Ca2%2b current and Ca2%2b transients under action potential clamp in guinea pig ventricular myocytes

Precise characterization of the magnitude and kinetics of transsarcolemmal Ca2%2b 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 physiological AP (AP clamp), we characterized the properties of the Ca2%2b 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%2b channels. T-type Ca2%2b current contributed to the fast component of I(Ca) and L-type Ca2%2b current contributed to both fast and slow components of I(Ca). Activation of β-adrenoceptors enhanced I(Ca) with a maximal effect lasting throughout the entire plateau of the AP. Measurements of cytosolic Ca2%2b transients using fura-2 indicated that the I(Ca) was responsible for triggering Ca2%2b release from the sarcoplasmic reticulum. The AP clamp provides a new approach for investigation of the relationship between I(Ca) and Ca2%2b transients under more physiological conditions.

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