The use of the superfusion approach with rat adrenal capsular cells to compare the steroidogenic potencies of angiotensin analogues, without the effects of peptide degradation

The superfusion approach was used to assay the relative potencies of (Asp1)angiotensin II (A II), (des-Asp1)angiotensin II (A III) and (Sar1)angiotensin II (SA II) in stimulating the steroidogenesis (corticosterone) of rat adrenal capsular (zona glomerulosa) cells without the effects of peptide degradation; an example of the advantages of such a procedure. At the rate of superfusion employed, there was no significant reduction in the concentration of A II or A III through the cell chamber. However, in the in vitro closed system A III was degraded more rapidly than A II. SA II would be expected to be more protected than A II. The potency ratios were found to be 0.30 +/- 0.05 (s.e.), four experiments for A III/A II and 2.23 +/- 0.16 (s.e.), six experiments for SA II/A II in the usual closed system with 1 h incubations. In the superfused system, however, the corresponding ratios were 0.82 +/- 0.11, five experiments and 1.19 +/- 0.23, four experiments. These results indicated that, as shown by the relative potencies in the superfusion system and therefore without the effects of degradation. A II, A III and SA II have about the same intrinsic activity. The reason for the relatively high activity of SA II in the closed system is its lower rate of degradation compared with A II and the low activity of A III is owing to its relatively high rate of degradation. It is concluded that, although the intrinsic activities of A III and A II are similar, because of the equivalent activity of SA II, it is probably not necessary for SA II, or therefore also A II, to be converted to A III for it to stimulate steroidogenesis in the system.