Comparison of the myometrial response to oxytocin during daylight with the response obtained during the early hours of darkness in the fetectomized rhesus monkey at 160-172 days gestational age.
Six rhesus monkeys, fetectomized at 122-129 days gestational age (dGA), were studied to evaluate photoperiod-dependent differences in the myometrial sensitivity to exogenous oxytocin in the absence of a live fetus. One to two weeks before the study, at 150-162 dGA, sixteen maternal arterial samples were taken every 4 h to assess plasma concentrations as well as the presence of 24-h rhythmicity in plasma estradiol and cortisol. At 160-172 dGA, oxytocin was administered via the maternal inferior vena cava to elicit regular myometrial contractions. Each animal was studied twice: once during daylight and once during the early hours of darkness. Oxytocin was infused in a pulsatile fashion during the first minute out of every five over four consecutive 30-min intervals, during which the dose rate was increased after each 6-h oxytocin pulse (400, 800, 2000, and 4000 pg.kg-1.min-1). The overall response of the myometrium of the fetectomized animals, evaluated as the number of contractions per pulse of oxytocin, was higher during the daytime study (p = 0.006). The response to oxytocin in intact pregnant animals is greatest during the early hours of darkness. Comparison of the overall myometrial response of the fetectomized monkeys with the response of intact pregnant animals showed that the daytime responses were similar, but that the nighttime response of the fetectomized animals was significantly lower (p < 0.0002). These contrasting findings may be due to differences in the maternal neuroendocrine environment depending on the presence or absence of a fetal adrenal steroid contribution acting directly on the myometrium or acting indirectly through provision of precursors for estrogen formation.(ABSTRACT TRUNCATED AT 250 WORDS)
Barbera
M
M
Department of Physiology, NYS College of Veterinary Medicine, Cornell University, Ithaca 14853.
Honnebier
O M
OM
Mecenas
C A
CA
Jenkins
S L
SL
Nathanielsz
P W
PW
D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D004576:Electromyography; D004958:Estradiol / Q000097:blood; D005260:Female; D005333:Fetus / Q000502:physiology; D005865:Gestational Age; D006854:Hydrocortisone / Q000097:blood; D008027:Light; D008253:Macaca mulatta; D009215:Myometrium / Q000187:drug effects* / Q000502:physiology; D010121:Oxytocin / Q000008:administration & dosage / Q000494:pharmacology*; D011247:Pregnancy; D011312:Pressure; D014590:Uterine Contraction / Q000187:drug effects / Q000502:physiology
D004958:Estradiol; D010121:Oxytocin; D006854:Hydrocortisone