration of furosemide- and bumetanide-sensitive Na+ fluxes contributed to enhanced ouabain-resistant Na+ influx into SHR erythrocytes in saline medium, whereas higher furosemide- or bumetanide-resistant Na+ efflux caused greater ouabain-resistant Na+ efflux in Mg(2+)-sucrose medium. Furosemide- and bumetanide-resistant Rb+ leaks were augmented in SHR erythrocytes. The association of the disclosed ion transport alterations with blood pressure was examined in 20 recombinant inbred strains derived from F2 SHR x BN hybrids. Ouabain-resistant Na+ uptake as well as furosemide- and bumetanide-resistant Na+ inward leaks (but not red blood cell Na+ content or furosemide- and bumetanide-sensitive Na+ net uptake) cosegregated with systolic and pulse pressures but not diastolic pressure of the recombinant inbred strains. In contrast, neither ouabain-resistant Na+ efflux nor any component of ouabain-resistant Rb+ uptake correlated positively with blood pressure of the recombinant inbred strains. Increased ouabain-resistant Na+ influx was compensated for by accelerated ouabain-sensitive Na+ extrusion because red blood cell Na+ content was not elevated in the hypertensive strains. Thus, high cell Na+ turnover rates might be related to genetic hypertension if an altered Na+ inward leak would be less effectively compensated for in tissues involved in cardiovascular regulation.

Kinetics of red cell Na+ and K+ transport in Prague hypertensive rats

Kinetics of ouabain-sensitive, furosemide-sensitive (FS), bumetanide-sensitive (BS) and -resistant Na+ and K+ transport were studied in erythrocytes of Prague hypertensive rats (PHR) and Prague normotensive rats (PNR). Maximal transport rates (Vmax) and apparent affinities for either intracellular Na+ or extracellular K+ (replaced by Rb+) were determined in red cells in which Na+ content varied around the physiological range and that were incubated in Na+ media. No major differences between PHR and PNR were disclosed in the kinetics of ion transport mediated by the Na(+)-K+ pump or BS inward Na(+)-K+ cotransport. FS Rb+ uptake was higher (due to a greater Vmax) in red cells of PHR as compared to PNR. In cells with a lowered Na+ content this elevation of FS Rb+ uptake was largely due to an augmented K(+)-Cl- cotransport which exhibits a low affinity for Rb+o and is blocked by 1 mM furosemide but not by 10 microM bumetanide. Red cells of PHR and PNR strains did not differ in either Na+ or Rb+ leaks. A slight increase of red cell Na+ content in PHR was evaluated in terms of the pump-leak concept. The present study did not reveal any obvious kinetic abnormalities of red cell cation transport the presence of which in tissues involved in blood pressure regulation would favor the development or the maintenance of genetic hypertension in PHR.

Natural variation in passive sodium permeability in human erythrocytes

The rate of influx of 22Na+ into human erythrocytes (RBC) varies greatly depending upon the donor. A high rate of influx may be related to a congenital predisposition to essential hypertension. In Northern Europeans, we find a threefold difference in the rate of 22Na+ influx between those with the least (LP) and most highly permeable (HP) RBC (from less than 0.15 to greater than 0.60 mmol Na+/liter RBC/hr). In order to further define determinants of these apparently hereditary differences in passive membrane Na+ transport, we identified two groups of normal laboratory and hospital personnel differing markedly (greater than twofold) in RBC 22Na+ influx rate. We find that the loop diuretics furosemide and bumetanide decrease by about 50% the influx of 22Na+ into HP RBC, but have a lesser influence on LP RBC. Impermeant polyanions such as citrate and pyrophosphate also specifically diminish 22Na+ influx into HP, but not LP, RBC. Therefore, the exaggerated 22Na+ influx into HP RBC probably occurs through a discrete pathway (perhaps via "Na/K/Cl cotransport"), which appears to be almost absent in LP RBC. The differences between HP and LP RBC most likely do not involve polymorphisms of RBC anion transport per se. The rate of RBC anion (35SO4(2-)) transport is the same in HP and LP RBC and is equally inhibited by furosemide and (to a lesser extent) bumetanide. Furthermore, the potent inhibitor of RBC anion transport, DIDS (diisothiocyanostilbene disulfonate) does not affect RBC Na+ permeability in either group. Nonetheless, the preferential reduction of Na+ permeation of HP RBC by loop diuretics may be of help in experimentally distinguishing HP from LP phenotypes. This information may be crucial in unraveling the structural basis of intrinsic differences in cell membrane Na+ permeability and their possible relationship to essential hypertension.

Na+-K+ cotransport and hypertension. Effect of piretanide and hydrochlorothiazide on red blood cell sodium concentration in acutely salt-loaded hypertensive and normotensive rats

Na+-K+ cotransport has been presumed to be a genetic marker or aetiological factor in essential hypertension in numerous studies. In spite of extended in vitro research, the role of this transport system in hypertension could not be proved. In the present study the action of the cotransport inhibitor piretanide and the non-loop diuretic hydrochlorothiazide (HCT) on red blood cell (RBC) sodium concentration was examined under in vivo conditions in spontaneously hypertensive rats (SHR) and normotensive Sprague Dawley rats (NSDR) during acute salt-loading. Before drug administration salt-loading resulted in an increase of RBC sodium concentration, the percentage of which was not different in SHR and NSDR. However, after administration of piretanide in oral doses of 8-32 mg/kg body weight, the percent increase in RBC sodium was lower in SHR than in NSDR. This effect which was found to be dose-dependent was not brought about by HCT. The data might be due to a piretanide inhibition of sodium-induced inward RBC Na+-K+ cotransport more pronounced in SHR than in NSDR.

Intrinsic difference in erythrocyte membrane in spontaneously hypertensive rats characterized by Na+ and K+ fluxes

The goal of this study was to determine whether the elevated flux of sodium and potassium through the erythrocyte membrane of spontaneously hypertensive rats (SHR) is due to an intrinsic difference in the cell membrane or to a humoral factor present in the plasma. Isolated and washed erythrocytes from SHR and normotensive Wistar Kyoto (WKy) and Sprague-Dawley (SD) rats, were incubated in 1) a physiological salt solution, 2) WKy or SD plasma and 3) SHR plasma. Incubations were performed at 4 degrees C for 23 h. Erythrocytes from SHR incubated in physiological salt solution had significantly greater Na+ and K+ fluxes than those from normotensive WKy and SD rats (P less than 0.005). Plasma from any of the three strains of rats, as compared to physiological salt solution, increased Na+ influx in the following order: SD greater than WKy greater than SHR. Erythrocyte K+ efflux was not altered by plasma. We conclude that the elevated flux of Na+ and K+ in SHR erythrocytes is due to an intrinsic difference in the cell membrane. The greater Na+ influx in plasma from any strain of rats is not correlated with the blood pressure of the rat. The lesser increase in Na+ influx in erythrocytes incubated in plasma from SHR masks the greater intrinsic membrane permeability in the SHR erythrocyte when Na+ fluxes are studied in whole blood. The elevated flux of Na+ and K+ through the erythrocyte membrane of SHR may reflect a general membrane defect that underlies the pathogenesis of elevated arterial pressure.

Modulation of Na+ transport systems in Wistar rat erythrocytes by excess dietary Na+ intake

Plasma Na+, erythrocyte Na+ content and the activity of Na+ transport systems of red cells were measured in Wistar rat fed a normal or high Na+ diet. Net Na+ and K+ fluxes of erythrocytes were also measured in the presence of plasma of rats fed with excess Na+. Na+-K+ cotransport and passive Na+ permeability were increased. Erythrocyte Na+ content was increased after 2 months but not after 8 days of high Na+ intake. No significant difference in plasma Na+ and pump activity could be detected after such a diet. No factor acting in Na+ extrusion was found to be present in plasma of salt loaded rats. These results indicate that Na+ intake may modulate Na+ transport systems, namely passive permeability and Na+-K+ cotransport and that increased Na+ erythrocyte content is not a causative factor.