An in vitro study on spontaneous contractility and drug responsiveness of porcine myometrium during three ovarian stages

The aim of the present study was to clarify changes in spontaneous contractility and drug responsiveness of the porcine uterus during ovarian stages. Myometrial smooth muscle strips were collected from slaughtered pigs in follicular, early luteal and functional luteal phases. Longitudinal muscle (LM) and circular muscle (CM) were mounted in organ bath and their spontaneous contractility and drug responsiveness were examined in vitro. During the equilibration periods (90min), all CM strips showed spontaneous contraction regardless of the ovarian stages, but the number of LM strips contracting spontaneously decreased in the functional luteal phase. Spontaneous contractility was characterized using frequency, amplitude and area under the curve (AUC). In LM strips, amplitude and AUC, but not frequency, changed during the ovarian stages. On the other hand, amplitude and frequency, but not AUC of CM strips, changed during the ovarian stages. Carbachol, fluprostenol and oxytocin caused contraction of the myometrium. LM was more sensitive to these contractile agents than was CM at all ovarian stages. Comparing the contractile responses in follicular and luteal phases, the magnitude of maximum contraction but not the EC50 value decreased in the functional luteal phase. Isoproterenol (LMCM) and BW245C (CMLM) inhibited spontaneous myometrial contraction in a muscle layer-dependent manner. EC50 Values and maximum inhibition of both drugs were not different in the follicular and luteal phases. In conclusion, a significant difference in spontaneous contractility during the ovarian stages was found in LM but not in CM. Responses to contractile drugs decreased from the follicular phase to luteal phase, but those to inhibitory drugs did not change in these two phases, suggesting that porcine uterine smooth muscle has low susceptibility to uterotonic agents in the luteal phase probably due to progesterone.