Progesterone potentiation of bupivacaine arrhythmogenicity in pentobarbital-anesthetized rats and beating rat heart cell cultures.

The effects of progesterone treatment on bupivacaine arrhythmogenicity in beating rat heart myocyte cultures and on anesthetized rats were determined. After determining the bupivacaine AD50 (the concentration of bupivacaine that caused 50% of all beating rat heart myocyte cultures to become arrhythmic), we determined the effect of 1-hour progesterone HCl exposure on myocyte contractile rhythm. Each concentration of progesterone (6.25, 12.5, 25, and 50 micrograms/ml) caused a significant and concentration-dependent reduction in the AD50 for bupivacaine. Estradiol treatment also increased the arrhythmogenicity of bupivacaine in myocyte cultures, but was only one fourth as potent as progesterone. Neither progesterone nor estradiol effects on bupivacaine arrhythmogenicity were potentiated by epinephrine. Chronic progesterone pretreatment (5 mg/kg/day for 21 days) caused a significant increase in bupivacaine arrhythmogenicity in intact pentobarbital-anesthetized rats. There was a significant decrease in the time to onset of arrhythmia as compared with control nonprogesterone-treated rats (6.2 +/- 1.3 vs. 30.8 +/- 2.5 min, mean +/- SE). The results of this study indicate that progesterone can potentiate bupivacaine arrhythmogenicity both in vivo and in vitro. Potentiation of bupivacaine arrhythmia in myocyte cultures suggests that this effect is at least partly mediated at the myocyte level.