INHIBITION OF IP(3) AND IP(3)-DEPENDENT CA2+ MOBILIZATION BY CYCLIC-NUCLEOTIDES IN ISOLATED GASTRIC MUSCLE-CELLS

The mechanisms by which cAMP and cGMP and agents that stimulate one (isoproterenol and nitroprusside) or both cyclic nucleotides (VIP) decrease cytosolic free Ca2+ ([Ca2+]i) and inhibit contraction were examined in dispersed, intact, and saponin-permeabilized gastric muscle cells. In these cells, the [Ca2+]i transient responsible for initial contraction is mediated by inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ release (K. N. Bitar, P. G. Bradford, J. W. Putney, Jr., and G. M. Makhlouf. Science Wash. DC 232: 1143-1145, 1986, and J. Biol. Chem. 261: 16591-16596, 1986). In intact muscle cells, dibutyryl cAMP and all three relaxant agents inhibited contraction, [Ca2+]i, and net Ca2+ efflux (i.e., Ca2+ release) in a concentration-dependent fashion. In permeabilized muscle cells, cAMP, cGMP, and all three relaxant agents 1) inhibited cholecystokinin (CCK)-induced IP3 production (maximal 38-48%), 2) inhibited CCK- and IP3-induced Ca2+ efflux (maximal 55-59%) and contraction (maximal 59-66%), and 3) stimulated Ca2+ uptake (maximal 25-30%), in a concentration-dependent fashion. cAMP and cGMP were equipotent inhibitors Of IP3 production and of CCK- and IP3-induced Ca2+ efflux and contraction, whereas cGMP was distinctly more potent as a stimulant of Ca2+ uptake. For all functions, maximal effects induced by cAMP and cGMP were similar to those induced by the three relaxant agents. Inhibition of Ca2+ release was the main determinant of inhibition of contraction; stimulation of Ca2+ uptake was relatively minor (<5% of Ca2+ efflux). Decrease in IP3 production did not contribute to inhibition of Ca2+ efflux and contraction since inhibition of IP3-induced Ca2+ efflux was similar to inhibition of CCK-induced IP3-dependent Ca2+ efflux. IP3's lack of effect implied considerable spareness in IP3's response to CCK-8. We conclude that when stimulus is maximal, inhibition of initial contraction by cyclic nucleotides is mediated by inhibition of IP3-dependent Ca2+ release.