Quantitative changes in rat renin secretory granules after acute and chronic stimulation of the renin system

In order to study the cellular mode of renin secretion, stereological methods were used to estimate number and volume of rat renin secretory granules during stimulation of the renin system. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min increased plasma renin concentration (PRC) twofold, but did not significantly change the number of renin granules per arteriole or the renin-containing volume of the arteriole. Chronic stimulation was achieved by a combination of low-salt diet and inhibition of angiotensin-converting enzyme (ACE) for 14 days, and resulted in a 36-fold increase in PRC, a 20-fold increase in the number of granules per arteriole, and a 17-fold increase in the arteriolar volume that contained renin. An acute decrease in renal perfusion pressure to 40 mmHg for 5 min in the chronically stimulated rats increased PRC further (1.6-fold), and significantly reduced the number of granules per arteriole by 4000 (45% reduction), but did not change the renin-containing arteriolar volume significantly. The average renin granule size was 0.35 microm3 with no significant differences among the groups. We conclude that recruited granular cells contribute significantly to renin release, and that all granular cells along the arteriole participate in secretory responses. The reduced number of renin granules after acute stimulation is compatible with exocytosis as the dominating mechanism of renin release.