Evolution of prolactin receptors in rabbit mammary gland during pregnancy and lactation

The numbers and affinity of prolactin receptors in the rabbit mammary gland were determined during pregnancy and early lactation under conditions in which the endogenous lactogenic hormone was depleted by means of the compound CB 154. In untreated rabbits the number of prolactin binding sites per mg protein increased from 25 +/- 3 (SE) fmol at Day 14 of gestation to 54.8 +/- 5.8 fmol/mg at Day 22, after which binding declined to 14.2 +/- 8.5 fmol/mg, then increased in late pregnancy and during lactation to 110.5 +/- 11.5 fmol/mg at Day 28. In animals treated with CB 154, binding was always higher than in non-treated animals, with a peak during pregnancy of 149 +/- 24 fmol/mg at Day 22. After declining in late pregnancy, the number of receptors was highest at Day 6 of lactation (257.4 +/- 34.6 fmol/mg). There is an almost linear increase in the weight of the mammary gland from Day 14 of pregnancy until Day 6 of lactation and this increase is unaffected (except at Day 6 of lactation) by CB 154 treatment. It was observed that prolactin receptors remain at a relatively low and constant level while mammary development (mammogenesis) takes place. The onset of milk secretion parallels a striking increase (greater than 500%) in the number of prolactin receptors (expressed per total mammary gland or per cell) after parturition has occurred. These results are discussed with emphasis on the mechanisms through which hormonal balances during pregnancy and lactation may modulate the amount of receptors per cell, hence its sensitivity to lactogenic hormones.