Effect of halothane on cardiac sarcoplasmic reticulum Ca2+-ATPase at low calcium concentrations.

The effect of halothane on cardiac sarcoplasmic reticulum Ca2+-ATPase activity was studied at low calcium concentrations (0.4-20 microM). Clinical concentrations of halothane (1%-3%, v/v) were found to depress Ca2+-ATPase activity more severely at lower calcium levels than at the higher calcium levels previously reported (greater than 0.1 mM). An increase in calcium concentration in the external medium of a preparation of isolated cardiac sarcoplasmic reticulum vesicles antagonized the halothane-induced depression of the Ca2+-ATPase activity. The depression of calcium-activated ATPase activity by halothane appears to take place by a competitive-type inhibition. The Ca2+-ATPase Vmax remained constant at 0.175 mumole/min/mg of protein with an increasing Km (0.47 microM-4.09 microM). Halothane depression of sarcoplasmic reticulum function may in part explain the ability of halothane to depress myocardial function.