Enflurane increases the adrenaline threshold for the development of slow responses in isolated canine trabeculae

To determine if enflurane has different effects on myocardial sensitivity to adrenaline than those previously reported for halothane, we have studied the enflurane-adrenaline interaction in isolated canine trabeculae using doses of adrenaline necessary to produce slow responses (adrenaline threshold for the development of slow responses, ATSR) as an indicator. The preparations were depolarized in Tyrode's solution containing potassium chloride 26 mmol litre-1, then adrenaline concentrations in the solution were increased stepwise. Enflurane 1% had no significant effect, but 2% and 4% significantly increased the ATSR two-fold and seven-fold, respectively. To investigate the influence of enflurane on the adrenaline-adrenoceptor interaction, we studied the effects on the ATSR of 2% enflurane alone or in combination with either prazosin 8 ng ml-1 or metoprolol 17 ng ml-1. Compared with the ATSR obtained with 2% enflurane alone, alpha 1-block with prazosin did not alter, but beta 1-block with metoprolol significantly increased the ATSR (three-fold). These effects on enflurane are qualitatively as well as quantitatively similar to those reported previously for halothane. Thus, assuming that the MAC value for enflurane is about 2-2.5 times greater than that for halothane, the effects of enflurane on slow channel conductance (antiarrhythmic effects of enflurane) might be about two to three times greater than those of halothane at equivalent depth of anaesthesia. Such differences may explain in part the clinical observation that ventricular arrhythmias are less likely with enflurane than with halothane.