[Absence of an increase in prolactin receptors on the day of estrus in the mammary gland of the nu IPL rat].

Ovine prolactin (o-PRL) binding to rat mammary gland membranes has been shown to vary during the estrous cycle in the normal rat. In this study we report the characteristics of o-PRL binding to mammary gland membranes throughout the estrous cycle in a new rat strain, IPL nude, which presents a total absence of lactation. Prolactin receptors were quantified in the 100 000 g pellet. The mean value of the affinity constant in IPL nude rat was 16.7 X 10(9) M-1 and no variation was observed throughout the estrous cycle. The binding capacity also remained unchanged and the values were situated between 11.6 +/- 1.1. fmoles/mg protein and 38.6 +/- 12.3 fmoles/mg protein, corresponding to the lowest values obtained in normal rat. On the day of estrus, the prolactin binding capacity in the mammary gland of the IPL nude rat was significantly different from that of normal rat. These findings confirm that prolactin certainly plays an important role in the induction or stimulation of the rat mammary gland function.