Cholinergic Short-term Conditioning and Activation of ATP-Sensitive K+Current in Cat Atrial Myocytes

In atrial myocytes, an initial exposure to acetylcholine (ACh1) exerts a short-term conditioning effect such that a second ACh exposure (ACh2) activates ATP-sensitive K+current (IK,ATP). The purpose of the present study was to determine the mechanism underlying the short-term conditioning induced by ACh that results in subsequent ACh-induced activation of IK,ATP. Cat atrial myocytes were studied using a nystatin-perforated patch whole cell recording method. Changes in -type Ca2+current (ICa, L) amplitude were used as an index of relative changes in cyclic AMP (cAMP). The results show that when atrial myocytes are treated with two consecutive exposures to 10μ ACh separated by a recovery interval, ACh2activates a larger increase in potassium conductance (gK+) than ACh1. The additional ACh2-induced increase in gK+is selectively blocked by 10μ glibenclamide, identifying the current as IK,ATP. Moreover, ICa,Lactivated immediately after the withdrawal of ACh1exhibited a transient increase in amplitude above control (+76%), consistent with rebound stimulation of cAMP. Rp-cAMPs (50μ ), a selective antagonist of cAMP-dependent protein kinase A, blocked the rebound stimulation of ICa,Land abolished ACh2-induced activation of IK,ATP. Thapsigargin (5μ ), an inhibitor of Ca2+ATPase in the sarcoplasmic reticulum (SR), abolished ACh2-induced activation of IK,ATPwithout decreasing rebound stimulation of ICa,L. Rebound stimulation of ICa,Land ACh2-induced activation of IK,ATPboth varied as a function of ACh1duration. We conclude that withdrawal of an initial ACh exposure elicits a rebound cAMP-mediated stimulation of SR Ca2+uptake. This mechanism induces a short-term conditioning in atrial myocytes such that a subsequent ACh exposure activates IK,ATP. The present results demonstrate novel cholinergic signaling mechanisms in the regulation of IK,ATP.