The natriuretic hormone and hypertension

Effects of Salt Rich Diet in the Obese Zucker Rats: Studies on Renal Function During Isotonic Volume Expansion

Obese Zucker rats (OZR) are hyperinsulenemic, hyperglycemic and dyslipidemic and develop salt dependent hypertension. Since salt sensitivity is considered to be due to impaired handling of renal sodium excretion, these studies were conducted in the obese and lean Zucker rats (LZR) anesthetized with Inactin to evaluate renal function under basal conditions and during acute isotonic fluid volume expansion (VE). Mean Arterial blood pressure (MBP), heart rate (HR), renal blood flow(RBF) and glomerular filtration rate (GFR) were not significantly different between the lean Zucker rats fed normal diet or that fed salt rich diet(8% NaCI). However, basal UV and UNaV were significantly greater in the LZR fed high salt. During VE essentially identical increases occurred in GFR, UV and UNaV in both the lean groups. In the OZR fed salt rich diet also, there were no significant changes in the heart rate, RBF and GFR. However, arterial blood pressure of the OZR fed salt rich diet was significantly greater than that of the OZR on the normal diet as well as that of both the lean groups. Also, as in the LZR, basal UV and UNaV were significantly greater in the salt fed obese rats. During volume expansion there were no impairments in the ability of the obese groups fed normal or salt rich diet to eliminate sodium and water during volume load. In fact, the net sodium and water excretions during and 60 min after VE in both the obese groups were significantly greater than that of corresponding lean groups. Furthermore, these values in the OZR fed salt rich diet were significantly greater than that of the obese rats on normal salt diet perhaps due to the contribution of pressure natriuretic mechanisms'. These data demonstrate that although OZR are salt sensitive, the renal mechanisms that would collectively respond to acute isotonic VE were fully functional. An unexpected and a novel finding in these studies is that the salt rich diet, in addition to increasing arterial blood pressure also significantly lowered plasma of insulin levels and enhanced glucose and cholesterol levels in the obese Zucker rats.

Alteration of erythrocyte membrane Na, K-ATPase in children with borderline or essential hypertension

The aim of this study was to evaluate the substrate (ATP) kinetics of erythrocyte membrane Na, K-ATPase in children with borderline or essential hypertension. Although the activity of Na, K-ATPase in the presence of in vivo concentrations of ATP was not significantly altered, kinetic studies showed an obvious inhibition of enzyme activity in the erythrocyte membrane of children with borderline or essential hypertension. Hanes plot analysis revealed a decrease of V(max) from 7.19 in erythrocytes from control subjects to 4.93 and 3.33 in those from children with borderline or essential hypertension, respectively. A mean value of the K(m) decreased from 0.10 in the control to 0.08 and 0.02 in children with borderline or essential hypertension, respectively. The energy status of erythrocytes, estimated by ATP, ADP and AMP levels, ATP/ADP ratio, and adenylate energy charge (AEC) was not significantly changed in the cells from hypertensive children. The use of a free radical-generating system (FeSO4/ascorbate) in vitro significantly reduced enzyme activity in the control erythrocytes while in those from hypertensive children it was abolished completely. The level of lipid peroxides was considerably higher (+ 37 per cent) in the plasma, while that of reduced glutathione was significantly lower both in the erythrocytes and the plasma of children with essential hypertension than in healthy children. These results indicate significant alterations of the antioxidant status which could be the cause of the inhibited Na, K-ATPase activity in erythrocyte membranes from hypertensive children.

Effect of plasma from essential hypertensives on vascular tone of aortic strips, isolated perfused mesentery and isolated perfused kidney

Different vascular models of normotensive Wistar rats, including aortic strips, isolated perfused mesentery and isolated perfused kidney, were used to study hemodynamic effects of plasma fractions obtained by gel filtration from the blood of essential hypertensive and normotensive subjects. Plasma fractions from essential hypertensives studied had been shown to increase blood pressure after intravenous injection in rats. In the aortic strips, 50 microliters of a hypertensive fraction (HF) elicited a calcium-dependent contraction of 0.14 +/- 0.035 mN (n = 20, p less than 0.05), which was inhibited by nifedipine, whereas tension of the strips was not significantly changed by normotensive fractions (NF) (n = 17). In the isolated perfused mesentery preparation, no significant change of perfusion pressure by HF or NF could be demonstrated (n = 10). In the isolated perfused kidney, a transient increase of perfusion pressure was induced by HF (19.5 +/- 16.6 mm Hg, n = 40, P less than 0.001) but not by NF. This increase was abolished in calcium-free, 2 mmol/l EGTA containing perfusion medium. The response was diminished, but not abolished by nifedipine. These data demonstrate vasopressor properties of plasma from essential hypertensives, which might be the consequence of a circulating vasoconstrictor substance in the blood of essential hypertensives.

Contribution of elevated peripheral resistance to acute volume loaded hypertension in dogs

The role of peripheral resistance in volume loaded hypertension was examined by measuring changes in hemodynamic and humoral factors during dextran infusion for one hour, and for three hours after stopping the infusion in 30 anesthetized dogs. The mean blood pressure was elevated significantly (118%, p less than 0.01), accompanied by increased cardiac output (170%, p less than 0.01) during volume loading. In the recovery period, cardiac output returned to the basal level while the total peripheral resistance gradually increased (118%, p less than 0.01) contributing to the maintenance of high blood pressure (108%, p less than 0.01). There was no significant correlation between the hematocrit change and the increase in peripheral resistance in the recovery period. The levels of plasma norepinephrine, epinephrine, renin activity and vasopressin were suppressed by volume loading. These results demonstrated the importance of increase in peripheral resistance for maintaining high blood pressure in this acute volume excess model. The participation of the renin-angiotensin system, the sympathetic nervous system and vasopressin were all excluded and vascular structural changes were most unlikely to be involved in this acute experiment. It remains unknown whether this conversion is attributable to the summation of local autoregulation or to some other factors.

Studies on the role of intracellular sodium and calcium in the centrally mediated pressor effects of CSF [Na+], ouabain and angiotensin II in anesthetized dogs

Cerebroventricular infusions of hyperosmotic Na+ solutions, ouabain and/or Angiotensin-II produced significant increases in the arterial blood pressure in chloralose anesthetized, vagotomized dogs. A lower concentration of ouabain (10(-6) M) which did not alter blood pressure, significantly potentiated the centrally mediated pressor effects of hyperosmotic Na+ and angiotensin-II. Hence, the data suggested that the magnitude of the central pressor effects of Angiotensin-II or hyperosmotic Na+ may depend upon net accumulation of sodium in the neuronal cells. Prior cerebroventricular infusions of felodipine, a "calcium antagonist," significantly inhibited the pressor actions of higher concentrations of ouabain as well as that of hyperosmotic Na+-solutions, indicating that cellular calcium may be essential for triggering these central effects. These studies collectively indicate that disturbances in the Na+-transport in the neuronal cells may account for the involvement of the central nervous system in various types of hypertension.

Abnormal leucocyte sodium transport in Chinese patients with essential hypertension and their normotensive offsprings

We studied changes in intracellular electrolytes and the rate constant of 22Na efflux from isolated leucocytes in Chinese patients with essential hypertension and their normotensive offsprings. The hypertensives had an increase of sodium content and a reduction of the total or ouabain-sensitive 22Na efflux from leucocytes. The normotensives born of hypertensive parents showed lower sodium content, higher potassium content and reduced rate constant of ouabain-insensitive 22Na efflux from leucocytes. The results suggest that the inhibition of cell sodium pump activity may be a marker of essential hypertension. Abnormal cell sodium transport observed in normotensives with family history of hypertension may have some etiological linking with the inheritance of hypertension.

Intracellular Na+ and Ca2+ in aortic smooth muscle cells after enzymatic isolation in spontaneously hypertensive rats

In aortic smooth muscle cells from spontaneously hypertensive and normotensive rats, which were separated from the blood vessels by enzymatic digestion, intracellular Na+ and Ca2+ activities were measured. The determinations were made by ion-selective electrodes. In smooth muscle cells from spontaneously hypertensive rats intracellular Na+ and Ca2+ activities were elevated as compared to cells from normotensive rats (P less than 0.001 for Na+ and P less than 0.01 for Ca2+). Since in cultured aortic smooth muscle cells from spontaneously hypertensive rats intracellular Ca2+ is known to be not significantly increased, primarily humoral factors may account for the changes in intracellular Ca2+ reported.