Effect of protein kinase C activation on cytoskeleton and cation transport in human erythrocytes. Reproduction of some membrane abnormalities revealed in essential hypertension

Certain manifestations of alterations of membrane cytoskeleton, protein kinase C activity, and ion transport were revealed in erythrocytes of patients with essential hypertension: 1) the average volume of erythrocytes is reduced by 4%; 2) about 7% of the total number of erythrocytes is represented by cup-shaped forms compared with 1.5 to 3.0% in the control group; 3) basal phosphorylation of Band 4.9 protein is increased 1.6-fold to 1.8-fold; 4) activity of protein kinase C is increased by 60 to 70%; 5) the rate of proton electrochemical gradient (delta mu H+)-induced Na+-H+ exchange is increased twofold. Treatment of erythrocytes of healthy donors with protein kinase C activator (12-O-tetradecanoylphorbol-13-acetate) leads to similar but more marked changes in cell shape (17% of cup-shaped forms), volume reduction (by 7%), an increase of Band 4.9 protein phosphorylation (threefold), and an increase in the rate of Na+-H+ exchange (fourfold). Protein kinase activation does not modify Na+-Li+ exchange and slightly increases (by 20-50%) Na+-K+ pump activity, Na+-K+ cotransport, and the rate of 45Ca influx. It may be assumed that the increase of protein kinase C activity is one of the most probable molecular mechanisms conditioning abnormalities of the membrane skeleton and Na+-H+ exchange in primary hypertension.     

Cation fluxes and Na+-K+-activated ATPase activity in erythrocytes of patients with essential hypertension

Recently it has been claimed that the active potassium influx in erythrocytes of patients with essential hypertension would be increased. In view of the diagnostic and possibly therapeutic potential of this claim, we have determined the Na+-K+ activated ATPase activity and the affinity of the enzyme for Na+, K+, and ATP in membranes isolated from erythrocytes of hypertensive (with and without medication) and normotensive subjects. Subsequently, the active (ouabain-sensitive) sodium and potassium fluxes and their ratios have been determined after treatment of intact erythrocytes either with cold or with p-chloromercuribenzene-sulfonate (PCMBS). Finally, in view of a subsequent claim that the furosemide-sensitive, ouabain-insensitive cation fluxes would be greatly reduced in erythrocytes of patients with essential hypertension, we have determined these fluxes in choline chloride medium containing ouabain with and without furosemide. For none of these parameters has any significant difference between hypertensive and normotensive subjects been found except for a decrease in the ouabain-sensitive K+ influx after cold treatment in hypertensives. This is also true for the hypertensive subjects who had a known hypertensive parent. It is concluded that the results do not support a role of Na+-K+ activated ATPase or the furosemide-sensitive cation carrier in the pathogenesis of essential hypertension, and that ouabain-sensitive and furosemide-sensitive cation flux determinations in erythrocytes do not seem to be useful for the diagnosis of this condition.     

(Na+ + K+) ATPase inhibitors and intracellular electrolytes in essential hypertension

White blood cell (WBC) Na+ and K+ concentrations, plasma (Na+ + K+)ATPase inhibition and blood pressure were determined in normotensive control subjects and patients with essential hypertension. While the untreated hypertensive group had significantly lower WBC K+ concentrations than the normotensive group (mean +/- SEM, 121.6 +/- 4.4 vs. 134.7 +/- 2.8 mEq/kg, p less than 0.05), no significant difference was observed in WBC Na+ concentrations between the 2 groups. The mean of plasma (Na+ + K+)-ATPase inhibition in untreated hypertensive patients was higher than that in normotensive controls (14.8 +/- 1.7 vs. 7.2 +/- 1.8%, p less than 0.05). The correlations between (Na+ + K+)ATPase inhibition and mean blood pressure and between WBC Na+/K+ ratio and mean blood pressure were significant (r = 0.278, p less than 0.05 and 0.270, p less than 0.05, respectively), but both were weak. However, untreated hypertensive patients with higher (Na+ + K+)ATPase inhibition had significantly higher WBC Na+/K+ ratios than untreated patients with less (Na+ + K+)ATPase inhibition. These results suggest a contribution of plasma (Na+ + K+)ATPase inhibition in the production of high blood pressure in a subset of patients with essential hypertension, which results in altered intracellular K+ concentrations.     

Sodium-lithium exchange and sodium-potassium cotransport in human erythrocytes. Part 2: A simple uptake test applied to normotensive and essential hypertensive individuals

The sodium-lithium (Na+-Li+) exchange and sodium-potassium (Na+-K+) cotransport activities were assessed on erythrocytes of 38 normotensive individuals, 18 patients with well-established essential hypertension, and five renal hypertensive patients, by means of an uptake assay method. With both transport systems, no significant differences in mean values and variance were observed among the three groups. Four of six low-renin essential hypertensive patients exhibited some of the lowest exchange and cotransport rates obtained among all individuals examined. The activity of both transport systems was slightly lower in women than in men.     

Red blood cell Na+,K+-ATPase in men with newly diagnosed or previously treated essential hypertension

Alterations of cellular function of Na+,K+-adenosine triphosphatase (ATPase; Na+-K+ pump) have been implicated in the pathophysiology of essential hypertension. Therefore, this aspect of red blood cell (RBC) Na metabolism was studied in black men with newly diagnosed, untreated essential hypertension (NEH) and a normotensive control group. RBC Na content, Na+-K+ pump number (ouabain binding sites), and pump activity were measured. No statistically significant differences were found between the two groups for any of these three parameters. However, a group of previously treated essential hypertensive subjects (PEH) who had been withdrawn from therapy in the preceding 6 weeks were also studied. This group differed significantly from the NEH subjects in regard to all RBC Na+-K+ pump parameters. Their RBC Na content (10.27 +/- 3.23 vs 7.77 +/- 2.52 mmol Na/LRBC; p = 0.006) was higher, and their Na+-K+ pump activity (166 +/- 50 vs 221 +/- 87 nmol inorganic phosphate/mg membrane protein/hr; p = 0.03) and Na+-K+ pump number (213 +/- 40 vs 284 +/- 85 binding sites/RBC; p = 0.001) were lower compared with those in NEH subjects. Although the PEH subjects were older and somewhat less hypertensive than their NEH counterparts, these factors were not found to influence the Na+-K+ pump parameters. These results indicate that chronic diuretic therapy of patients with essential hypertension is associated with a reduced number of RBC Na+-K+ pumps. Since RBCs are not considered target cells for diuretics, the effects of these drugs on RBC electrolyte metabolism may occur at the time of erythropoiesis by the production of RBCs with fewer Na+-K+ pumps.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhanced red cell sodium permeability in patients with borderline hypertension

Red cell membrane sodium permeability was studied in 41 untreated patients with essential hypertension (20 borderline hypertensives and 21 established hypertensives) and 21 age matched normotensive subjects by means of the measurement of unidirectional passive influx of 22Na+ into ouabain-treated erythrocytes. The mean value (+/- SD) of 22Na+ influx was greater in the hypertensives than in the normotensives (0.183 +/- 0.047 vs 0.152 +/- 0.047 mmol/l . cells/hr, respectively, p less than 0.02). Among the patients with essential hypertension, the borderline hypertensives demonstrated a higher 22Na+ influx than the established hypertensives (0.207 +/- 0.043 vs 0.160 +/- 0.038 mmol/l . cells/hr, respectively, p less than 0.001), and 22Na+ influx positively correlated with plasma renin activity (r = 0.44, p less than 0.005). In 16 of 20 borderline hypertensives, 5 year blood pressure changes were examined retrospectively, and a positive correlation was observed between mean blood pressure increase and 22Na+ influx value in these subjects (r = 0.64, p less than 0.01). These results suggest that passive sodium influx may be altered in the course of the development of hypertension in relation to the changes in blood pressure level and that enhanced sodium permeability may be a characteristic of the early stage of essential hypertension.     

Cation fluxes and (Na+ + K+)-activated ATPase activity in erythrocytes of patients with essential hypertension

Various claims, partially conflicting, have been made in recent years for abnormalities in cation pumps and fluxes in erythrocytes of patients with essential hypertension. In view of the obvious significance of such abnormalities for diagnostic purposes, and possibly for our understanding of the pathophysiology of essential hypertension, we have investigated these claims. We have determined the following parameters of erythrocytes from essential hypertensives and normotensives: 1. (Na+ + K+)-ATPase activity and the Km values for Na+, K+ and ATP; 2. ouabain-sensitive fluxes of Na+ and K+ in Na+-enriched cells after cold treatment and after treatment with p-chloromercuribenzenesulphonate; 3. furosemide-sensitive, ouabain-insensitive cotransport efflux of Na+ + K+. No significant differences were observed, except for a slight decrease in the ouabain-sensitive K+ influx after cold treatment in hypertensives. Hence, we conclude that determination of these parameters in erythrocytes does not seem to be useful for the diagnosis of essential hypertension.     

The Relationship of Plasma Ionized Calcium to Cardiovascular Disease Endpoint and Family History of Hypertension

Plasma ionized calcium and total calcium were measured on 271 individuals from 34 Utah pedigrees divided into groups defined by the method of pedigree ascertainment: 1) hypertension clusters, 2) early stroke death clusters and 3) clusters of early heart attack deaths. Normotensive individuals were also categorized by family history of hypertension. Members of stroke cluster pedigrees had higher mean plasma ionized Ca²⁺ than either hypertension pedigrees (p < 0.05) or coronary artery disease pedigrees which had the lowest concentrations (p < 0.001). Within the normotensive group, those subjects with a positive family history of hypertension exhibited significantly higher plasma ionized Ca²⁺ (2.18 ± 0.10 (S.D.) mEq/1) than individuals without a family history of hypertension (2.12 ± 0.08 mEq/1, p < 0.01). In medicated hypertensives, both ionized (p < 0.05) and total (p < 0.01) plasma calcium were higher than calcium levels in the normotensive negative family history subjects. Plasma ionized Ca²⁺ in the adult normotensives (N=134) had significant age corrected positive correlations with plasma sodium (r = 0.25, p < 0.01), potassium (r=0.29, p<0.001) and erythrocyte sodium-lithium countertransport values (r=0.20, p<0.05). These findings provide additional evidence that plasma ionized calcium concentrations may be important to help define the heterogeneity of hypertension in adult Americans.     

Outward Na+-K+-Cl- cotransport function in erythrocytes from essential hypertensives

We have performed a kinetic analysis of the interaction of the outward Na+-K+-Cl- cotransport system with intra-cellular Na+ in erythrocytes from 30 normotensive controls and 72 patients with essential hypertension. Neither maximal rate of ouabain-resistant, bumetanide-sensitive sodium efflux (Vmax) nor intracellular Na+ content required for half-maximal stimulation (K50%) were significantly different between normotensives and hypertensives. Nevertheless, using 95% confidence limits of the K50% in the normotensive group as a cut-off point, 21 (29.17%) essential hypertensive patients exhibited values above the upper normal limit of 20.11 mmol/l cells, revealing a decreased apparent affinity of outward Na+-K+-Cl- cotransport for internal Na+ ('Co-' hypertensives). The Vmax of Na+-K+-Cl- cotransport exhibited a great variability among hypertensives but 'Co-' patients tended to have increased values of this parameter when compared with the remaining essential hypertensives (959 +/- 84 vs 652 +/- 39 mumol/l cells/h, P = 0.0024). Mean BP values were significantly lower in the 'Co-' subset (121.4 +/- 1.6 mmHg), compared with the remaining 51 hypertensive patients (126.4 +/- 1.3 mmHg, P = 0.0297). We conclude that an abnormal function of outward Na+-K+-Cl- cotransport is present in 19% to 40% of Spanish patients with essential hypertension and this abnormality may be implicated in the mechanisms of hypertension.     

Sodium and potassium ion transport accelerations in erythrocytes of DOC, DOC-salt, two-kidney, one clip, and spontaneously hypertensive rats. Role of hypokalemia and cell volume

Sodium (Na+) and potassium (K+) transport by the furosemide-sensitive Na+-K+ transport system, the Na+-K+ pump, and the cation leak(s) were studied in erythrocytes from DOC-water, DOC-salt, two-kidney, one clip (Sprague-Dawley), and spontaneously hypertensive rats (Wistar-Kyoto). Rubidium (Rb+) was used as a tracer for K+. After 4 weeks of DOC-salt hypertension, inward K+ (Rb+) transport by the furosemide-sensitive system was increased threefold, and the inward Na+ leak and the red cell Na+ content were elevated by about 50%. The rise in cell Na+ accelerated K+ inward and Na+ outward transport by the Na+-K4 pump, DOC-water hypertension caused similar but less pronounced changes. In two-kidney, one clip hypertension, the Na+ leak and the Na+-K+ pump rates were slightly elevated, and furosemide-sensitive Rb+ uptake tended to be increased. In spontaneously hypertensive rats, furosemide-sensitive Rb+ uptake was accelerated by 50%. The marked hypokalemia in DOC-water and DOC-salt hypertension was associated with a slight loss of red cell K+ and an increase in mean cellular hemoglobin content (MCHC), indicative of cell shrinkage. Hypokalemia induced by dietary K+ deficiency caused alterations in red cell cation transport, content, and cell volume which were qualitatively similar but more pronounced than those seen in DOC-salt hypertension. Osmotic shrinkage in vitro induced a severalfold acceleration of furosemide-sensitive Rb+ uptake, similar to that observed in rat erythrocytes shrunken in vivo in K+-deficient states. It is concluded that the acceleration of furosemide-sensitive K+ (Rb+) transport in erythrocytes of mineralocorticoid hypertensive rats is largely caused by the hypokalemia and consecutive red cell K+ loss and shrinkage, respectively. Mean cellular hemoglobin content (MCHC) is thus a parameter that must be considered in studies on Na+ and K+ transport across the membrane of rat erythrocytes.     

Handgrip Increases Endothelin-1 Secretion in Normotensive Young Male Offspring of Hypertensive Parents

Objectives. We tested the hypothesis that an abnormal response of plasma endothelin-1 (ET-1) is elicited by handgrip exercise (HG) in young normotensive offspring of hypertensive parents.Background. It has been hypothesized that ET-1 is involved in blood pressure control and plays a pathophysiologic role in the development of clinical hypertension.Methods. Two groups of healthy male subjects, 11 with hypertensive parents (group A) and 10 without a family history of hypertension (group B), underwent 4 min of HG at 50% maximal capacity. Heart rate and blood pressure and plasma levels of ET-1, epinephrine and norepinephrine were measured at baseline, peak HG, and after 2 (R2) and 10 (R10) min of recovery.Results. Group A had higher norepinephrine levels than group B throughout the test (baseline 181 ± 32 [SEM] vs. 96 ± 12 pg/ml, p < 0.05; peak HG 467 ± 45 vs. 158 ± 12 pg/ml, p < 0.000001; R2 293 ± 46 vs. 134 ± 8 pg/ml, p < 0.01; R10 214 ± 27 vs. 129 ± 10 pg/ml, p < 0.0005); no significant difference in epinephrine levels was detected. Compared with group B subjects, group A had higher baseline ET-1 levels (1.07 ± 0.14 vs. 0.59 ± 0.11 pg/ml, p < 0.02), which increased to a greater extent at peak HG (1.88 ± 0.31 vs. 0.76 ± 0.09 pg/ml, p < 0.005) and R2 (2.46 ± 0.57 vs. 1.31 ± 0.23 pg/ml, p < 0.05) and remained elevated at R10 (3.16 ± 0.78 vs. 0.52 ± 0.09 pg/ml, p < 0.002). Multivariate analysis demonstrated that only a family history of hypertension (chi-square = 7.59, p = 0.0059) and ET-1 changes during HG (chi-square = 4.23, p = 0.0398) were predictive of blood pressure response to HG and that epinephrine and norepinephrine were not.Conclusions. The response to HG in offspring of hypertensive parents produced increased ET-1 plasma levels and resulted in a sustained ET-1 release into the bloodstream during recovery compared with offspring of normotensive parents. This may be an important marker for future clinical hypertension.     

Assay of a Circulating Sodium Pump Inhibitor in Patients with Essential Hypertension and Normotensive Subjects

The plasma levels of a sodium pump inhibitor (Na⁺PI) were measured by a modified method of Hamlyn et al, using dog kidney Na⁺, K⁺-ATPase. When the level of Na⁺PI was expressed as the % inhibition of the enzyme and compared with that of a control solution, it was found to be 9.0 ± 0.7% in 43 untreated patients with essential hypertension. This was significantly higher than 5.0 ± 0.4% for 56 normotensive subjects (p < 0.01). Male patients with essential hypertension showed the highest mean value of 10.5 ± 1.1%, disclosing an apparent sex difference in the patient group (p< 0.01). Only in female patients was there a significant positive correlation between the inhibitor's level and the mean blood pressure (r = 0.649, p < 0.01). These results provided additional evidence for increased Na⁺ PI in the plasma of patients with essential hypertension, which might bear an important role in the pathogenesis of the disease.     

Circulating inhibitor of the Na+-K+ pump in essential hypertension. Physiological and pharmacological variations

The presence of a circulating Na+ pump inhibitor has been assessed in 112 subjects by studying the effects of deproteinized plasma on ouabain binding to erythrocytes and/or inhibition of Na+-K+-ATPase activity. High levels of an inhibitor possessing some digitalis-like properties, were associated with essential hypertension, hypertensive heredity, treatment of hypertension with beta-blocking agents and high sodium intake. Low levels were found in hypertensives on diuretics, patients with chronic renal failure and normotensive controls. These observations are consistent with a possible role of this circulating inhibitor in the control of sodium balance and in hypertension.     

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.     

Na+-H+ exchange and other ion-transport systems in erythrocytes of essential hypertensives and spontaneously hypertensive rats: a comparative analysis

The activity of ion-transport systems and Ca2+-induced erythrocyte haemolysis were compared between patients with essential hypertension and two strains of spontaneously hypertensive rats. Previous data on the increased rate of Na+-Li+ countertransport in erythrocytes of essential hypertensives were confirmed in this study. However, identification of Na+-Li+ countertransport in rat erythrocytes remained a complicated person because of the high rate of sodium-independent efflux of Li+. The rate of Na+-H+ exchange increased by 50-80% both in spontaneously hypertensive Wistar-Kyoto rats (SHR) and in patients with essential hypertension. No difference between Milan hypertensive strain rats (MHS) and Milan normotensive strain rats (MNS) was found. The rate of Na+,K+ cotransport increased in SHR and MHS erythrocytes compared with rats of the control strains [normotensive Wistar-Kyoto rats (WKY) and MNS; 30-50 and 90-110%, respectively]. No difference in this parameter was found between patients with essential hypertension and healthy subjects. Erythrocytes of patients with essential hypertension and of SHR were characterized by a higher sensitivity of their K+ channels to the increased concentration of intracellular Ca2+. This parameter did not change in MHS erythrocytes. Ca2+-induced haemolysis increased four- to fivefold in MHS erythrocytes compared with MNS and did not change in erythrocytes of SHR and patients with essential hypertension. The conclusion from these data is that the SHR strain is a more adequate model of human essential hypertension than the MHS.     

Relationship between red cell sodium transport, blood pressure, and family history of hypertension

Numerous studies have demonstrated that Na+/Li+ countertransport is increased in erythrocytes from hypertensive patients. Since Na+/Li+ countertransport is conducted through the physiologically occurring Na+/Na+ exchange, we studied the latter pathway in 20 subjects with essential hypertension and 20 normotensive subjects matched for age and sex. Ten hypertensives and six normotensives had a positive family history of hypertension. Ouabain (0.1 mM) and furosemide (0.1 mM) were used to assess the active Na+ efflux and Na+-K+-Cl- pathway. There was no significant difference between hypertensive and normotensive subjects in any of the three pathways studied. Among the 16 subjects with a positive family history of hypertension, the mean value for external Na+-dependent Na+/Na+ exchange was significantly higher than in 24 subjects with no family history of hypertension (0.0457 +/- 0.0337 versus 0.0283 +/- 0.0202; P less than 0.05). This study suggests that an inherited membrane transport defect may exist for Na+/Na+ exchange in families of hypertensive subjects.     

Platelet and erythrocyte Mg2+, Ca2+, Na+, K+ and cell membrane adenosine triphosphatase activity in essential hypertension in blacks.

OBJECTIVE: To assess the relationship between intracellular Mg2+, Ca2+, Na+ and K+ and cell membrane adenosine triphosphatase (ATPase) activity in normotensive and hypertensive blacks. DESIGN: Intracellular cations and cell membrane ATPase activity were studied in black patients with untreated essential hypertension and age-, weight- and height-matched normotensive controls. Platelet, erythrocyte and serum Mg2+, Ca2+, Na+ and K+ levels as well as platelet and erythrocyte membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities were measured in all subjects. METHODS: Intracellular Na+ and K+ were measured by flame photometry and Mg+ and Ca+ by atomic absorption spectrophotometry. Cell membrane ATPase activity was determined by a colorimetric method. RESULTS: The hypertensive group consistently demonstrated depressed activity of each ATPase studied, with significantly lower serum Mg2+, serum K+, erythrocyte Mg2+ and platelet Mg2+ levels compared with the normotensive group. Platelet Na+ and Ca2+ and erythrocyte Ca2+ were significantly elevated in the hypertensive group. In the hypertensive group, mean arterial pressure (MAP) was inversely correlated with platelet and erythrocyte membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase. Serum Mg2+, serum Ca2+ and platelet Mg2+ were negatively correlated with MAP in the hypertensive group whilst erythrocyte and platelet Ca2+ were positively correlated. In the normotensive group, platelet Mg2+ and MAP were negatively, and erythrocyte Ca2+ and MAP, positively correlated. CONCLUSIONS: Black patients with essential hypertension have widespread depression of cell membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities with serum and intracellular Mg2+ depletion and cytosolic Na+ and Ca2+ overload, which may reflect an underlying membrane abnormality in essential hypertension. These cellular abnormalities may be related to the defective transport mechanisms that in turn may be aggravated by Mg2+ depletion.     

A higher cellular sodium turnover rate in cultured skin fibroblasts from blacks

Differences in cellular Na+ and K+ regulation may relate to the pathogenesis of essential hypertension and the predisposition of blacks to this disease. To explore these tenets, we examined several aspects of cellular Na+ homeostasis in serially passed, cultured skin fibroblasts from 30 subjects (15 hypertensive blacks and whites and normotensive subjects matched for sex, age, and race.) Fibroblasts from blacks demonstrated higher cellular Na+ turnover rates than did those from whites. This difference was expressed by accelerated Na+-K+ pump activity (ouabain-sensitive Na+ washout rate, 3.46 +/- 0.216 for blacks vs 1.84 +/- 0.283 mEq/L/min for whites; p = 0.0006) and a higher rate of cellular accumulation of Na+ in the presence of ouabain (0.964 +/- 0.0743 vs 0.562 +/- 0.0440 mEq/L/min for blacks and whites, respectively; p = 0.0045). Associated with these findings, fibroblasts from blacks had higher cellular Na+ concentration than did those from whites (9.78 +/- 0.512 vs 7.50 +/- 0.400 mEq/L; p = 0.0170, as measured by atomic absorption, and 7.84 +/- 0.470 vs 5.03 +/- 0.980 mEq/L; p = 0.0141, as derived from the equilibrium distribution ratio of 22Na+). It is concluded that blacks differ from whites with respect to cellular Na+ turnover rate, which is evidenced by an increased Na+ influx and accelerated Na+-K+ pump activity in their fibroblasts. Our findings support the tenet that innate racial differences in cellular Na+ regulation may underlie the predisposition of blacks to hypertension.     

Endogenous digitalislike circulating substances in spontaneously hypertensive rats

Circulating digitalislike compounds have been proposed to be involved in some Na+-dependent types of experimental hypertension and in human essential hypertension. The level of circulating Na+-K+ pump inhibitor(s) was investigated in the spontaneously hypertensive rat of the Okamoto strain (SHR), its normotensive control, Wistar-Kyoto rat (WKY), and the regular Wistar rat using the following criteria: the ability of whole plasma to inhibit the total active Na+ efflux from Wistar rat erythrocytes and to cross-react with digoxin antibodies and the ability of plasma extracts to inhibit Na+,K+-adenosine triphosphatase (ATPase) activity of membranes from rat kidney. SHR plasma inhibited the net Na+ efflux from Wistar erythrocytes by up to 27% compared with WKY or Wistar plasma. For a given number of cells, the inhibition increased with the amount of available plasma. Cross-reactivity with digoxin antibodies was twice as high in SHR as in WKY or Wistar plasma. It was already enhanced in 3- to 4-week-old rats. Plasma extracts from SHR significantly inhibited Na+,K+-ATPase activity when compared with WKY extracts (75.6 +/- 2.6 vs 89.3 +/- 2.4 mumol Pi/mg/hr; p less than 0.01) but did not differ from Wistar plasma extracts. These results strongly suggest that circulating digitalislike compound(s) are present in elevated amounts in SHR as early as 3 to 4 weeks of age, but their exact participation in blood pressure elevation or maintenance remains to be clarified.     

Plasma renin and aldosterone in youngsters with insulin-dependent diabetes mellitus

We studied plasma renln activity (PRA) and aldosterone in three groups of subjects. Group 1 consisted of seven Type I diabetics with microalbuminuria (>25 μg/min), age 12.0–19.5 yr (mean ± SD: 15.4 ± 2.2), duration of disease 6.5–10.1 yr (7.9 ± 1.9), HbA₁c 9.6–16.0% (12.6 ± 2.9). Group 2 consisted of seven sex and age-matched diabetics, duration of disease 3.7–9.0 yr (6.0 ± 2.3), HbA₁c less than 8%, microalbuminuria less than 10 μg/min, and microangiopathy-free. Group 3 consisted of seven healthy subjects. After overnight recumbency the PRA in group 1 patients was significantly higher than that for group 2 (3.926 ± 4.54 ng/ml/h vs. 1.416 ± 0.44; p < 0.05) or for group 3 (3.926 ± 4.54 vs. 1.11 ± 0.82; p < 0.007). After physical exercise the group 1 PRA value (10.199 ± 9.62) was higher than in either group 2 (2.821 ± 1.77; p < 0.005) or group 3 (1.61 ± 0.803; p < 0.0006). Poor metabolic control and the presence of microalbuminuria can play a role in perturbations of the Renin-Angiotensin system. The presence of microalbuminuria can be an important indicator of mildly impaired renal function and may influence PRA production.