T-type and L-type calcium currents in freshly dispersed human uterine smooth muscle cells.

OBJECTIVE

Our purpose was to characterize the types of voltage-activated calcium currents that are found in human uterine myocytes and to determine the effects of magnesium and nifedipine on these currents.

STUDY DESIGN

Electrophysiologic experiments were performed on freshly isolated human uterine smooth muscle cells by means of the nystatin modification of the whole-cell patch clamp technique.

RESULTS

Two types of voltage-activated calcium currents that are similar to the T-type and L-type calcium currents observed in cardiac myocytes were identified in freshly dispersed, pregnant human uterine myocytes. Magnesium at 8 mmol/L reduced uterine myocyte T-type currents by 68% +/- 17% but did not reduce L-type currents. Nifedipine at 10(-6) mol/L blocked the L-type currents but had no effect on T-type currents.

CONCLUSION

Freshly isolated human uterine smooth muscle cells exhibit subtypes of calcium currents that are analogous to those found in cardiac myocytes. The uterine myocyte T-type current may be primarily involved with action potential transmission and the L-type current primarily with increasing intracellular free calcium by bulk calcium transport. The differing physiologic effects of magnesium and nifedipine on the calcium current subtypes suggest that for the treatment of preterm labor the primary effect of magnesium therapy is to decrease the frequency of contractions and of nifedipine the strength.