HORMONAL PROLONGATION OF LACTATION FOR 75 DAYS AFTER LITTER WITHDRAWAL IN POSTPARTUM RATS1

Abstract

Litters were removed from rats on the 4th day after parturition, and hormones were injected daily in an attempt to prolong milk secretion and retard mammary involution. Subcutaneous injections of 1 mg prolactin, 1 I.U. oxytocin (twice daily) or 0.5 mg hydrocortisone acetate maintained secretory activity and partially inhibited lobulo-alveolar involution for 10 to 20 days, while physiological saline, 1 mg growth hormone, 20 [mu]g thyroxine, or 0.2 unit insulin were ineffective in these respects. Daily injections of prolactin with oxytocin or hydro-cortisone acetate were more effective than prolactin alone, while in rats receiving all 3 hormones in combination milk could be expressed manually from the nipples for 75 days after litter removal. Injections of this triad of hormones did not prevent a gradual decline in secretion or progressive involution of lobuloalveolar tissue. This is believed to be due mainly to non-removal of secretion from the mammary gland, although it is possible that hormones other than the three injected are also necessary for optimal lactation.