It may now be possible to identify certain intracellular events that impact specifically on secretion-granule fusion to the plasma membrane or on granule lysis. Secretion vesicles in isolated rat islets appear to translocate somatostatin (SRIF) receptors from the Golgi apparatus to the plasma membrane. We have proposed that secretion granule fusion to the plasma membrane can be determined by measuring recruitment of SRIF receptors to the surface membrane. Granule lysis can be assessed by measuring insulin release. To activate cyclic AMP (cAMP)-dependent pathways, we employed isobutylmethylxanthine (IBMX, 400 μM), glucagon (10 μM), and forskolin (20 μM), a diterpene activator of adenylate cyclase. These agents evoked rapid release of insulin (from 0.41 ± 0.02 to 1.88 ± 0.02; 0.41 ± 0.02 to 1.93 ± 0.08; and 0.41 ± 0.02 to 1.66 ± 0.03 μU/islet/ min, respectively, P < 0.001). There was no concomitant recruitment of SRIF receptors. Somatostatin (10 μg/ml), which inhibits cAMP-stimulated protein phosphorylation, suppresses insulin release evoked by IBMX, glucagon, or forskolin (inhibition: 80, 75, or 82%, respectively). In contrast, trifluoperazine (10 μM), an inhibitor of calmodulin, did not suppress insulin release induced through cAMP-dependent pathways. Trifluoperazine suppresses glucose-induced insulin release and the recruitment of SRIF receptors to the surface membrane, suggesting the possible role of calmodulin in promoting secretion-granule fusion with the plasma membrane.
These data suggest that granule fusion to the plasma membrane may be a calmodulin-directed function. In contrast, the major site of action of CAMP in polypeptide hormone secretion may be in promoting lysis of secretion granules and associated hormone discharge.
