To investigate cAMP-dependent regulation of somatostatin secretion and gene expression in the islets of Langerhans, we have correlated the effects of forskolin, theophylline, and (Bu)2cAMP (dbcAMP) on the secretion of somatostatin-like immunoreactivity (SLI), cAMP generation, and somatostatin mRNA (S-mRNA) accumulation by cultured rat islet cells and a rat somatostatin-producing islet tumor cell line (1027 B2). Additionally, we have compared these effects with those of phorbol esters. Forskolin induced large acute increases in cAMP levels in islet cells, whereas theophylline produced modest sustained elevations in cAMP. During 4-h exposure to islet cells, forskolin, theophylline, and dbcAMP produced time- and dose-related increases of up to 14-fold in SLI release and up to 5-fold in S-mRNA levels. The rate of increase in S-mRNA paralleled secretion and occurred with the following order of potency: forskolin > dbcAMP > theophylline. The analog 1,9-dideoxyforskolin, which is unable to activate adenylyl cyclase, produced a small increase in SLI release without affecting S-mRNA. The effects of short term increases in islet cAMP levels and SLI release on long term changes in S-mRNA accumulation were investigated in a 48-h study with forskolin. Pretreatment of islet cells for 30 min with forskolin evoked large acute increases in cAMP levels and SLI release. S-mRNA rose in a biphasic pattern, with an acute increase at 30 min followed by a secondary increase at 12-48 h. In 1027B2 cells, forskolin and theophylline generated large increases in cAMP levels. Despite this, the two agents as well as dbcAMP produced only slight (20-35%) stimulation of SLI release and S-mRNA accumulation. Phorbol 12-myristate 13-acetate and phorbol 12,13-dibutyrate evoked dose-dependent stimulation of SLI secretion of up to 4-fold from islet cells without altering S-mRNA. Both secretion and S-mRNA were unresponsive to phorbol esters in 1027 B2 cells. We conclude that 1) cAMP-dependent mechanisms regulate somatostatin secretion and gene transcription in normal islet cells; 2) acute increases in intracellular cAMP levels lead to immediate stimulation of secretion associated with both acute and long term increases in S-mRNA production; 3) the higher potency of forskolin compared to that of dbcAMP and theophylline suggests that this agent activates S-mRNA accumulation and SLI release through both cAMP-independent as well as cAMP-dependent mechanisms; 4) 1027 B2 cells lack cAMP-regulated somatostatin secretion and gene transcription due to a post-cAMP defect; and 5) phorbol esters stimulate SLI secretion from normal islet cells, but not from 1027 B2 cells, and are without effect on S-mRNA in either normal or neoplastic islet D-cells.
