Angiotensin II's Role in Mediating Angiotensin I- and Tetradecapeptide-Induced Steroidogenesis by Rat Glomerulosa Cells

To evaluate the role of angiotensin II (A II) in mediating the steriodogenic response to angiotensin I (A I) and tetradecapeptide, rat glomerulosa cells were incubated with each peptide in the presence or absence of an angiotensin-converting enzyme inhibitor (captopril or the nonapeptide bradykinin-potentiating factor). Both A I and tetradecapeptide increased aldosterone secretion in a dose-dependent fashion, but were considerably ess effective (P < 0.001) than the same dose (2.4 × 10_-9 M) of A II. In addition, both A I and tetradecapeptide caused a dose-dependent increase in A II accumulation in the incubation media, indicating that part of their steriodogenic effect is indirect via conversion to the octapeptide.

While captopril (1.0 × 10^-4 M) almost completely blocked A I (2.4 × 10^-8M)-induced A II accumulation, it caused only a 50% reduction (P < 0.01) in aldosterone output. The nonapeptide-converting enzyme inhibitor (2.3 × 10^-6 M) produced a similar blockade. This lack of complete inhibition of A I-induced steroidogenesis suggests that A I also has a direct effect on the glomerulosa cells, i.e. 50% of the activity of A I is due to intrinsic activity. On the other hand, converting enzyme inhibitors did not affect tetradecapeptide-induced aldosterone output or A II accumulation, making it impossible to determine if it has a direct steroidogenic effect. The failure of converting enzyme inhibitors to modify tetradecapeptide-induced accumulation of A II suggests that an enzyme other than converting enzyme is responsible for its generation.