Mechanisms underlying the inotropic action of halothane on intact rat ventricular myocytes

The mechanisms contributing to the negative inotropic effect of halothane were studied in isolated rat ventricular myocytes. Contraction and intracellular Ca2+ transients were measured optically in these cells. The initial application of halothane (2% or 0.5 mmol litre(-1)) led to short-lived increases in the Ca2+ transient and contraction, which were abolished by ryanodine. Continued application of halothane led to a sustained decrease in contraction: this resulted from: (i) a decrease in myofilament Ca2+ sensitivity; (ii) a decrease in the Ca2+ transient; and (iii) a decrease in the Ca2+ content of the sarcoplasmic reticulum. Although halothane reduced action potential duration, the sustained negative inotropic effect was similar when action potentials or voltage clamp pulses of constant duration were used to trigger contractions. In cells exposed to nifedipine 0.5 mu mol litre(-1) (which decreases the L-type Ca2+ current, I-Ca), Ca2+ transients, sarcoplasmic reticulum Ca2+ content and fractional release (the fraction of sarcoplasmic reticulum Ca2+ content released during each stimulus) were reduced. Halothane 0.5 mmol litre(-1) (which also decreases Ic,) decreased Ca2+ transients to a lesser extent and reduced sarcoplasmic reticulum Ca2+ content to a greater extent than nifedipine, whereas fractional release was unchanged compared with control. These data suggest that halothane sensitizes Ca2+-induced Ca2+ release from the sarcoplasmic reticulum in addition to reducing I-Ca.