In the presence of 62 μu. or more of [3H]oxytocin/ml there was specific uptake of oxytocin by lactating rat mammary glands in vitro within 400 s under conditions similar to those used in the biossay of oxytocin with rat mammary strips in vitro. This uptake was blocked by pre-incubation with non-radioactive oxytocin. A similar, rapid, specific uptake of oxytocin by uterine tissue in vitro was observed. There was no specific uptake of oxytocin by non-target tissues such as heart and skeletal muscle. Measurements of inulin and water spaces of the tissues showed that, over these short periods of time, diffusion into mammary tissue was much less than into the other tissues. The ratios of uptake of [3H]oxytocin: [3H]inulin and [3H]oxytocin: [3H]water were much higher for mammary tissue than those for other tissues used, indicating a preferential (tissue-specific) uptake. Uterine tissue from stilboestrol-primed rats also showed a preferential uptake of oxytocin, though not as great as that for mammary tissue. It is suggested that the specific uptake of oxytocin by mammary and uterine tissue is due to binding to specific receptors.
There was a variation in the specific uptake of oxytocin with the day of lactation of the mammary tissue, and specific uptake was only observed after the 8th day. This could indicate synthesis of receptors during lactation. In a similar way, synthesis of receptors may occur in the non-pregnant uterus due to the influence of exogenous oestrogens, leading to the increase in specific uptake by non-pregnant uterine tissue for oestrogen-primed rats. There is some evidence of more than one type of binding site for oxytocin. Biological action may only be associated with one of these sites.