Effect of ca2+ loading on response of depolarized guinea pig myocytes to critically timed premature stimulations. Nordin, Charles. Department of Medicine, Albert Einstein College of Medicine, Bronx, New York
APStracts 2:0377H, 1995.
Single premature stimulations during trains of nondriven action potentials induced by depolarization normally cause a transient hyperpolarization of diastolic membrane potential prior to the subsequent spontaneous upstroke. However, rare, marked transient depolarizations have also been reported. This paper presents experimental data and computer simulations which characterize transient depolarization following premature stimulations and investigate the role of intracellular [Ca2+] in generating this unusual response. In experiments with isolated guinea pig myocytes, transient depolarizations (range 4-58 mV) consistently occurred if stimulations were delivered between 100 and 160 msec following the upstroke of spontaneous action potentials during exposure to K+ free Tyrode's solution, which raises intracellular [Ca2+]. In contrast, no transient depolarizations developed when stimulations were delivered during injection of constant inward current or brief exposure to very low dose Ba2+ (250-500 NM). The experimental response to K+ free Tyrode's solution was reproduced by a computer model of the transmembrane current and intracellular Ca2+ flux of an isolated guinea pig ventricular myocyte (24) following reduction of extracellular [K+] below 1 mM. Transient depolarization was generated primarily by Na-Ca exchange. Simulations using only those equations governing intracellular Ca2+ cycling revealed that bursts of Ca2+ into the myoplasm after Ca2+ loading caused a transient increase in trough myoplasmic [Ca2+] when the coupling interval following the upstroke of a myoplasmic [Ca2+] oscillation was nearly identical to those coupling intervals which caused pacing-induced transient depolarization of membrane potential following the upstroke of an action potential. These results suggest that transient depolarizations following nondriven action potentials arise from critically timed, stimulus-induced perturbation of intracellular [Ca2+] oscillations associated with Ca2+ overload. Simulations using a multicellular model suggest that critically timed premature stimulations can initiate trains of depolarized, nondriven action potentials in otherwise quiescent, Ca2+ overloaded heterogeneous syncytia by a similar mechanism. 

Received 21 March 1995; accepted in final form 1 August 1995.
APS Manuscript Number H269-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 15 September 1995.