A proposed mechanism of bupivacaine-induced contraction of human umbilical artery smooth muscle cells

This in vitro study using ring preparations of human umbilical vessels and cultured human umbilical artery smooth muscle cells was designed to determine: (a) the mechanism of bupivacaine-induced contraction of ring preparations, and (b) whether similar concentrations of bupivacaine release Cal 2+ in cultured smooth muscle cells. Isometric tension was recorded from ring preparations of human umbilical veins and arteries in an isolated tissue chamber. Separate fluorescence and electrophysiology studies were done with cultured human umbilical artery smooth muscle cells. Bupivacaine-evoked contractions of ring preparations were either tonic or twitch in nature. The contraction of ring preparations was dependent on extracellular Cal 2+ and sensitive to nifedipine inhibition. Bupivacaine also increased intracellular Cal 2+ in patterns consistent with tonic or phasic tension responses seen in isometric recordings. In addition, the membrane-resting potential was depolarized by bupivacaine. Since similar concentrations of bupivacaine caused both contraction and a rise in intracellular Ca 2+, the bupivacaine-evoked contraction was the result of increased cell Cal 2+ and the source of this Ca 2+ was the extracellular space.