Vascular renin-like activity and blood pressure maintenance in the rat. Studies of the effect of changes in sodium balance, hypertension and nephrectomy.

Aortic renin-like activity and plasma renin concentration were measured in rats subjected to dietary salt loading or depletion. Homogenates prepared from rat aortic tissue generated angtotensin I from plasma substrate at both pH S3 and pH 6.5. Aortic renin-like activity measured at incubation pH 6.5 and plasma renin concentration changed in parallel, rising with salt restriction and falling with salt loading. However, the capacity of aortic bomogenate to generate angiotensin I at an incubation pH of 53 did not snow any signiflcant change with alteration of sodium balance. In addition, administration of the converting enzyme inhibitor (CEI) SQ20,881 produced a greater fall of blood pressure in salt-depleted than salt-loaded animals. In order to determine whether arterial or plasma renin was the important variable in blood pressure regulation, the depressor response to CEI was studied in GoMMatt two-kidney, one dip hypertensive rats before and 1, 2, 6 and 24 hours after bilateral nephrectomy. Aortic and plasma renin concentration were measured in a second series of hypertensive rats at the same times after bilateral nephrectomy. Although plasma renin concentration showed tbe expected fall to basal levels within 1 hour, a signiflcant depressor response to CEI was observed at 1 and 2 hours after bilateral nephrectomy, with a smaller fall at 6 hours. When a competitive antagonist of angiotensin II (saralasin) was infused, a significant fall in Mood pressure occurred 1 hour after bilateral nephrectomy, and no further fall was induced by injection of CEI. Thus, the fall of blood pressure induced by CEI was due to renin-angiotensin blockade and not an additional hypotensive effect Aortic renin-like activity measured at an incubation pH of 6.5 fell slowly after bilateral nephrectomy and was significantly lower only at 6 and 24 hours. This closely parallels the reduction of depressor response observed with converting enzyme inhibitor. However aortic renln-Uke activity measured at pH 5 J showed no significant change at anytime after bilateral nephrectomy except for activity after 1 hour, which was significantly elevated. This evidence supports more indirect studies which have suggested that arterial renin-like activity derived from the kidney is important in blood pressure maintenance by tbe local generation of angiotensin II, and that this activity has a longer half-life than plasma renin. Enzymic activity detected at a low incubation pH probably represents a tissue enzyme that is not important in blood pressure maintenance. (Hypertension 1: 643-649, 1979)