Sodium transport inhibition, cell calcium, and hypertension. The natriuretic hormone/Na+-Ca2+ exchange/hypertension hypothesis

Atrial natriuretic factor determinations and chronic sodium homeostasis

To examine the physiological role of atrial natriuretic factor (ANF) in the maintenance of sodium homeostasis under various conditions, we performed experiments in rats across a wide range of sodium intake, in rats with chronic renal insufficiency at extremes of sodium intake, and in rats given desoxycorticosterone acetate. After three weeks of a very low sodium diet, regular diet, or regular diet plus 1% saline as drinking water, no difference in plasma values of immunoreactive atrial natriuretic factor (IR-ANF) were identified, while rats at the lowest level of sodium intake had elevated atrial values. Normal rats, and rats with 5/6 nephrectomy has plasma values of IR-ANF which were no different irrespective of their sodium intake, nor were atrial values in these rats different. Although mineralocorticoid "escape" could be documented by changes in urine sodium excretion, neither plasma nor atrial IR-ANF values showed differences either at 24 or 72 hr after "escape". The data are consistent with previous observations that ANF serves the purpose of affecting rapid adjustments to large alterations in circulating fluid volume. Chronic high sodium intake, adaptation to renal insufficiency, and adjustment to the effect of mineralocorticoid do not appear to be associated with increased circulating plasma concentrations of IR-ANF.

Saralasin blocks the effect of angiotensin II and extracellular fluid saline expansion on the Na-K-ATPase inhibitor release in rats

A low molecular weight substance which behaves like ouabain as inhibitor of brain membrane Na-K-ATPase and 3H-ouabain binding was found in plasma after saline expansion of extracellular fluid or angiotensin II infusion into the third brain ventricle in the rat. Intracerebroventricular infusion of angiotensin II antagonist, saralasin, blocks the increase of the Na-K-ATPase inhibitor produced by infusion of angiotensin II into the third ventricle or extracellular fluid saline expansion.

Na-K ATPase activity of erythrocyte membrane correlated with urinary Na/K ratio in normotensive adolescents with a family history of hypertension

In female adolescents with a normal blood pressure range, the correlation between urinary sodium excretion and Na-K ATPase activity of erythrocyte membrane was investigated. 34 subjects with a family history of hypertension showed a significantly positive correlation between body mass index and mean blood pressure (r = 0.64, p less than 0.01), and a significantly lower urinary Na/Cr and Na/K ratio compared with 37 subjects without the same family history (p less than 0.05 and p less than 0.01, respectively). The positive correlation between urinary Na/K ratio and Na-K ATPase activity of erythrocyte membrane was demonstrated in only subjects with a family history of hypertension (r = 0.41, p less than 0.05). The authors discuss these significant correlations in terms of the abnormal membrane cation fluxes associated with essential hypertension.

Effects of calcium loading on blood pressure in spontaneously hypertensive rats: attenuation of the vascular reactivity

The effects of oral calcium loading on the blood pressure of spontaneously hypertensive rats (SHR) (n = 20) and age-matched normotensive Wistar-Kyoto rats (WKY) (n = 16) were investigated. Calcium loading was performed by adding 1.5% CaCl2 (Calcium chloride) to the drinking water. Calcium loading attenuated the development of hypertension in the SHR but not in the WKY, and at the end of a 3 week experiment, systolic blood pressure was 171+2 vs 197+3 mmHg (P less than 0.01). In spite of this reduction of blood pressure, there were no significant changes in sodium-water balance, plasma levels of norepinephrine and epinephrine, plasma renin activity, plasma aldosterone concentration and serum electrolytes due to the calcium treatment. On this basis, the depressor mechanism of calcium loading in the SHR was studied by observing the pressor response to norepinephrine and the vascular reactivity to norepinephrine in hind limb perfusion. It was found that both the pressor response and the vascular reactivity were significantly attenuated by the calcium treatment in the SHR but not in the WKY. These results suggest that the antihypertensive effects of calcium treatment in SHR may depend mainly on attenuation of the vascular reactivity.

Comparative effects of diltiazem and hydrochlorothiazide in blacks with systemic hypertension

The blood pressure-lowering effects of a calcium-entry blocker, diltiazem (240 or 360 mg/day), and a thiazide diuretic, hydrochlorothiazide (50 or 100 mg/day), were studied in 20 black hypertensive patients. Supine blood pressure decreases of -34/-18 mm Hg for diltiazem and -29/-21 mm Hg for hydrochlorothiazide were noted after 14 weeks of therapy. Differences between drugs were not significant. Blood pressures were normalized in all 4 of the monotherapy nonresponders when the 2 drugs were combined. Significant adverse effects were not noted. These data suggest that diltiazem is an effective antihypertensive agent in black patients. As monotherapy, its blood pressure-lowering effect is equivalent to hydrochlorothiazide.