Roles of beta 1- and beta 2-adrenoceptors in the mechanism of halothane myocardial sensitization in dogs.

The authors investigated the comparative roles of beta 1- and beta 2-adrenoceptors in myocardial sensitization by halothane in dogs. The arrhythmogenic dose (AD) of isoproterenol was determined in the presence of various doses of phenylephrine during halothane anesthesia in dogs, and the influences of 1-metoprolol (beta 1-antagonist) and ICI-118,551 (beta 2-antagonist) on the AD were examined. In the presence of 1-metoprolol, the AD of isoproterenol was significantly greater than the control, but in the presence of ICI-118,551, the AD of isoproterenol was lower. Blood pressure during the arrhythmias was higher in the presence of ICI-118,551 than that in controls. In addition, the AD of ritodrine (beta 2-agonist) was also determined at various doses of phenylephrine. The interaction between phenylephrine and ritodrine in inducing arrhythmias showed hyperbolic isoboles. However, 1-metoprolol completely inhibited the occurrence of arrhythmias induced by ritodrine and phenylephrine. The results suggest that myocardial beta 1-adrenoceptors play an essential role in the genesis of arrhythmias during halothane anesthesia in dogs, whereas beta 2-adrenoceptors do not.