CHOLINERGIC INHIBITION OF CA2-PIG GASTRIC AND TRACHEAL SMOOTH-MUSCLE CELLS( CURRENT IN GUINEA)

1. Cholinergic regulation of L-type Ca2+ channels was investigated in freshly dissociated guinea-pig gastric and tracheal smooth muscle cell s. Acetylcholine (ACh, 50 mu M) decreased Ca2+ channel current (I-Ca) by 37 +/- 3% (mean +/- S.E.M., 46 cells). 2. ACh reduced I-Ca at all v oltages, with no shift in the current-voltage relationship. Effects of ACh were rapid (within 5 s) and repeatable, with multiple application s reproducibly inhibiting I-Ca in the continued presence of extracellu lar Ca2+ and in the presence of protein kinase C inhibitors. 3. The in volvement of Ca2+ stores in this inhibition was investigated using Ca2 +-free solution or cyclopiazonic acid (CPA) to deplete the stores. ACh initially inhibited I-Ca in Ca2+-free solution (Na+ as charge carrier , 53 +/- 4% decrease, 18 cells) with subsequent responses significantl y attenuated (n = 9). CPA (1 mu M) reduced, then abolished, the effect s of ACh on I-Ca (n = 5). 4. When studied in cell-attached patches (Ba 2+ as charge carrier), ACh reduced Ca2+ channel open probability in tw enty-two of thirty-six cells, consistent with the involvement of a dif fusible cytosolic messenger. 5. ACh also inhibited I-Ca in tracheal mu scle cells (reduction of 38 +/- 6% in 1 mM Ca2+, 4 cells; 77 +/- 3% in Ca2+-free solution, 7 cells). Furthermore, in cells where ACh elicite d oscillating Ca2+-activated Cl- current, oscillatory inhibition of I- Ca was also observed (3 cells). 6. In summary ACh causes rapid and rev ersible inhibition of I-Ca in gastric and tracheal muscles. Ca2+ store s were required to initiate this effect, with the rapid onset and osci llatory inhibition consistent with Ca2+ inhibition of the channel. Sup pression of I-Ca would reduce Ca2+ entry during cholinergic excitation .