Effects of Action Potential Duration on Excitation-Contraction Coupling in Rat Ventricular Myocytes

Abstract

Abstract Although each of the fundamental processes involved in excitation-contraction coupling in mammalian heart has been identified, many quantitative details remain unclear. The initial goal of our experiments was to measure both the transmembrane Ca 2+ current, which triggers contraction, and the Ca 2+ extrusion due to Na + -Ca 2+ exchange in a single ventricular myocyte. An action potential waveform was used as the command for the voltage-clamp circuit, and the membrane potential, membrane current, [Ca 2+ ] i , and contraction (unloaded cell shortening) were monitored simultaneously. Ca 2+ -dependent membrane current during an action potential consists of two components: (1) Ca 2+ influx through L-type Ca 2+ channels (I Ca-L ) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to ≈−25 mV and continues during diastole. This slow inward tail current can be abolished completely by replacement of extracellular Na + with Li + , suggesting that it is due to electrogenic Na + -Ca 2+ exchange. In agreement with this, the net charge movement corresponding to the inward component of the Ca 2+ -dependent current (I Ca-L ) was approximately twice that during the slow inward tail current, a finding that is predicted by a scheme in which the Ca 2+ that enters during I Ca is extruded during diastole by a 3 Na + –1 Ca 2+ electrogenic exchanger. Action potential duration is known to be a significant inotropic variable, but the quantitative relation between changes in Ca 2+ current, action potential duration, and developed tension has not been described in a single myocyte. We used the action potential voltage-clamp technique on ventricular myocytes loaded with indo 1 or rhod 2, both Ca 2+ indicators, to study the relation between action potential duration, I Ca-L , and cell shortening (inotropic effect). A rapid change from a “short” to a “long” action potential command waveform resulted in an immediate decrease in peak I Ca-L and a marked slowing of its decline (inactivation). Prolongation of the action potential also resulted in slowly developing increases in the magnitude of Ca 2+ transients (145±2%) and unloaded cell shortening (4.0±0.4 to 7.6±0.4 μm). The time-dependent nature of these effects suggests that a change in Ca 2+ content (loading) of the sarcoplasmic reticulum is responsible. Measurement of [Ca 2+ ] i by use of rhod 2 showed that changes in the rate of rise of the [Ca 2+ ] i transient (which in rat ventricle is due to the rate of Ca 2+ release from the sarcoplasmic reticulum) were closely correlated with changes in the magnitude and the time course of I Ca-L . These findings demonstrate that Ca 2+ release from the sarcoplasmic reticulum can be modulated by the action potential waveform as a result of changes in I Ca-L .