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D. G. Porter and A. D. Watts

Summary. Intravenous bolus injections of pig relaxin which produced short-lived peaks of the hormone equivalent in concentration to those observed at term promptly rendered the uterus almost totally quiescent and the inhibition persisted for about 2·5 h. During this time the uterus remained responsive to oxytocin. The main effect of relaxin was to reduce the frequency of intrauterine pressure cycles rather than the amplitude. In contrast progesterone, which also inhibited myometrial activity, took between 6 and 24 h to exert its maximum effect by reducing both amplitude and frequency of IUP cycles and it also abolished the responsiveness of the myometrium to oxytocin. Its actions were reversible but recovery took between 54 and 140 h. Oestradiol benzoate had no significant effect on myometrial activity in 21 out of 26 treatments. At 24 h after the 5 remaining treatments, however, myometrial activity was virtually zero. No evidence was obtained of a biphasic effect of oestradiol on myometrial activity as reported for the rat and ewe. This work demonstrates that purified pig relaxin is an active myometrial inhibitor in the oestrogen-treated ovariectomized non-pregnant pig in vivo.

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A. D. Watts, A. P. F. Flint, G. R. Foxcroft, and D. G. Porter

Summary. Recordings of electromyographic and mechanical activities of the uterus of intact and ovariectomized minipigs during the last days of pregnancy, combined with a frequent blood sampling regimen, indicated that the evolution of electrical activity was dependent upon the clearance both of progesterone and relaxin from the plasma. Increases in dihydro-keto-prostaglandin-F-2α (DHKF-2α) occurred only after the initial decline in progesterone, and were positively correlated with increases in electrical activity only after circulating relaxin titres had fallen. These results demonstrate that relaxin provides for the inhibition of the myometrium during the period when progesterone titres decline. The temporal relationship of the decline of progesterone and the rise in DHKF-2α suggests that uterine PGF-2α may not initiate luteolysis in the sow at term.

Keywords: relaxin; myometrium; parturition; prostaglandins; pig

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W. A. Bennett, T. L. Watts, W. D. Blair, S. J. Waldhalm, and J. W. Fuquay

Summary. Microtransducers sensitive to changes in internal diameter were chronically implanted in the oviducts of 5 dairy cows. Motility patterns were recorded throughout 9 oestrous cycles. Cyclic variations in patterns of motility were observed and compared with circulating concentrations of plasma progesterone. Luteal-phase motility patterns were of low amplitude and frequency. The frequency and amplitude of motility increased 3–5 days before behavioural oestrus. This activity consisted primarily of longitudinal muscle contractions, with an interspacing of circular muscle activity occurring during oestrus. Patterns of activity after oestrus were similar to those before oestrus, with activity decreasing 3–5 days after oestrus. Transducers implanted bilaterally in 2 animals permitted observation of asynchronous patterns between right and left oviducts. Preliminary data suggested a higher level pf activity in the oviduct ipsilateral to the active ovary. These variations may be due to a local effect, possibly mediated by the functional ovary or the ovum.

Keywords: oviduct; motility; oestrous cycle; progesterone; cow