Excitatory response of guinea-pig myometrium to intravenous noradrenaline declines towards term.
Guinea-pigs were anaesthetized at three stages of pregnancy. Intrauterine pressure was recorded for a 1-h control period and during 10-min intravenous infusions of noradrenaline at rates of 1.0 and 10 micrograms/(min X kg). The mean and maximum amplitude of contractions occurring during the infusions was compared with that of contraction cycles registered in the control period. At 18-21 and 35-43 days post coitum, noradrenaline invariably evoked a rapid and sustained rise in intrauterine pressure, the amplitude of the contractions being greater than during spontaneous contraction cycles recorded in the control period. In late pregnancy, 59-68 days p.c., infusion of 1.0 micrograms noradrenaline/(min X kg) failed to elicit a clear response; contractions occurring during infusion of 10 micrograms noradrenaline/(min X kg) had amplitudes similar to those of the control period and were without a sustained contracture. The absence of denervation hypersensitivity, despite the occurrence of sympathetic denervation in the course of pregnancy, may be due to a generalized effect on excitation-contraction coupling, possibly caused by relaxin.
D000320:Adrenergic Fibers / Q000502:physiology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D005260:Female; D006168:Guinea Pigs; D007275:Injections, Intravenous; D009215:Myometrium / Q000187:drug effects / Q000502:physiology*; D009638:Norepinephrine / Q000008:administration & dosage / Q000494:pharmacology*; D011247:Pregnancy; D013268:Stimulation, Chemical; D013997:Time Factors; D014590:Uterine Contraction / Q000187:drug effects*; D014599:Uterus / Q000294:innervation
D009638:Norepinephrine