Differences in cardiodepressant effects of halothane and isoflurane after inotropic stimulation in guinea-pig papillary muscles.

The effects of halothane and isoflurane on action potentials and force of contraction were studied in guinea-pig isolated papillary muscles in order to investigate the cardiodepressant action of the anesthetics in the presence of clinically relevant inotropic drugs. In control conditions, equipotent concentrations of the two volatile anesthetics depressed force of contraction to a similar degree. Halothane (2%) slightly shortened the action potential duration, whereas isoflurane (2.8%) did not influence the shape of the action potential. The muscles were then treated with three different inotropes, digitoxin, amrinone and 3-isobutyl-1-methylxanthine (IBMX) prior to exposure to the anesthetics. After pretreatment with digitoxin and amrinone, halothane was significantly more cardiodepressant than isoflurane. With IBMX, no difference in negative inotropic effect was observed. Neither volatile anesthetic significantly changed the action potential duration after pretreatment with the inotropic drugs. We suggest that halothane is more cardiodepressant than isoflurane in the presence of drugs which enhance force of contraction both by an increase in calcium influx and calcium release because it impairs force of contraction not only via inhibition of the calcium current but also via interference with calcium release.