An in vitro study on spontaneous myometrial contractility in the mare during estrus and diestrus.

Uterine smooth muscle specimens were collected from euthanatized mares in estrus and diestrus. Longitudinal and circular specimens were mounted in organ baths and the signals transcribed to a Grass polygraph. After equilibration time and 2 g preload, their physiologic isometric contractility was recorded for a continuous 2.0 h. Area under the curve, frequency and time occupied by contractions were studied. Differences between cycle phases, between muscle layers, and over the recorded time periods were statistically evaluated using linear mixed-effect models. In the mare, physiologic contractility of the uterus decreased significantly over time for all variables evaluated (time as covariate on a continuous scale). For area under the curve, there was a significant effect of muscle layer (longitudinal > circular). For frequency, higher values were recorded in estrus for circular smooth muscle layer, whereas higher values were seen in longitudinal smooth muscle layers during diestrus. In longitudinal layer and in diestrus, more time was occupied by contractions than in circular layer, and in estrus. This study is describing physiologic myometrial motility in the organ bath depending on cycle phase.