Properties of endothelin-induced contractions in the rabbit non-pregnant and pregnant myometria.

Endothelin produced concentration-dependent contractions in both non-pregnant and pregnant myometria. These contractions consisted of two components; rhythmic and slowly developing contractions. The rhythmic contractions were abolished by calcium channel antagonists and in calcium-free Krebs-Ringer solution. On the contrary, the slowly developing contraction was little affected by calcium channel antagonists, and considerably reduced in calcium-free Krebs-Ringer solution. These results indicate that the rhythmic contractions are dependent on extracellular calcium and mediated via calcium influx through voltage-dependent calcium channel, and that the slowly developing contraction is dependent on another mechanism than extracellular calcium influx and may be due to the release of calcium from intracellular calcium-stores. Endothelin was more potent in the non-pregnant myometrium than in the pregnant one (-log EC50 (M) values were 9.07 +/- 0.11 in the non-pregnant and 8.41 +/- 0.06 in the pregnant, respectively). In the presence of calcium channel antagonists and in calcium-free Krebs-Ringer solution, endothelin-induced contractions in the non-pregnant myometrium were more markedly depressed than those in the pregnant one. These findings suggest that endothelin receptors exist in rabbit myometrium, and that a functional change at the level of endothelin receptors, the calcium channels to regulate extracellular calcium influx or calcium mobilizing system from intracellular calcium-stores may occur in accordance with being pregnant.