Differing mechanisms of inhibition of calcium increases in human uterine myocytes by indomethacin and nimesulide

Objective: Indomethacin, an inhibitor of cyclooxygenase types 1 and 2, and nimesulide, a cyclooxygenase 2 selective inhibitor, are both well-known inhibitors of prostaglandin production. It has been assumed that the tocolytic mechanism of nimesulide and indomethacin is only through decreased prostaglandin production. The purpose of this study was to test the hypothesis that either nimesulide or indomethacin, or both, has a mechanism of action on human myocytes other than inhibition of prostaglandin production. Study Design: Human uterine myometrium was obtained from consenting patients during cesarean deliveries. Myocytes were cultured, plated, and loaded with a calcium-dependent fluorescent dye, calcium green 1. The relative concentrations of intracellular free calcium were determined by measurement of time-dependent fluorescence changes by means of a video fluorimeter. In all experiments, cells were stimulated with prostaglandin F2α, 30 μmol/L. Experiments were performed without pretreatment (control) or with pretreatment consisting of indomethacin, 10 μmol/L, or nimesulide, 30 μmol/L. The percentages of cells demonstrating calcium increases were counted and compared by means of the Fisher exact test. A P VALUE = .05 was considered significant. Results: After prostaglandin F2αexposure, 33% of cells showed an increase in intracellular free calcium under control conditions. When pretreated with nimesulide, 39% of cells responded to prostaglandin F2α (P = .59). When pretreated with indomethacin, only 16% of cells responded to prostaglandin F2α (P = .019). Conclusions: Pretreatment with nimesulide failed to reduce the fraction of cells that responded to prostaglandin F2α. In contrast, pretreatment with indomethacin significantly reduced the fraction of responding cells. These data suggest that, in vitro, indomethacin exhibits a mechanism of tocolysis other than inhibition of prostaglandin synthesis. (Am J Obstet Gynecol 2001;184:1100-3.)