Effects of halothane on membrane currents associated with contraction in single myocytes isolated from guinea-pig ventricle.

1. The effects of halothane on electrical activity and contraction were investigated in single myocytes isolated from guinea-pig ventricle. 2. Halothane depressed the plateau and shortened the duration of action potentials. 3. Halothane also reduced the amplitude of inward calcium currents and of additional inward current activated by cytosolic calcium under voltage-clamp conditions. 4. Contractions (measured by an optical technique) accompanying either action potentials or calcium currents were reduced by halothane. However, the extent of attenuation of contraction was greater than when a similar level of calcium channel blockade was induced by application of verapamil. 5. Actions of halothane on calcium-activated tail currents in double-pulse experiments were consistent with reduction by halothane of the cytosolic calcium transient, perhaps as a consequence of reduced uptake of calcium into sarcoplasmic reticulum stores. 6. It is concluded that the actions of halothane on inward currents contribute to its effects on action potentials. The reduction in contraction caused by halothane may result partly from a reduced influx of calcium to trigger contraction, and partly by a reduced release of calcium from sarcoplasmic reticulum stores.