Changes in the levels of aldosterone synthase cytochrome P-450, a recently identified enzyme in rat adrenals, were studied in response to the renin-angiotensin system and K stimuli. As examined by an immunoblot technique, the zona glomerulosa mitochondria from rats fed on a low Na-normal K diet (8.6 mmol Na+and 207 mmol K+/kg of diet) or a low Na-high K (0.2 M KC1 in drinking water) diet for 4-10 days con-tained significantly higher amounts of aldosterone synthase cytochrome P-450 than those from rats fed on a normal diet (86 mmol Na+and 207 mmol K+/kg of diet). Activities of the enzyme were also found to increase by about 10-fold on day 10. In concert with these changes, both plasma renin activity and plasma aldosterone concentration increased, indicating that the renin-angiotensin system was activated in these rats. Feeding with a normal Na-high K diet also induced significantly higher levels of both amount and activity of aldosterone synthase cytochrome P-450 together with an elevated serum K concentra-tion on day 4, though they all decreased to near the control level on the following days. On the other hand, when enalapril malate, an angiotensin I-converting enzyme inhibitor, was administered to the low Na-normal K rats, the increases in the amount and activity of the enzyme as well as in plasma aldosterone concen-tration were suppressed altogether. However, the enalapril ad-ministration to the low Na-high K rats suppressed the increases only partially. These results indicate that the aldosterone syn-thase cytochrome P-450 is an ultimate target of the regulation of aldosterone biosynthesis by angiotensin II and K.
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