Guanylin a novel regulatory peptide possibly involved in the control of Ca2+-dependent agonist-stimulated aldosterone secretion in rats

Guanylin is a 15-amino acid peptide, which activates guanylate cyclase (GC) and plays a major role in the regulation of water and electrolyte secretion by intestinal mucosa. The expression of guanylin prohormone has been recently demonstrated in the rat adrenal gland, and this prompted us to investigate whether guanylin, like other peptides secreted by adrenal medulla, affects the function of the adrenal cortex. Autoradiography demonstrated the presence of [125I]guanylin binding sites in the zona glomerulosa (ZG), but not zona fasciculata-reticularis. Guanylin did not change either basal or ACTH-stimulated steroid secretion of dispersed rat adrenocortical cells, but concentration-dependently (from 10(-10) M to 10(-8) M) inhibited aldosterone response of ZG (capsular) cells to both angiotensin-II (ANG-II) and K+. Guanylin (10(-8) M) blocked the aldosterone secretagogue effect of the Ca2+-channel activator BAYK-8644, and the Ca2+-ionophore ionomycin counteracted the inhibitory action of this peptide on the secretory responses of capsular cells to ANG-II and K+. As expected, guanylin did not affect cyclic-AMP release by capsular cells, but evoked a sizeable increase in cyclic-GMP production. Both the inhibitor of GMP synthase decoyinine and the GC-inhibitor LY-83583, although suppressing cyclic-GMP release, did not affect guanylin-evoked inhibition of K+-stimulated aldosterone secretion. Collectively, these findings allow us to conclude that guanylin: i) inhibits aldosterone secretion of rat ZG cells by interfering with the agonist-induced activation of voltage-gated Ca2+-channels, the stimulation of guanylate cyclase conceivably playing a negligible role; and ii) could be included in that group of regulatory peptides, secreted by medullary chromaffin cells, which are able to counteract an exceedingly high aldosterone secretion.