Differences in the myocardial depressant action of thiopental and halothane.

The negative inotropic effects of thiopental (10-30 mg/L) and halothane (0.5-1.5%) were compared in rabbit papillary muscles under various stimulation conditions to gain insight into the action of these anesthetics on the availability of Ca2+ for the activation of myocardial contractile activity. The negative inotropic effect of thiopental was more pronounced at short (0.5 sec) than at long (1 sec) beat-to-beat intervals under steady-state conditions, and thiopental's effect on potentiated state contractions was less than that on steady-state contractions. For all variables studied, the effect of halothane was opposite that of thiopental. These results suggest that thiopental reduces the influx of extracellular Ca2+ and the amount of Ca2+ in sarcolemmal sites and slows the transport of intracellular Ca2+ within the sarcoplasmic reticulum from sites of uptake to sites of release without markedly diminishing the amount of intracellular Ca2+. Halothane does not appreciably affect the transport but does diminish the amount of Ca2+ within the sarcoplasmic reticulum that is available for the activation of myocardial contractile activity.