Effects of acetylcholine on potassium movements in the guinea-pig sinus node.

Factors influencing the effect of acetylcholine (ACh) on potassium movements in the sinus node have been studied using tracer potassium (42K) and a microelectrode technique. The following results were obtained. ACh has qualitatively different effects on 42K uptake depending upon concentration, i.e., small concentrations (approximately equal to 10(-9) M) of ACh may decrease and larger concentrations (greater than or equal to 10(-7) M) increase 42K uptake. The ACh induced increase in 42K uptake is slightly but significantly greater when the rate of discharge is maintained constant by electrical drive than when the rate is slowed by ACh as quantitated by transmembrane recordings. In the presence of atropine, the ACh-induced increase in 42K uptake is abolished and replaced by a small but consistent decrease. Blockage by nicotine enhances the ACh-induced increase in 42K uptake and blockade by d-tubocurarine diminishes it. The effect of ACh on 42K uptake is greater in high (5.4 mM) than in low (0.54 mM) [Ca]0. These results suggest the conclusion that ACh acts upon a muscarinic recpetor to increase and on a nicotinic receptor to decrease potassium uptake. The net effect depends upon the concentration of ACh. The action of ACh on potassium uptake is modulated by [Ca]0.