Evidence for a nicotinic receptor in the sinus node.

The sinus node of the guinea pig was perfused in vitro and potassium uptake was studied in the absence and in the presence of several nicotinic agonists and antagonists. The following results were obtained: (1) the decrease in K uptake caused by a low (10(-9)M) concentration of acetylcholine was changed to an increase when acetylcholine was given in the presence of nicotinic blockade; (2) a small concentration (10(-10)M) of atropine potentiated the decrease in K uptake induced by a low concentration of acetylcholine and 10(-8)M atropine abolished it: (3) a small concentration of nicotine (10(-10)M) decreased K uptake; (4) 1,1-dimethyl-4-phenylpiperazinium (DMPP) did not modify K uptake; (5) the lack of effect of DMPP was not affected by hexamethonium; (6) trimethylphenylammonium (TMPA) decreased K uptake; and (7) the inhibitory effect of TMPA was abolished by decamethonium. It is concluded that a nicotinic receptor in the sinus node mediates the inhibitory action of a low concentration of acetylcholine on K uptake.