Some properties of the circular myometrium of the sheep throughout pregnancy and during labour.

Spontaneous electrical and mechanical activity were measured in strips of circular myometrium, stretched to in vivo length, obtained from ewes during pregnancy and parturition. The resting tension was high (280 mN/mm2) early in pregnancy and decreased gradually during the course of gestation to reach low levels (20 mN/mm2) during labour. Strips showed continuous electrical and mechanical activity up to around day 50 of pregnancy. The variability in the amplitude of the action potentials, the occurrence of many small fluctuations in membrane potential and the short space constant (0.9 mm) could be explained by impaired conduction in these tissues. During the second half of pregnancy, muscle strips showed regular bursts of spike activity and contractions with a frequency of about 4/min. The space constant (lambda) increased gradually to around 2 mm at term. The membrane time constant (tau m) remained constant at approximately 130 ms throughout gestation. Tissues from ewes in labour were not spontaneously active but action potentials could be evoked by depolarizing current pulses. The space constant increased to 4.6 +/- 0.4 mm and tau m increased to 510 +/- 50 ms at this time. Verapamil (10(-6) M) rapidly abolished electrically evoked activity in strips obtained from ewes in labour. Verapamil abolished spontaneous activity in strips from animals up to day 145 of pregnancy, though a higher concentration (2 X 10(-5) M) was required. However, action potentials could be evoked by depolarizing current pulses and these were accompanied by contractions that were 50-80% of those observed in control solution. Cobalt (2mM) had an effect on spontaneous and electrically induced activity that was similar to that of verapamil. The threshold for the initiation of an action potential was increased in all tissues in the presence of the blockers. Verapamil decreased lambda by about 50% in the later stages of pregnancy and during labour. During labour alone verapamil reduced tau m by about 50%. These results show that the onset of labour in the ewe is associated with rapid and dramatic changes in both the passive and active properties of the circular muscle of the uterus.