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 gluc ose-induced insulin release by prostaglandin E(2) (PGE(2)) in transfor med beta-cells prompted us to look for the presence of prostaglandin-r egulatable GTP-binding proteins (G-proteins) on the secretory granules of normal pancreatic islets. PGE(2) (but not PGF(2 alpha), PGA(2), PG B(2) or PGD(2)) stimulated in a concentration-dependent manner a high- affinity GTPase activity in the secretory-granule-enriched fractions o f both normal rat and human islets. Similar results were found after s ucrose-density-gradient-centrifugation-based isolation of secretory gr anules to those after a differential-centrifugation procedure. Half-ma ximal stimulation occurred at 800 nM PGE(2), a concentration known to inhibit both phases of glucose-induced insulin secretion from pure bet a-cell lines. The GTPase stimulatory effect of PGE(2) was blocked virt ually totally by Ptx pretreatment; it was not due to an effect on subs trate binding since no measurable effect of PGE(2) on binding of guano sine 5'-[gamma-[S-35]thio]triphosphate was observed in cognate fractio ns. Other Ptx-sensitive inhibitors of insulin secretion (such as adren aline or clonidine) also stimulated GTPase activity, suggesting that o ne (or more) inhibitory exocytotic G-proteins (i.e. a putative G(E1)) is located on the secretory granules. These studies demonstrate, for t he first time in an endocrine gland, the presence of a regulatable G-p rotein, 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.