SIMULTANEOUS PRESENCE OF CAMP AND CGMP EXERT A COORDINATED INHIBITORYEFFECT ON THE AGONIST-EVOKED CA2-CELLS( SIGNAL IN PANCREATIC ACINAR)

The stimulation of the pancreatic acinar cells by physiological secret agogues, such as acetycholine (ACh), activates a well-established intr acellular signalling pathway, which involves the generation of Inosito l 1,4,5-trisphosphate (InsP(3)) and the release of Ca2+ from intracell ular stores. Caffeine, which inhibits this agonist-evoked Ca2+ respons e reversibly and competitively also blocks the Ca2+ signal generated b y the non-specific activation of the membrane guanine nucleotide-bindi ng proteins (G-proteins). Removal of caffeine is associated with an in crease of intracellular [Ca2+] ([Ca2+](i)) and the spatial and tempora l characteristics of this Ca2+ signal are identical to those of the si gnal generated by the initial agonist stimulation. Caffeine is also a potent non-specific inhibitor of various cellular phosphodiesterases ( PDE) and its inhibitory effect can be reproduced by other PDE inhibito rs, chemically related (theophylline) or not (papaverine). Various pro tocols designed to increase the concentration of either of the major i ntracellular cyclic nucleotides [adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP)] failed to repr oduce the full extent of the caffeine inhibition: at maximal agonist c oncentration (1 mu M ACh) increases of either cAMP or cGMP did not aff ect the Ca2+ signal, whereas at submaximal doses of agonist (0.1-0.3 m u M ACh) they induced partial inhibition. Here we show that only the s imultaneous increase of the cellular concentrations of both cyclic nuc leotides (either simultaneous or sequential) are effective in mimickin g the blocking effect of caffeine and other non-specific PDE inhibitor s. These data indicate, thus, that, in addition to other independent i ntracellular effects, cAMP and cGMP can exert a co-ordinated inhibitor y effect of the agonist-evoked Ca2+ signal in pancreatic acinar cells.