Effects of halothane on electrophysiologic properties and cyclic adenosine 3',5'-monophosphate content in isolated guinea pig hearts

We studied the effects of halothane on the electrophysiologic and biochemical properties of both Langendorff perfused hearts and single ventricular myocytes isolated from guinea pigs. Isometric contractions of left venricles in perfused hearts, elicited by atrial pacing, decreased to 14% of control after exposure to 2% halothane-equilibrated perfusate. Subsequently the slow inward Ca²⁺ current (I(Ca)) was recorded in isolated myocytes with a whole cell voltage clamp technique. I(Ca), recorded in response to 100-ms depolarizations from -40 mV to 0 mV, was decreased by 2% halothane to 28.4% of control. Halothane-induced I(Ca) depression did not exhibit use dependency. To define a possible site at which halothane acts, we measured the cyclic adenosine 3',5'-monophosphate (cAMP) content of single ventricular myocytes using a radioimmunoassay. Two percent halothane decreased myocardial cAMP content to 68.9% of control. Further addition of dibutyryl cAMP (10^-3 mol/L) partially reversed the depressed contractility during 2% halothane administration in perfused hearts. In conclusion, the present study demonstrated that the decrease of myocardial cAMP by halothane was due to a direct action, at least partly, and not to other factors such as catecholamines, and suggested that the decreases in contractility and I(Ca) were induced possibly through the decrease in cellular cAMP.