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Background: Arachidonic acid (AA) generated by the activation of phospholipase A2. and 12hydroxyeicasateraenoic acid (12-HEIE), a lipoxygenase metabolite of AA stimulates glucose induced insulin secretion in pancreatic islets, whereas prostaglandin E. (PGE2), a cyclooxygenase metabolite of AA, inhibits it. This effect of PGE2 is largely reversible by pertussis toxin, suggesting a possible involvement of Gi or Go-like proteins. The purpose of this study was to investigate the regulation of guanine nucleotide binding protein (G-protein) activity by AA and its metabolites (12-HETE and PGE2) in the secretory granule of normal rat pancreatic islets. Methods: After isolation and subcellular fractionation of pancreatic islets from male Sprague Dawley rats, measurements of GTPase activity using [y-P32]GTP hydrolysis and GTP binding by [y-S35] GTPvS in secretory granules were performed. Results: l. AA inhibited a high affinity low Km and a low affinity high Km GTPase activity in the normal rat islet secretory granule in a concentration dependent manner. Half maximal inhibition was demonstrable at 90 pM AA concentration known to stimulate both phases of glucose-induced insulin secretion. 2. PGE2 stimulated a high affinity low Km and a low affinity high Km GTPase activity in the normal rat islet secretory granule in a concentration dependent manner and this effect was more prominent in the high affinity low Km GTPase. Half maximal stimulation was demonstrable at 0.8uM PGE2, a concentration known to inhibit both phases of glucose-induced insulin secretion from pure beta cell lines and pancreatic islets. 3. PGE2 as well as other inhibitors of insulin secretion (such as epinephrine or clonidine) also stimulated high and low Km GTPase activity and these effects on low Km GTPase were recovered by pertussis pretreatment. Of the five prostaglandins (PGF2, PGA2, PGD2, and PGB2), only PGE2. stiniulated GTPase activity significantly (p<0.05). 4. 12-HETE had no effecl on GTPase activity. 5. AA increased GTP binding significantly(p< 0.05), but 12-HETE and PGE, had no measurable effects on GTP binding. Conclusion: In secretory granules of normal rat pancreatic islet cells, AA might have their stimulatory effect on insulin secretion via inhibition of GTPase activity and increased GTP binding, whereas PGE might represent their action via stimulation of low Km GTPase activity.