INHIBITORS OF SPASMOGEN-INDUCED CA2-MUSCLE CELLS FROM SMALL-INTESTINE( CHANNEL SUPPRESSION IN SMOOTH)

1 Whole-cell patch-clamp recordings were made from smooth muscle cells isolated from the longitudinal muscle layer of guinea-pig ileum. Carb achol (acting at muscarinic receptors) or histamine (acting at H-1 his tamine receptors) suppressed Ca2+ channel current. The effect of eithe r agonist had an initial transient component followed by a sustained c omponent. 2 Wortmannin inhibited transient and sustained components of carbachol-induced Ca2+ channel current suppression: half-effective in hibitory concentrations (IC50) were 1.1 mu M and 0.6 mu M for the two components respectively. Wortmannin also inhibited the transient phase of carbachol-induced cationic current (IC50 1.6 mu M) and Ca2+-depend ent K+-current (IC50 1.7 mu M). Wortmannin did not appear to produce a ny direct block of cationic channels or Ca2+ channels. 3 Intracellular application of the phospholipase inhibitor D609 (tricyclodecan-9-ylxa nthogenate) inhibited transient and sustained components of histamine action on the Ca2+ channel current: the IC50 was about 130 mu M for bo th components. Carbachol action on Ca2+ channels was also inhibited by D609. D609 had no significant direct blocking effect on Ca2+ channels , cationic channels activated by carbachol, or Ca2+-activated K+-curre nt in response to flash-photolysis of caged-inositol 1,4,5-trisphospha te. 4 Micromolar concentrations of wortmannin and D609 are inhibitors of both components of spasmogen-induced Ca2+ channel suppression. The data suggest that both components are mediated by a common, or similar , signal transduction element which is a phospholipase C (PLC) or phos pholipase D (PLD) isoform.