Regulation of contraction and relaxation by membrane potential in cardiac ventricular myocytes

Control of contraction and relaxation by membrane potential was investigated in voltage-clamped guinea pig ventricular myocytes at 37°C. Depolarization initiated phasic contractions, followed by sustained contractions that relaxed with repolarization. Corresponding Ca²⁺ transients were observed with fura 2. Sustained responses were ryanodine sensitive and exhibited sigmoidal activation and deactivation relations, with half-maximal voltages near -46 mV, which is characteristic of the voltage- sensitive release mechanism (VSRM) for sarcoplasmic reticulum Ca²⁺. Inactivation was not detected. Sustained responses were insensitive to inactivation or block of L-type Ca²⁺ current (I(Ca-L)). The voltage dependence of sustained responses was not affected by changes in intracellular or extracellular Na⁺ concentration. Furthermore, sustained responses were not inhibited by 2 mM Ni²⁺. Thus it is improbable that I(Ca- L) or Na⁺/Ca²⁺ exchange generated these sustained responses. However, rapid application of 200 μM tetracaine, which blocks the VSRM, strongly inhibited sustained contractions. Our study indicates that the VSRM includes both a phasic inactivating and a sustained noninactivating component. The sustained component contributes both to initiation and relaxation of contraction.