CHOLINERGIC ACTIVATION OF A NONSELECTIVE CATION CURRENT IN CANINE GASTRIC SMOOTH-MUSCLE IS ASSOCIATED WITH CONTRACTION

1. The effects of acetylcholine on membrane electrical properties of single smooth muscle cells dissociated from the circular layer of the canine gastric corpus were investigated using the nystatin perforated patch technique. Cells retained their ability to contract during recording, making it possible to correlate changes in membrane potential and membrane currents with contractions. 2. Acetylcholine caused depolarization from -59 +/- 8 mV to -18 +/- 7 mV (means +/- S.D., n = 12) with no generation of action potentials. The depolarization was associated with a membrane conductance increase, consistent with acetylcholine activating an inward current. In addition, acetylcholine caused contraction of cells to 58% of initial length. Both depolarization and contraction were reversible and were antagonized by atropine. 3. Under voltage clamp, acetylcholine activated inward current associated with increased current noise. The current-voltage relationship of the acetylcholine-induced current was studied at steady-state voltages and with voltage ramp commands. The inward current was largest between -40 and -20 mV and reversed direction to outward close to 0 mV (reversal potential, E(rev) = +3 +/- 9 mV). Reduction of external Na+ concentration to 21 mM shifted E(rev) to -42 +/- 5 mV, as predicted for a non-selective cation current. The conductance activated by acetylcholine (g(ACh)) increased sigmoidally with depolarization, with about 2.5 nS activated at 0 mV. 4. Cells consistently contracted upon stimulation with acetylcholine, even when studied under voltage clamp at potentials as negative as -100 mV. This was consistent with muscarinic receptor activation causing release of Ca2+ from internal stores. When cells were bathed in Ca2+-free solutions, the first application of acetylcholine elicited normal inward current and contraction. Thereafter, both inward current and contractions were greatly diminished or absent, suggesting that the stores of Ca2+ had been depleted. 5. Caffeine caused reversible contraction and activation of inward current similar to that elicited by acetylcholine. 6. It is concluded that muscarinic stimulation of canine gastric smooth muscle cells involves activation of a non-selective cation conductance and is consistently accompanied by contraction. The release of Ca2+ from internal stores may be a common trigger for both events.