Metabolism and Biological Activities of Angiotensin II and des-Asp1-Angiotensin II in Isolated Adrenal Glomerulosa Cells

Abstract

The steroidogenic activities and receptor-binding properties of angiotensin II and des-Asp¹-angiotensin II were analyzed and compared with the metabolism of each peptide during incubation with isolated rat and canine adrenal glomerulosa cells. In isolated rat glomerulosa cells, equimolar concentrations of each peptide stimulated aldosterone production similarly over the first 20 min of incubation. However, at later times, the des-Asp¹ heptapeptide was consistently less active, with one third the potency of the octapeptide at 40 min, and only one tenth that of the octapeptide after 120 min. In dog glomerulosa cells, the des-Asp¹ heptapeptide was less rapidly inactivated and retained one third the potency of the octapeptide after 120 min of incubation. In each species, the maximum aldosterone response to the two peptides was similar at all time intervals studied up to 120 min. Also, no additivity between maximal concentrations of angiotensin II and des-Asp¹-angiotensin II was observed. Assay of angiotensin II receptors with radioiodinated and tritiated peptides in rat glomerulosa cells and adrenal particles showed identical receptor concentration for each peptide, with somewhat lower affinity for the heptapeptide (K_a = 0.7 × 10⁹ M^-1 for the heptapeptide vs. 2.7 × 10⁹ M^-1 for angiotensin II). The metabolism of angiotensin in isolated glomerulosa cells was analyzed by thin layer chromatography and assay of peptides in the incubation medium. Extensive degradation of angiotensin II during incubation with rat cells was detected, with relatively little conversion to the 2–8 and 3–8 peptides. RIA of medium peptides showed decreases to 50% and 30% for angiotensin II and des-Asp¹-angiotensin II at 40 min, and to 26% and 10% at 120 min, respectively. Similar results were derived with radioreceptor assay and bioassay by stimulation of aldosterone production in fresh glomerulosa cells. In contrast, metabolism of angiotensin during incubation with dog adrenal glomerulosa cells was less rapid, and both peptides were degraded at the same rate. Analysis of labeled peptides eluted from rat glomerulosa cells after in vivo or in vitro binding of [¹²⁵I]iodoangiotensin II showed that more than 90% of the bound radioactivity was composed of the octapeptide. These studies have revealed extensive metabolism of angiotensin peptides by isolated rat glomerulosa cells, with more rapid degradation of des-Asp¹-angiotensin II than of the native octapeptide. Such peptide metabolism contributes to the apparent discrepancy in biological potencies of angiotensin II and des-Asp¹-angiotensin II on aldosterone production in isolated rat glomerulosa cells. It is also evident that angiotensin II acts upon rat and dog glomerulosa cells to evoke aldosterone production without prior conversion to the heptapeptide.