Oxytocin receptors in rat involuting mammary gland

Oxytocin-receptor concentrations in the rat mammary gland were determined by Scatchard analyses with [3H]oxytocin. There was about a 100-fold increase in the number of receptors per mammary gland between the 1st day of pregnancy and late lactation. The number of receptors then fell markedly during postweaning mammary regression, but rose again during a second pregnancy and lactation cycle. The changes in oxytocin-receptor number corresponded to changes in alkaline phosphatase activity per mammary gland. These results strongly support data suggesting that alkaline phosphatase, like oxytocin receptors, is a specific marker for mammary myoepithelial cells. Despite the fall in oxytocin-receptor number per mammary gland during postweaning regression, the concentration of receptors, expressed per milligram of protein, increased 10-fold over lactating levels on the 6th day of regression. Thereafter, receptor concentrations declined, but were still elevated about fivefold over lactating levels on the 15th day of regression. It is likely that the increased concentration of receptors was due to a decrease in the relative amount of nontarget secretory cells. The factors that regulate the concentration of oxytocin receptors on mammary myoepithelial cells are presently unknown; however, the involuting mammary system may be practical for obtaining enriched populations of oxytocin-sensitive myoepithelial cells.