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

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 Ca2+ current, which triggers contraction, and the Ca2+ extrusion due to Na(+)-Ca2+ 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, [Ca2+]i, and contraction (unloaded cell shortening) were monitored simultaneously. Ca(2+)-dependent membrane current during an action potential consists of two components: (1) Ca2+ influx through L-type Ca2+ channels (ICa-L) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to approximately -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(+)-Ca2+ exchange. In agreement with this, the net charge movement corresponding to the inward component of the Ca(2+)-dependent current (ICa-L) was approximately twice that during the slow inward tail current, a finding that is predicted by a scheme in which the Ca2+ that enters during ICa is extruded during diastole by a 3 Na(+)-1 Ca2+ electrogenic exchanger. Action potential duration is known to be a significant inotropic variable, but the quantitative relation between changes in Ca2+ 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 Ca2+ indicators, to study the relation between action potential duration, ICa-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 ICa-L and a marked slowing of its decline (inactivation). Prolongation of the action potential also resulted in slowly developing increases in the magnitude of Ca2+ transients (145 +/- 2%) and unloaded cell shortening (4.0 +/- 0.4 to 7.6 +/- 0.4 microns). The time-dependent nature of these effects suggests that a change in Ca2+ content (loading) of the sarcoplasmic reticulum is responsible. Measurement of [Ca2+]i by use of rhod 2 showed that changes in the rate of rise of the [Ca2+]i transient (which in rat ventricle is due to the rate of Ca2+ release from the sarcoplasmic reticulum) were closely correlated with changes in the magnitude and the time course of ICa-L.(ABSTRACT TRUNCATED AT 400 WORDS)

Myocardial infarction in rat eliminates regional heterogeneity of AP profiles,ItoK+currents, and [Ca2+]itransients

“…The underlying basis for the regional differences in contraction is currently unknown but may be related to a heterogeneous transmural expression of Ca 2ϩ handling proteins (26, 31). Alternatively, APD might also play a key role because the action potential profile is an important determinant of the inotropic state of the heart in both normal (7,42,54) and hypertrophied (10, 30) rat ventricular myocytes.In this study, we examined the regional changes in APD, I to density, and [Ca 2ϩ ] i transient magnitude and MI and correlated these differences with the expression of K ϩ channel genes encoding for I to (i.e., Kv1.4, Kv4.2, and Kv4.3). Our results show that gradients in APD, I to density, Kv4.2 expression, and peak [Ca 2ϩ ] i exist between the right ventricular free wall (RVW) and interventricular septum (SEP), and these differences are largely eliminated after myocardial infarction (MI).…”

“…The underlying basis for the regional differences in contraction is currently unknown but may be related to a heterogeneous transmural expression of Ca 2ϩ handling proteins (26, 31). Alternatively, APD might also play a key role because the action potential profile is an important determinant of the inotropic state of the heart in both normal (7,42,54) and hypertrophied (10, 30) rat ventricular myocytes.…”

Mechanisms of the negative inotropic effects of sphingosine-1-phosphate on adult mouse ventricular myocytes

“…It is well established that S1P can shorten APD in mammalian atrial myocytes (43). Furthermore, it is also known that shortening APD in rat ventricular myocytes can reduce [Ca 2ϩ ] i and decrease cell shortening (8). In an attempt to relate these experimental findings to our experimental results, we have utilized a mathematical model of the mouse ventricular myocyte AP and [Ca 2ϩ ] i homeostatis (7).…”

“…11A). These ranges of APD were used because they approximated experimental data, which has been shown to affect calcium (8) and to be involved with S1P application (43), respectively. We first established that a single voltage command or a train of 10 voltage commands showed no time-dependent changes in either APD, I CaL , or [Ca 2ϩ ] i (data not shown).…”

“…In ventricular myocytes, contraction strength is modulated by changes in I CaL or other maneuvers, which alter amplitude or duration of the Ca 2ϩ transient (3,8). We, therefore, explored the relationship between APD and I CaL .…”