Mechanism by which wortmannin and LY294002 inhibit catecholamine secretion in the rat adrenal medullary cells

The effects of wortmannin and LY294002, inhibitors of PI3-kinase, in secretagogue-stimulated rat adrenal chromaffin cells loaded with Calcium Green-1 were studied by simultaneously measuring changes in the fluorescence intensity of the indicator (Ca-response) and in the release of catecholamine (secretory response). Before application of these agents, the profile of the secretory response evoked by a 10-min stimulation with 30 mM K+ was approximated by the Mh (2.6 on average) power of that of the Ca-response. Both agents dose-dependently inhibited the high-K+-elicited Ca-response and secretory response in a similar mode to which the Mh power relation was preserved despite the occurrence of profound changes in the shapes and sizes of these two responses. The L-type Ca2+-channel blocker PN200-110 inhibited the high-K+-evoked responses in a similar fashion. Thus, it is likely that wortmannin and LY294002 inhibit high-K+-evoked CA secretion by inhibiting a Ca2+-influx through voltage-dependent Ca2+ channels. Although regulation of L-type Ca2+ channel activity via PI3-kinase has been reported in vascular myocytes, this possibility may be limited in the present case since the doses of LY294002 and wortmannin used to inhibit the secretory response are much higher than IC50's for inhibition of PI3-kinase with these agents. Compared with the high-K+-elicited responses, muscarine-evoked Ca-responses and secretory responses were more strongly inhibited by wortmannin, but less affected by LY294002. The differential effects suggest that the inhibition of the muscarine-evoked secretion by these agents is not associated with the inhibition of PI3-kinase. (C) 2001 Harcourt Publishers Ltd.