Cholinergic inhibition of Ca2+ current in guinea‐pig gastric and tracheal smooth muscle cells.

1. Cholinergic regulation of L‐type Ca2+ channels was investigated in freshly dissociated guinea‐pig gastric and tracheal smooth muscle cells. Acetylcholine (ACh, 50 microM) decreased Ca2+ channel current (ICa) by 37 +/‐ 3% (mean +/‐ S.E.M., 46 cells). 2. ACh reduced ICa at all voltages, with no shift in the current‐voltage relationship. Effects of ACh were rapid (within 5 s) and repeatable, with multiple applications reproducibly inhibiting ICa in the continued presence of extracellular Ca2+ and in the presence of protein kinase C inhibitors. 3. The involvement of Ca2+ stores in this inhibition was investigated using Ca(2+)‐free solution or cyclopiazonic acid (CPA) to deplete the stores. ACh initially inhibited ICa in the Ca(2+)‐free solution (Na+ as charge carrier, 53 +/‐ 4% decrease, 18 cells) with subsequent responses significantly attenuated (n = 9). CPA (1 microM) reduced, then abolished, the effects of ACh on ICa (n = 5). 4. When studied in cell‐attached patches (Ba2+ as charge carrier), ACh reduced Ca2+ channel open probability in twenty‐two of thirty‐six cells, consistent with the involvement of a diffusible cytosolic messenger. 5. ACh also inhibited ICa in tracheal muscle cells (reduction of 38 +/‐ 6% in 1 mM Ca2+, 4 cells; 77 +/‐ 3% in Ca(2+)‐free solution, 7 cells). Furthermore, in cells where ACh elicited oscillating Ca(2+)‐activated Cl‐ current, oscillatory inhibition of ICa was also observed (3 cells). 6. In summary, ACh causes rapid and reversible inhibition of ICa in gastric and tracheal muscles. Ca2+ stores were required to initiate this effect, with the rapid onset and oscillatory inhibition consistent with Ca2+ inhibition of the channel. Suppression of ICa would reduce Ca2+ entry during cholinergic excitation.