Submitted on December 12, 2003
Accepted on March 16, 2004
Effect of photoperiod on the mechanical response of the pregnant rabbit uterus to oxytocin
Jesus G Ninomiya-Alarcon1, Robyn Hudson2, Gloria Reyes-Guerrero3, Baltazar Barrera-Mera4, and Rosalinda Guevara-Guzman4*
1 Departamento de Fisiologia, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico; Departamento de Fisiologia, Universidad Quetzalcoatl, Irapuato, Guanajuato, Mexico
2 Departamento de Biologia Celular y Fisiologia, Instituto de Investigaciones Biomedicas, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico
3 Departamento de Fisiologia, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico; Departamento de Biologia de la Reproduccion, Universidad Autonoma Metropolitana-Iztapalapa, Mexico, Distrito Federal, Mexico
4 Departamento de Fisiologia, Universidad Nacional Autonoma de Mexico, Mexico, Distrito Federal, Mexico
* To whom correspondence should be addressed. E-mail: rguevara{at}servidor.unam.mx.
We present findings suggesting that photoperiod is important in determining the sensitivity of the late-pregnant rabbit uterus to oxytocin. Longitudinal myometrial strips were taken from term-pregnant and estrous rabbits at different times of a 16L/8D light/dark cycle (lights on 06:00 - 22:00 h; n = 5/group), mounted in an organ bath for isometric myographic recording, and the strength of contractions registered in response to the application of oxytocin (OT) or KCl. Strength of contractions was dose dependent and was up to 200 times greater and at doses 3 - 4 orders of magnitude lower in tissue taken from pregnant animals during the light phase (07:00 and 13:00 h) than during the dark phase (24:00 and 04:00 h). Strips from non-pregnant estrous females also showed greater sensitivity and contractile force when taken in the light (07:00 h) than in the dark (04:00 h), although the differences were not significant. Consistent with the influence of photoperiod on uterine sensitivity to OT, strips taken from two groups of pregnant females (n = 5/group) maintained on a L/D cycle advanced 12 h, showed significantly greater sensitivity and force in response to OT during the new subjective light than during the new subjective dark phase. The photoperiod-dependent contractile response to OT was specific and not simply the result of a change in general mechanical properties of the muscle since administration of KCl resulted in dose-dependent contractions of similar magnitude in both the light and dark phase. These results are consistent with the fact that rabbits, like other nocturnal mammals, typically give birth during the day.
