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Summary. Milk samples were taken from 10 plains viscacha between 9 and 64 days post partum. Mean concentrations (±s.e.) were 17 ± 1·1 mM-Na; 32 ± 1·6 mM-K; 35 ± 2.2 mM-Cl; 116 ± 3·3 mM-lactose (total reducing sugar) (all in 8 samples); < 10–220 mg citrate/1 (range of 4 samples); 15·7 ± 0·64 g total nitrogen/1 (3 samples). The Na:K ratio was 1:1·95 ± 0·17. It was estimated that the fat concentration was between 116 and 182 g/1.
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The aim of this study was to show that the pig uterus synthesizes oxytocin. Uteri were obtained from 2–7 pigs at regular intervals during the oestrous cycle, throughout pregnancy, at parturition and in lactational anoestrus. Localization of mRNA encoding oxytocin was by in situ hybridization and oxytocin concentrations were measured by radioimmunoassay. As reproductive status changed, mRNA encoding oxytocin varied significantly (P < 0.05). Uterine tissue type was a significant factor in determining synthesis of mRNA encoding oxytocin (P < 0.001). In luminal epithelia, concentrations of mRNA encoding oxytocin were greater at oestrus than during day 14 of the luteal phase (P < 0.01) or at any stage of pregnancy (P < 0.05), with concentrations minimal at parturition. This trend was also exhibited in uterine circular muscle. In longitudinal muscle, concentrations of mRNA encoding oxytocin were lower during late pregnancy than at oestrus (P < 0.05) or during the luteal phase (P < 0.05). Concentrations were minimal at parturition. The oxytocin content in endometrial and myometrial tissue was positively correlated across reproductive status (P < 0.02, r = 0.402, n = 35). These data are the first indication that the uterine endometrium and musculature of the pig express mRNA encoding oxytocin. The luminal epithelium of animals at oestrus was particularly rich in mRNA encoding oxytocin, whilst late pregnant and parturient animals did not show a rise in mRNA encoding oxytocin. Local uterine synthesis of oxytocin may therefore be more important in control of the oestrous cycle than in pregnancy or at parturition in pigs.
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Prostaglandin F2α (PGF2α)-induced release of ovarian oxytocin was investigated to determine whether the effect in vivo was local. [3H]PGF2α infused downstream into a single ovarian lymphatic was transferred into the adjacent ovarian vasculature (estimated transfer 1.1 and 1.7%, two experiments). When unlabelled PGF2α was infused in a similar manner (76 pmol min−1), there was a prompt eightfold increase in ovarian oxytocin release from the adjacent ovary containing a corpus luteum, but no effect on the opposite corpus luteum, showing that the effect was local. Instillation of 2% lignocaine into the ovarian vascular pedicle did not affect PGF2α-induced oxytocin release, supporting the idea that neural mechanisms are not involved. Repeated doses of PGF2α given close-arterially produced a successive reduction in oxytocin release. This effect was prevented by a prior infusion of insulin-like growth factor-I (IGF-I), which itself gave a small, but significant, increase in oxytocin release. The results show that PGF2α in ovarian lymphatics acts locally and directly to stimulate ovarian oxytocin secretion, that repeated exposure of the corpus luteum to pulses of PGF2α can result in tachyphylaxis, and that this latter effect can be ameliorated by IGF-I infused in vivo.
