Red cell sodium in DOCA-salt hypertension: a Brattleboro study

The influence of erythrocyte maturity on ion transport and membrane lipid composition in the rat

Significant relationships between ion transport and membrane lipid composition (cholesterol, total phospholipids and sphingomyelins) were found in erythrocytes of salt hypertensive Dahl rats. In these animals mean cellular hemoglobin content correlated negatively with Na(+)-K(+) pump activity and Na(+) leak but positively with Na(+)-K(+) cotransport activity. Immature erythrocytes exhibit lower mean cellular hemoglobin content (MCHC) than mature ones. The aim of the present study was to find a relationship between erythrocyte maturity, membrane lipid composition and ion transport activity in Wistar rats aged three months which were subjected to repeated hemorrhage (blood loss 2 ml/day for 6 days) to enrich circulating erythrocytes with immature forms. Immature and mature erythrocyte fractions in control and hemorrhaged rats were separated by repeated centrifugation. Hemorrhaged rats had increased number of reticulocytes but reduced hematocrit and MCHC compared to control rats. Immature erythrocytes of hemorrhaged rats differed from mature ones of control animals by elevated Na(+)-K(+) pump activity, reduced Na(+)-K(+) cotransport activity and increased Rb(+) leak. These ion transport changes in immature erythrocytes were accompanied by higher concentration of total phospholipids in their cell membranes. Membrane phospholipid content correlated positively with Na(+)-K(+) pump activity and cation leaks but negatively with Na(+)-K(+) cotransport activity. Moreover, they were also negatively related with MCHC which correlated negatively with Na(+)-K(+) pump activity and Rb(+) leak but positively with Na(+)-K(+) cotransport activity. Thus certain abnormalities of erythrocyte ion transport and membrane lipid composition detected in hypertensive animals might be caused by higher incidence of immature cells.

Relationships between membrane lipids and ion transport in red blood cells of Dahl rats

Distinct changes of membrane lipid content could contribute to the abnormalities of ion transport that take part in the development of salt hypertension in Dahl rats. The relationships between lipid content and particular ion transport systems were studied in red blood cells (RBC) of Dahl rats kept on low- and high-salt diets for 5 weeks since weaning. Dahl salt-sensitive (SS/Jr) rats on high-salt diet had increased blood pressure, levels of plasma triacylglycerols and total plasma cholesterol compared to salt-resistant (SR/Jr) rats. Furthermore, RBC of SS/Jr rats differed from SR/Jr ones by increased content of total membrane phospholipids, but membrane cholesterol was not changed significantly. SS/Jr rats had higher RBC intracellular Na+ (Na(i)+) content and enhanced bumetanide-sensitive Rb+ uptake. RBC membrane content of cholesterol and phospholipids correlated positively with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and also with Rb+ leak. The content of phosphatidylserines plus phosphatidylinositols was positively associated with RBC Na(i)+ content, with the activity of Na+-K+ pump and Na+-K+-2Cl- cotransport and with Rb+ leak. The content of sphingomyelins was positively related to Na+-K+-2Cl- cotransport activity and negatively to ouabain-sensitive Rb+-K+ exchange. We can conclude that observed relationships between ion transport and the membrane content of cholesterol and/or sphingomyelins, which are known to regulate membrane fluidity, might participate in the pathogenesis of salt hypertension in Dahl rats.

Erythrocyte ion transport and membrane lipid composition in young and adult rats with NO-deficient hypertension

The aim of our study was to search for abnormalities of sodium and potassium transport in erythrocytes of male Wistar rats subjected to chronic L-NAME treatment (40 mg/kg/day) for 4 weeks either from weaning (4-week-old) or in adulthood (12-week-old). Sodium content, Na(+),K(+)-pump and Na(+),K(+)-cotransport activity, cation leaks as well as membrane cholesterol and phospholipid contents were determined in fresh erythrocytes. Chronic inhibition of NO synthase elicited similar blood pressure rise in both age groups which did not differ in the degree of NO synthase inhibition. No significant ion transport abnormalities were disclosed in erythrocytes of young NO-deficient rats, whereas erythrocyte Na(+) content, outward Na(+),K(+)-cotransport and inward Na(+) leak were significantly reduced in adult hypertensive animals compared to age-matched controls. It should be noted that the erythrocytes of adult control rats were characterized by higher activity of Na(+),K(+)-pump and Na(+),K(+)-cotransport, increased Na(+) and Rb(+) leaks and elevated membrane cholesterol content compared to those of young normotensive controls. Increased Na(+) leak and elevated membrane cholesterol content but reduced membrane phospholipid content were revealed in erythrocytes of adult hypertensive rats when compared to young hypertensive rats. It can be concluded that young and adult Wistar rats did not differ in the extent of NO synthase inhibition and blood pressure rise elicited by chronic L-NAME treatment. Our results exclude the important participation of classical sodium transport abnormalities in the pathogenesis of this NO-deficient form of experimental hypertension.

Plasma triglycerides and red cell ion transport alterations in genetically hypertensive rats

Ion transport abnormalities in essential hypertension are often associated with concomitant changes of lipid metabolism, but this information is missing in rats with genetic hypertension. We therefore studied the alterations of red cell Na+ and K+ transport and their relationship to blood pressure and plasma lipids (cholesterol and triglycerides) in Prague hereditary hypertriglyceridemic (HTG) rats, Lyon hypertensive (LH) rats, and HTG x Lewis F2 hybrids. In both hypertensive models and F2 hybrids, red cell Na+ content (Na+(i)) was positively related to plasma triglycerides but not to plasma cholesterol levels. Na+(i) elevation was more pronounced in HTG than in LH rats, probably due to higher plasma triglycerides in the former strain. The two hypertensive strains differed in bumetanide-sensitive Na+ transport, which was augmented in HTG rats with low plasma cholesterol but suppressed in LH rats characterized by high cholesterol levels. In the two genetic models, there was a positive association of blood pressure with Na+ leak, and this was also confirmed by the cosegregation of these parameters in F2 hybrids. We conclude that the enhancement of Na+ leak represents the major ion transport abnormality in rats with genetic hypertension. The alterations in plasma lipids are important determinants of abnormal red cell ion transport in hypertensive models studied. Although the detailed mechanism of their participation in ion transport regulation is still not completely understood, triglyceride-dependent changes in membrane microviscosity seem to be responsible for the modulation of particular ion transport pathways.

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.

Complete dissociation of DOCA-salt hypertension and red cell ion transport alterations

Our previous study revealed major ion transport alterations that resulted in a pronounced elevation of red cell Na+ content in DOCA-salt treated homozygous vasopressin-deficient (DI) Brattleboro rats in which only a moderate increase of systolic blood pressure occurred. In contrast, no changes of red cell Na+ content were observed in heterozygous vasopressin-secreting (non-DI) Brattleboro rats with a severe DOCA-salt hypertension. Using a chronic supplementation of DI rats with an antidiuretic agonist dDAVP (1-desamino-8-D-arginine vasopressin) we did not demonstrate any significant changes of red cell ion transport in dDAVP-treated DI rats with a fully developed DOCA-salt hypertension. The absence of ion transport alterations seems to be mainly due to dDAVP-induced correction of altered K+ metabolism seen in DOCA-salt treated DI animals. It can be concluded that DOCA-salt hypertension can develop even without red cell ion transport alterations which are usually caused by cell K+ depletion.