CA2+ CURRENT AND CA2+ TRANSIENTS UNDER ACTION-POTENTIAL CLAMP IN GUINEA-PIG VENTRICULAR MYOCYTES

Precise characterization of the magnitude and kinetics of transsarcolemmal Ca2+ influx during an action potential (AP) is essential for a complete understanding of excitation-contraction coupling in heart. Using a voltage-clamp protocol that simulated a phyisiological AP (AP clamp), we characterized the properties of the Ca2+ current (I(Ca)) in guinea pig ventricular myocytes. The AP-generated I(Ca) showed a complex time course that was different from I(Ca) generated by a square pulse. I(Ca) activated rapidly during the upstroke of the AP and then partially inactivated during the plateau. The fast component of I(Ca) reached a peak value of -7.6 +/- 1.0 pA/pF at 2.40 +/- 0.30 ms after depolarization, followed by a slow component with a peak value of -2.9 +/- 0.4 pA/pF during the plateau. I(Ca) generated by an AP was composed of both L- and T-type Ca2+ channels. T-type Ca2+ current contributed to the fast component of I(Ca) and L-type Ca2+ current contributed to both fast and slow components of I(Ca). Activation of beta-adrenoceptors enhanced I(Ca) with a maximal effect lasting throughout the entire plateau of the AP. Measurements of cytosolic Ca2+ transients using fura-2 indicated that the I(Ca) was responsible for triggering Ca2+ release from the sarcoplasmic reticulum. The AP clamp provides a new approach for investigation of the relationship between I(Ca) and Ca2+ transients under more physiological conditions.