Effect of suckling and hormones on contractility of involuting rat mammary gland

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Abstract
Rat mammary glands (filled intraductally with milk) contracted in response to iv and topical application of oxytocin and to direct mechanical stimulation during an 11-day involution period that was induced by removal of the pups on postpartum day 4. The intramammary pressure (IMP) response was linear over a 10- to 14-fold dosage range of iv oxytocin and was similar the first 5 days of involution; the dose of oxytocin required to elicit a given IMP, however, was increased three to five times by day 4. After day 5 the effective dosage range of oxytocin remained unchanged, but the maximal IMP which could be obtained was reduced quickly and significantly. Alveolar involution and the reduction in maximal IMP did not occur in teatligated glands provided that the contralateral glands were suckled. Daily injections of prolactin or growth hormone for 7 or 11 days after pup removal mimicked the effects of suckling; hydrocortisone acetate retarded alveolar involution but did not prevent the reduction in IMP or further increase the elevation of IMP observed with prolactin treatment only.