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Department of Pharmacology and Clinical Pharmacology, St George's Hospital Medical School, London.

1. Single smooth muscle cells were isolated from the longitudinal muscle layer of the guinea-pig ileum and within 10 h Ca(2+)-currents (ICa) were recorded using the whole-cell patch clamp technique. 2. Histamine (10 microMs) and bradykinin (BK, 1 microM) suppressed ICa; the effect had two phases: a rapid and transient suppression of ICa followed by a sustained suppression. Acetylcholine and substance P appeared to have similar effects but these were not investigated in detail. 3. The effects of histamine and BK on ICa were established by high intracellular concentrations of the Ca2+ buffer EGTA (30 mM) or 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) (5 mM) in the absence of Ca2+ added to the pipette solution. When [Ca2+]i was strongly buffered to 125 or 190 nM by BAPTA-Ca2+ mixtures in the pipette the transient suppression of ICa was blocked but the sustained effect still occurred. This indicated that the transient effect was caused by a rise in [Ca2+]i. The sustained effect, in contrast, did not seem to be caused by a rise in [Ca2+]i but did show Ca2+ dependence because it did not occur if [Ca2+]i was abnormally low. 4. Application of caffeine (10 mM) to deplete stored Ca2+ or intracellular heparin (1 mM) to block the action of D-myo-inositol 1,4,5-trisphosphate (IP3) to release stored Ca2+ prevented the transient but not the sustained suppression of ICa. Heparin also blocked the transient Ca(2+)-activated K+ current in response to histamine or BK. Both transient and sustained suppressions of Ca2+ channel activity were observed in the absence of extracellular Ca2+ when current was carried mostly by Na+ ions. 5. Intracellular guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 10 or 100 microM) induced a gradual decline of ICa upon which transient decreases of current were superimposed. Histamine caused a larger than normal inhibition of ICa and no recovery occurred on wash-out. Intracellular guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S; 1 mM) abolished the effects of histamine and BK on ICa.(ABSTRACT TRUNCATED AT 400 WORDS)

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