Interaction between halothane and propranolol on oxytocin-induced uterine contractions

Halothane and other volatile anesthetics were found to relax uteri maximally contracted by oxytocin and to enhance the relaxant effect of propranolol and other agents (tetracaine, quinidine and chlorpromazine) with local anesthetic properties. Concentration-effect curves for inhibition by propranolol or tetracaine were shifted to the left by halothane. Methoxyflurane, chloroform and diethyl ether also increased the effect of propranolol. The tissue/medium ratio for propranolol was increased by halothane. The uptake of propranolol was saturable and was decreased at 2.5 degrees C, suggesting an active transport mechanism. The increased uptake of propranolol in the presence of halothane was probably insufficient to account for the synergism noted. Inhibition by propranolol and tetracaine was inversely related to the concentration of calcium in the medium. The inhibitory effect of propranolol, tetracaine, quinidine and chlorpromazine appeared to be related to their ability to inhibit calcium transport.