Acute Progesterone Treatment Has No Effect on Bupivacaine-Induced Conduction Blockade in the Isolated Rabbit Vagus Nerve

Pregnancy decreases anesthetic requirements during regional anesthesia. Using an in vitro animal model, this study attempts to elucidate the mechanism of hormonal effects on nerve conduction in desheathed rabbit vagus nerve. The acute effects of progesterone administration on neural blockade induced by bupivacaine were investigated in terms of changes in compound action potentials of A, B, and C fibers. No change in baseline compound action potential was found after 30 min of perfusion of the nerve with progesterone. Exposure of the nerve to progesterone before exposure to bupivacaine did not significantly increase the degree of conduction blockade produced by bupivacaine, and a radioactive assay demonstrated that progesterone was taken up acutely by neural tissue over a 45-min measurement period. These results indicate that although progesterone was taken up in significant amounts by neural tissue, an acute exposure does not increase the sensitivity of the nerves to bupivacaine. Hence, the increased sensitivity of nerves to local anesthetics seen with pregnancy or with chronic progesterone treatment requires some period of time to occur. The mechanism is therefore unlikely to be a direct effect of progesterone on the cell membrane but may involve hormonal effects on protein synthesis