Effect of growth hormone on mammary gland growth and lactation in mice with special reference of the relation with prolactin.

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1. GH and Mammary Gland Growth1) GH and normal mammary gland growth: Twice daily injections of 5 or 50 ng SMS to C3H/He virgin mice between 25 and 55 days of age resulted in a significant inhibition of normal mammary gland growth associated with the reduced plasma GH level. Similar results were also obtained by the neonatal single injection of 4 mg monosodium glutamate (MSG) to SHN female mice. Meanwhile, plasma prolactin (PRL) level and the pattern of estrous cycles were affected little by either treatment. There was the significant simple correlation between plasma GH level and normal mammary gland growth, but not the partial correlation when plasma PRL level was fixed.2) GH and preneolastic mammary hyperplastic alveolar nodule (HAN) formation: Single neonatal MSG treatment inhibited HAN formation. However, GH level had neither single nor partial correlation with HAN formation.3) GH and spontaneous mammary tumor development: Spontaneous mammary tumor development was affected little by reduced plasma GH level by MSG.2. GH and lactationIn C3H/He mice, chronic decline of plasma GH level by twice daily injections of 50 ng SMS suppressed normal mammary gland growth on day 12 of lactation, however, there was no difference in lactational performance between the SMS-treated and the control groups.All findings strongly suggest that while GH is an important hormone for normal mammary gland growth and lactation, its role is often masked by the other mammotropic hormones, PRL.