Inhibitors of spasmogen-induced Ca2+ channel suppression in smooth muscle cells from small intestine

1. Whole-cell patch-clamp recordings were made from smooth muscle cells isolated from the longitudinal muscle layer of guinea-pig ileum. Carbachol (acting at muscarinic receptors) or histamine (acting at H₁ histamine receptors) suppressed Ca²⁺ channel current. The effect of either agonist had an initial transient component followed by a sustained component.
2. Wortmannin inhibited transient and sustained components of carbachol-induced Ca²⁺ channel current suppression: half-effective inhibitory concentrations (IC₅₀) were 1.1 μM and 0.6 μM for the two components respectively. Wortmannin also inhibited the transient phase of carbachol-induced cationic current (IC₅₀ 1.6 μM) and Ca²⁺-dependent K⁺-current (IC₅₀ 1.7 μM). Wortmannin did not appear to produce any direct block of cationic channels or Ca²⁺ channels.
3. Intracellular application of the phospholipase inhibitor D609 (tricyclodecan-9-ylxanthogenate) inhibited transient and sustained components of histamine action on the Ca²⁺ channel current: the IC₅₀ was about 130 μM for both 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⁺-current in response to flash-photolysis of caged-inositol 1,4,5-trisphosphate.
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 phospholipase D (PLD) isoform.