Abstract
It has been proposed that many factors are involved in the regulation of synthesis and secretion of aldosterone by the zona glomerulosa of the adrenal cortex. According to the propositions generally accepted, the renin-angiotensin system is regarded as the most important one and the site of action of angiotensin on steroidogenesis is believed to be mainly in the earlier step above pregnenolone but not in the vicinity of the final step of aldosterone formation. This hypothesis can, however, not satisfactorily explain why angiotensin II administered in vivohas a relatively specific aldosteronotropic effect. Moreover, in the rat, there have been much conflicting results about the role of renin-angiotensin system played in steroidogenesis of the adrenal cortex. Under these circumstances, the authors attempted to study a possible action of the renin-angiotensin system on the later steps of steroidogenesis in vitro by estimating percentage conversion of the radioactive precursor to polar steroids in the adrenal from rats fed on a low-sodium diet, those on a high-sodium diet and rats with unilateral renal hypertension, produced according to Koletsky's method. The adrenals from rats under various experimental conditions were minced and incubated for 2 hours at 37°C in Krebs-Ringer bicarbonate solution containing 0.1 % sodium fumarate after 40 minutes preincubation, added together with 14C-4. pregnenolone, 14C-4-progesterone or 14C-4-cholesterol. The radioactive steroids yielded during the incubation were separated by means of paperchromatography of toluol-propyleneglycol system and were scanned. The radioactivities of 5 steroids, which were identified tentatively as 18-OH-B, aldosterone, 18-OH-DOC, corticosterone, desoxycorticosterone out of the 8 steroids converted from radioactive progesterone, and the radioactivity of the precursor, progesterone unchanged, were counted by the liquid scintillation counter.
The rats fed on a low-sodium diet for 7 days showed a significant increase (p<0.001) in plasma renin activity (PRA), and their adrenals produced a significantly enhanced conversion of radioactive precursor to 18-OH-13 (p<0.05) and to aldosterone (p<0.001). On the other hand, the rats given 1% sodium chloride solution showed a significant decrease in PRA (p<0.001), and the production of 18-OH-B and aldosterone by their adrenals was diminished. Addition of angiotensin II in the doses of 5 to 50μg every 30 minutes to the incubation medium showed a stimulatory effect on the conversion of steroid presursor to 18-OH-B, aldosterone, 18-OH-DOC and B. The rats with aorticonstri tion developed a marked hypertension (158±3mmHg) in acute stage with a con omitant in rease in PRA (p<0.001), and their adrenals produced an appreciably enhanced conversion of the precursor to 18-OH-B and aldosterone, but the differences between the control and the constri tion groups were not significant. In ohronic stage, the rats with the aortic constriction showed only a slightly supernormal increase in PRA, despite of continuing high blood pressure (160±9mmHg). In addition, the corticoid pattern converted from the radioactive precursor differed from that produced by the rat adrenal in acute stage of hypertension; there was a significantly enhanced conversion of the precursor to 18-0H-DOC(p<0.05). About 50% of angiotensin II added to the incubation medium was destroyed by the adrenal mince or homogenate within 1 hour. The fact suggests that most of the previous works on failure to produce the stimulatory effect of angiotensin II on aldosterone formation in vitro is due to a considerably high angiotensinase activity of rat adrenal gland.
