Delayed rectifier k current in rabbit atrial myocytes . Muraki, K., Y. Imaizumi, M. Watanabe, Y. Habuchi, and W. R. Giles. Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences, Nagoya City University, Department of Laboratory Medicine, Kyoto Prefecture University, and Departments of Medical Physiology and Medicine, University of Calgary
APStracts 2:0127H, 1995.
The role of delayed rectifier K current(s) (IK) in rabbit left atrium were examined by applying the whole-cell voltage clamp technique to isolated single myocytes. Right-triangular waveforms which mimic the shape of atrial action potentials (APs) and selective blockers were used to compare the contribution of IK with other K currents to repolarization of the APs. IK measured at 34 oC in atrial myocytes was very small; the maximum peak amplitude of the tail current (IK -tail) at -40 mV was approx. 50 pA. The IK-tail was almost abolished in most cells (80 %) by the application of 1 [mu]M E4031, a class III anti-arrhythmic drug. The E4031-sensitive current (IE4031) recorded using the triangular command waveform showed strong inward rectification, and had a maximum amplitude of approximately 30 pA near -40 mV. Total outward current elicited by triangular command pulses depended strongly on stimulation frequency. The main frequency-dependent component was a Ca-independent transient K current, (It). It elicited by the triangular pulse at 1 Hz was substantially reduced by 4-aminopyridine (4-AP) at potentials positive to 0 mV, but was not changed significantly by E4031 (1 [mu]M); 100 [mu]M E4031 reduced It by approximately 30 %. The shape of the APs which were recorded from a single rabbit atrial cell strongly depended on the pulse frequency. Application of 1 [mu]M E4031 increased action potential duration (APD) in over 50 % of cells examined but had little effect on the resting membrane potential (RMP). Application of 0.1 mM BaCl2 also lengthened APD and reduced RMP by about 20 mV. These results suggest that rabbit left atrial myocytes have an E4031-sensitive IK which contributes to the late phase of AP repolarization at potentials between -10 and -70 mV. The 4-AP-sensitive It is the major outward current responsible for the early phase (positive to -10 mV) of AP repolarization even at 1-2 Hz. 

Received 16 August 1994; accepted in final form 7 March 1995.
APS Manuscript Number H735-4.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 April 1995.