STIMULATION BY PROSTAGLANDIN-E(2) OF A HIGH-AFFINITY GTPASE IN THE SECRETORY GRANULES OF NORMAL RAT AND HUMAN PANCREATIC-ISLETS

Recent reports of a pertussis-toxin (Ptx)-sensitive inhibition of glucose-induced insulin release by prostaglandin E(2) (PGE(2)) in transformed beta-cells prompted us to look for the presence of prostaglandin-regulatable GTP-binding proteins (G-proteins) on the secretory granules of normal pancreatic islets. PGE(2) (but not PGF(2 alpha), PGA(2), PGB(2) or PGD(2)) stimulated in a concentration-dependent manner a high-affinity GTPase activity in the secretory-granule-enriched fractions of both normal rat and human islets. Similar results were found after sucrose-density-gradient-centrifugation-based isolation of secretory granules to those after a differential-centrifugation procedure. Half-maximal stimulation occurred at 800 nM PGE(2), a concentration known to inhibit both phases of glucose-induced insulin secretion from pure beta-cell lines. The GTPase stimulatory effect of PGE(2) was blocked virtually totally by Ptx pretreatment; it was not due to an effect on substrate binding since no measurable effect of PGE(2) on binding of guanosine 5'-[gamma-[S-35]thio]triphosphate was observed in cognate fractions. Other Ptx-sensitive inhibitors of insulin secretion (such as adrenaline or clonidine) also stimulated GTPase activity, suggesting that one (or more) inhibitory exocytotic G-proteins (i.e. a putative G(E1)) is located on the secretory granules. These studies demonstrate, for the first time in an endocrine gland, the presence of a regulatable G-protein, strategically located on the secretory granules where it might regulate the exocytotic cascade distal to both plasma-membrane events and the generation of soluble mediators of insulin secretion.