Mapping changes in uterine contractility during the ovulatory cycle with wide-area calcium imaging in live mice.

Uterine contraction patterns vary during the ovulatory cycle and during gestation, but prior measurements have produced limited and conflicting information on these patterns. We combined a virally delivered genetically encoded calcium reporter (GCaMP8m) and ultra-widefield imaging in live nonpregnant mice, and characterized uterine calcium dynamics at organ-scale throughout the estrus cycle. At all stages, activity emanated from multiple loci of initiation (LOIs) and had irregular patterns in time. The average frequency of calcium events varied strongly with cycle stage, being lowest in proestrus (0.01 ± 0.02 Hz) and estrus (0.02 ± 0.01 Hz), higher in metestrus (0.09 ± 0.02 Hz), and highest in diestrus (0.16 ± 0.04 Hz; means ± s.e.m., p < 7 ×10 -6 ). In proestrus and metestrus, the LOIs showed a strong preference for the rostral end of the uterine horn (82% and 74% of events, respectively), whereas LOIs in metestrus and diestrus were multifocal and evenly distributed across the horn, with propagation equally likely in the rostral and caudal directions. Event velocities in proestrus (1.6 ± 1.2 mm/s) and estrus (1.6 ± 1.0 mm/s) were faster than in metestrus (0.13 ± 0.01 mm/s) and diestrus (0.13 ± 0.02 mm/s). These results establish that uterine activity rises following ovulation, peaking in diestrus, and that there is no anatomically fixed uterine pacemaker, although the ovarian end of the uterus exhibits pacer-like properties in proestrus and estrus.