Effects of action potential duration on excitation-contraction coupling in rat ventricular myocytes. Action potential voltage-clamp measurements
Bouchard, R.A.; Clark, R.B.; Giles, W.R.
Circulation Research 76(5): 790-801
ISSN/ISBN: 0009-7330 PMID: 7728996 Accession: 008557557
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 apprxeq -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 gm). 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.