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 physiological AP (AP clamp), we characterized the properties of the Ca2+ current (ICa) in guinea pig ventricular myocytes. The AP-generated ICa showed a complex time course that was different from ICa generated by a square pulse. ICa activated rapidly during the upstroke of the AP and then partially inactivated during the plateau. The fast component of ICa 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. ICa generated by an AP was composed of both L- and T-type Ca2+ channels. T-type Ca2+ current contributed to the fast component of ICa and L-type Ca2+ current contributed to both fast and slow components of ICa. Activation of beta-adrenoceptors enhanced ICa with a maximal effect lasting throughout the entire plateau of the AP. Measurements of cytosolic Ca2+ transients using fura-2 indicated that the ICa was responsible for triggering Ca2+ release from the sarcoplasmic reticulum. The AP clamp provides a new approach for investigation of the relationship between ICa and Ca2+ transients under more physiological conditions.
Arreola
J
J
Department of Pharmacology, School of Medicine and Dentistry, University of Rochester, New York 14642.
Dirksen
R T
RT
Shieh
R C
RC
Williford
D J
DJ
Sheu
S S
SS
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000502:physiology*; D015220:Calcium Channels / Q000502:physiology; D004553:Electric Conductivity; D004594:Electrophysiology; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D007545:Isoproterenol / Q000494:pharmacology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D011943:Receptors, Adrenergic, beta / Q000502:physiology
D015220:Calcium Channels; D011943:Receptors, Adrenergic, beta; D007545:Isoproterenol; D002118:Calcium