Received June 13, 2007
Revised July 13, 2007
Accepted after revision August 20, 2007

¹ University of Otago

^* To whom correspondence should be addressed. E-mail: chris.bolter{at}stonebow.otago.ac.nz.

	Abstract

Using Langendorff preparations of the guinea-pig heart, we have examined the participation of the acetylcholine (ACh)-activated potassium channel, IK,ACh, in the bradycardia produced by electrical stimulation of the vagus (parasympathetic) nerve and muscarinic agonists (ACh and bethanecol, bolus i.a.). Hearts from young animals (160-250 g), were perfused with Krebs-Henseleit solution, and pacemaker frequency was determined from the P-wave of an ECG. Tertiapin-Q was used to block IK,ACh. Vagal stimulation (10-s trains at 2, 5 and 10 Hz) produced graded reductions in atrial rate that were substantially attenuated, and to a similar extent, by 300 nM and 1 µM tertiapin-Q (to 0.42 ± 0.12, mean ± SD of the control values; P < 0.001). ACh (3 nmoles) produced brief graded bradycardias that were also attenuated by tertiapin-Q (0.24 ± 0.24); P = 0.006). Similar results were obtained when experiments were repeated in 2 mM Cs+ (to block If). Bethanecol (30, 50 and 70 nmoles), a muscarinic agonist with no appreciable nicotinic activity, produced sustained bradycardias that were attenuated by 300 nM tertiapin-Q (0.36 ± 0.21; P < 0.0001). The responses to vagal stimulation and ACh developed more slowly in tertiapin-Q, indicating that a rapidly acting mechanism had been blocked. Responses to vagal stimulation were faster in 2 mM Cs+. Together, these observations show that ACh released from parasympathetic nerve varicosities exerts a considerable part of its effect on the pacemaker by activating IK,ACh, and acts in a manner not readily distinguishable from that of directly applied muscarinic agonists.

Key Words: Acetylcholine, Sino-atrial node, Vagus