Abstract

Rat islets express several isoforms of adenylyl cyclase (AC), and the regulation of AC activity in isolated islets by Ca²⁺ and protein kinase C (PKC) was investigated. At basal 2.8 mmol/l glucose, the muscarinic receptor agonist carbamylcholine chloride (CCh) evoked a concentration-dependent increase in cAMP generation with a maximum increase at least 4.5-fold above control. In contrast, forskolin and glucagon-like peptide 1 fragment 7-36 amide increased cAMP accumulation 23-fold and almost 10-fold, respectively. Cholecystokinin 26-33 sulfated amide (CCK) also stimulated cAMP production by up to eightfold, as did the phorbol ester, phorbol 12,13-dibutyrate (PDBu). PDBu and CCh or CCK responses were not additive. The effects of phorbol ester, CCh, and CCK were inhibited by as much as 75% by the PKC inhibitors GF 109203X and Ro-32-0432 and after PKC downregulation. In the absence of extracellular Ca²⁺, PDBu-, CCh-, and CCK-induced cAMP production was inhibited by ∼50% in each case. Chelation of intracellular Ca²⁺ with 1,2-bis(o-amino-5-fluorophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetraacetoxymethyl ester (BAPTA/AM) inhibited CCh- and CCK-stimulated cAMP generation by ∼50% but did not inhibit the stimulatory effect of PDBu. Stringent Ca²⁺ depletion by removal of extracellular Ca²⁺ and inclusion of BAPTA/AM allowed for increased cAMP production in response to CCh and CCK; PKC inhibitors and PKC downregulation prevented this stimulation. Glucose stimulation also increased islet cAMP production, but PDBu did not potentiate the glucose response. The results suggest that Ca²⁺ influx, Ca²⁺ mobilization, and PKC activation play important roles in the modulation of AC activity in pancreatic islets.