The response of the mammary gland of rats to various hormones injected daily between days 16 and 32 of lactation was studied. One unit of oxytocin, given 3 times daily, resulted in 37 and 44 % more mammary DNA and RNA content to day 32 of lactation than in saline-injected controls; however, oxytocin did not significantly increase litter weight gain or mammary RNA/DNA ratio. Cumulative litter weight gain decreased markedly in both oxytocin and saline injected rats as lactation advanced. Consequently, the endogenous supply of oxytocin appeared to be sufficient for removal of the reduced amounts of milk synthesized during advanced lactation. Cortisol-21-acetate (0.5 mg), ovine prolactin (2 mg) and ovine growth hormone (1 mg) were injected singularly and in all possible combinations between days 16 and 32 to determine if these treatments would maintain mammary nucleic acid content and lactational performance. Cortisol-21-acetate maintained mammary DNA and RNA content to day 32 of lactation and markedly retarded the normal decline during prolonged lactation in cumulative litter weight gain. Prolactin given with cortisol- 21-acetate or with cortisol-21-acetate plus growth hormone reduced the mammary DNA content, but stimulated the synthetic activity of the mammary cells (RNA/DNA). Prolactin and growth hormone injected separately or together did not exert any marked galactopoietic influence on litter weight gain. Since mammary nucleic acid content and litter weight gains decrease during prolonged lactation and adrenal glucocorticoid supplementation will retard these declines, it is suggested that adrenal secretions may be rate limiting in milk synthesis during prolonged lactation in the rat. (Endocrinology86: 237, 1970)
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THATCHER et al. (1970) studied this question.