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 (AII), (des-Asp¹) angiotensin II (AIII) and (Sar¹) angiotensin II (SAII) 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 AII or AIII through the cell chamber. However, in the in vitro closed system AIII was degraded more rapidly than AII. SAII would be expected to be more protected than AII. The potency ratios were found to be 0.30 ± 0.05 (s.e.), four experiments for AIII/AII and 2.23 ± 0.16 (s.e.), six experiments for SAII/AII 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. AII, AIII and SAII have about the same intrinsic activity. The reason for the relatively high activity of SAII in the closed system is its lower rate of degradation compared with AII and the low activity of AIII is owing to its relatively high rate of degradation. It is concluded that, although the intrinsic activities of AIII and AII are similar, because of the equivalent activity of SAII, it is probably not necessary for SAII, or therefore also AII, to be converted to AIII for it to stimulate steroidogenesis in the system.