Renin release during acute reduction of arterial pressure in normotensive subjects and patients with renovascular hypertension

In normotensive subjects, acute reduction of mean arterial pressure to from 60 to 75 mm Hg by infusion of sodium nitroprusside caused significant increase in renin activity of renal venous plasma and also in the renal-systemic difference of renin activity. At the same time, the products of the renal-systemic difference of renin activity and renal plasma flow increased significantly, whereas renin substrate activity of plasma was unchanged, indicating that there was an increase in renin release during reduction in pressure. Renin activity of renal venous plasma, expressed in logarithms, showed a significant correlation with the degree of reduction in pressure; an increase in renin activity became significant when mean arterial pressure was reduced to below a level of 70 to 75 mm Hg. There was a striking difference in the renal response to reduction in pressure between patients with renovascular hypertension and normotensive subjects. In 10 renovascular hypertensive patients, significant increase in renin release occurred from the involved kidney at mean arterial pressures ranging from 90 to 137 mm Hg; the threshold at which renin release increased was shifted to a range much higher than that in normotensive subjects. Furthermore, the magnitude of renin release from the involved kidney was significantly greater when compared to that in normotensive subjects. In contrast, in the contralateral uninvolved kidney, no significant release of renin was detected during reduction in pressure. The renal mechanism controlling renin secretion appears to be operative at higher systemic arterial pressure levels and with enhanced responsiveness in the involved kidney of renovascular hypertensive patients; the findings are consistent with the hypothesis that the renin-angiotensin system participates in maintaining hypertension in this disease.

Effect of angiotensin on the pressor response to tyramine in normotensive subjects and hypertensive patients

Arterial blood pressure was recorded directly in 11 normotensive subjects, in 11 patients with essential hypertension and in 10 patients with renovascular hypertension. The pressor response to intravenous injections of norepinephrine, tyramine, angiotensin and ephedrine was compared in these three groups. An infusion of angiotensin was then given and the pressor response to injections of norepinephrine, tyramine, and ephedrine was again measured; in some cases, a comparison of the effects of the cold pressor test was made.

In normotensive subjects, angiotensin infusion caused a significant increase in the responses to tyramine and ephedrine but not to norepinephrine; this occurred with subpressor doses and was unrelated to the level of blood pressure. In patients with renovascular hypertension, the mean pressor responses to injections of tyramine and ephedrine and the ratio of response to tyramine to that to norepinephrine were significantly increased, and the infusion of angiotensin failed to cause any further increase in these responses. In contrast, in patients with essential hypertension, the average response to injections of tyramine was not significantly increased, but during angiotensin infusion the response to tyramine increased as in normotensive subjects. It is proposed that increased response to tyramine, and lack of the potentiating effect of angiotensin demonstrated in renovascular hypertension, may be caused by increased formation of endogenous angiotensin due to renin secretion in this disease.

Cardiovascular reactivity in acute and chronic renal hypertensive dogs

Both before and after pentobarbital anesthesia, cardiovascular responsiveness to norepinephrine, epinephrine and serotonin was increased and that to angiotensin was decreased when experimental renal hypertension was first elicited. Response to tyramine was strikingly enhanced.

Several weeks later, during the chronic phase of hypertension, there was a slight and scarcely significant heightened response to norepinephrine, a clearly increased response to tyramine, and no change in that to serotonin, dimethylphenylpiperazinium iodide, and 3-hydroxy-phenyl-ethanolamine HCl.

Study of the problem of variability of responsiveness showed relatively little difference in mean response to any of five pressor drugs with or without anesthesia and after sham operation.

Hypertension produced by infusion of angiotensin into normal anesthetized dogs did not change the response to norepinephrine but more than doubled that to tyramine. When acute renal hypertension was produced in the same dogs, angiotensin infusion failed to cause further change in response to either substance. Infusion of angiotensin into dogs with chronic renal hypertension caused increased response to tyramine but not to norepinephrine.

The perfused hind legs of dogs with acute renal hypertension showed clearly reduced responses to angiotensin with no concurrent change in responses to norepinephrine. The pressor response to serotonin was increased while that to tyramine was unchanged. During chronic hypertension responses were like the normal values.

Hypertension caused by infusion of angiotensin decreased greatly the vasoconstrictor response to superimposed injections of angiotensin into the perfused leg circuit while norepinephrine responses were unchanged. Response to another peptide, vasopressin, was reduced like that to angiotensin.

These studies suggest that during acute renal hypertension increased quantities of angiotensin are present and that there is a decrease in neural tone. The finding that during the chronic phase of hypertension, the response to tyramine alone remained elevated, suggests the presence of small amounts of angiotensin, i.e., enough to increase the pressor effectiveness of endogenous norepinephrine released by tyramine. During the chronic phase of hypertension, peripheral resistance appears relatively more dependent on neural control.