Simultaneous presence of cAMP and cGMP exert a co-ordinated inhibitory effect on the agonist-evoked Ca2+ signal in pancreatic acinar cells

Abstract

The stimulation of the pancreatic acinar cells by physiological secretagogues, such as acetycholine (ACh), activates a well-established intracellular signalling pathway, which involves the generation of Inositol 1,4,5-trisphosphate (InsP3) and the release of Ca2+ from intracellular stores. Caffeine, which inhibits this agonist-evoked Ca2+ response reversibly and competitively also blocks the Ca2+ signal generated by the non-specific activation of the membrane guanine nucleotide-binding proteins (G-proteins). Removal of caffeine is associated with an increase of intracellular [Ca2+] ([Ca2+]i) and the spatial and temporal characteristics of this Ca2+ signal are identical to those of the signal 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 inhibitors, chemically related (theophylline) or not (papaverine). Various protocols designed to increase the concentration of either of the major intracellular cyclic nucleotides [adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP)] failed to reproduce the full extent of the caffeine inhibition: at maximal agonist concentration (1 microM ACh) increases of either cAMP or cGMP did not affect the Ca2+ signal, whereas at submaximal doses of agonist (0.1-0.3 microM ACh) they induced partial inhibition. Here we show that only the simultaneous increase of the cellular concentrations of both cyclic nucleotides (either simultaneous or sequential) are effective in mimicking the blocking effect of caffeine and other non-specific PDE inhibitors. These data indicate, thus, that, in addition to other independent intracellular effects, cAMP and cGMP can exert a co-ordinated inhibitory effect of the agonist-evoked Ca2+ signal in pancreatic acinar cells.

Dual regulation of the ATP-sensitive potassium channel by caffeine

“…Inhibition of PDE leads to accumulation of intracellular cyclic nucleotides. It has been shown that inhibition of PDE mimics the effect of caffeine on the agonist-evoked Ca 2ϩ response in pancreatic acinar cells (12) and on the contractility of gallbladder muscle (46). In the present study, we showed that several nonselective as well as cGMP-or cAMP-specific PDE inhibitors were capable of reproducing the caffeine-induced enhancement of Kir6.2/SUR1 single-channel currents in intact HEK293 cells (Fig.…”

“…It has been suggested that simultaneous increases of intracellular cAMP and cGMP by caffeine inhibit the agonist-evoked intracellular Ca 2ϩ response in pancreatic acinar cells (12). We thus performed the following experiments to determine whether accumulation of cAMP by inhibition of cAMP-hydrolyzing PDE affects the K ATP channel function.…”

β-Adrenergic potentiation of endoplasmic reticulum Ca2+ release in brown fat cells

“…Sensitization of IP 3 -mediated Ca 2ϩ release by increases in cAMP is not unique to brown fat and has been reported in other cell types, including hepatocytes (5,12,31,34), pancreatic ␤-cells (19), articular chondrocytes (9), and parotid acinar cells (30). The effect of cAMP increases in brown fat on IP 3 -mediated Ca 2ϩ release contrasts with other cell types, such as megakaryocytes, pancreatic acinar cells, and nonvascular smooth muscle, in which cAMP inhibits IP 3 -mediated Ca 2ϩ release (1,7,35). Not all cells responding to 10 M phenylephrine with Ca 2ϩ increases also responded to isoproterenol with Ca 2ϩ increases.…”

Regulation of slowly activating potassium current (IKs) by secretin in rat pancreatic acinar cells

“…However, the concentration of secretin used in the AR4‐2J cell line (1 n m ) was much higher than the one used in our study (50 p m ). Moreover, another previous study indicates that the stimulation of the cAMP system partly suppress the ACh‐evoked [Ca 2+ ] i increase (Camello et al 1996). After excluding the above possibilities, it is supposed that the synergistic effect might occur at the channel protein level, and precise knowledge about the mechanism of I Ks modulation by Ca 2+ and by cAMP is necessary to understand the synergistic effect.…”