Mammary resistance: a possible controlling factor in milk ejection

Abstract

At 8 and 13 days post partum, rats have the same total milk availability; yet, in response to suckling they release a greater amount of milk on day 13 than on day 8. Increased sensitivity to suckling in the more advanced lactators may result from a greater release of oxytocin or from changes in the mammary glands as lactation advances. The present study explores this latter possibility in anaesthetized dams at 8-9 and 13-15 days of lactation. Milk release and intramammary pressure were measured in anaesthetized dams in response to various doses of oxytoxin. Milk release was determined from the body weight gain of pups which had been fasted for 5 h before suckling on dams which had been isolated for 5 h. This parameter was significantly greater in 13- to 15-day lactators than in 8- to 9-day lactators over the range of oxytocin doses examined. In contrast, intramammary peak pressure and its dissipation time were significantly larger in the 8- to 9-day lactators than in the 13- to 15-day lactators. The compliance of the mammary glands was indirectly assessed at the two stages of lactation. When a constant pressure pulse was introduced into a cannulated gland, the resulting pressure peak was significantly greater in 8-day than in 13-day lactators, indicating a greater resistance in the former. Taken together, these results indicate that when endogenous oxytocin is inhibited (by anaesthesia) the greater milk release observed at the later stage of lactation in response to various doses of oxytocin may be due to a decline in mammary resistance as lactation progresses.

Inhibition of oxytocin release and milk let-down in postpartum primiparous cows is not abolished by naloxone

“…The sensitivity recurred after several milkings and then, the milk ejection occurred regularly at much lower OT plasma levels as during the first milkings. Similar findings are reported from the rat (Lau & Henning, 1987), where the sensitivity of the mammary gland to OT increased from 8 days post partum to 13 days post partum.…”

Breastfeeding Challenges and the Preterm Mother-Infant Dyad: A Conceptual Model

“…[130][131][132][133] Peripheral inhibition of milk synthesis and ejection may occur as stimulation of the central and/or peripheral autonomic systems can lead to vasoconstriction resulting in decreased hormonal delivery to the mammary alveoli and myoepithelium, respectively. 60,[134][135][136] The time delay between lactogenic hormone release and the resulting milk synthesized in the mammary alveoli would appear prolonged hours. 137 This contrasts with the release of milk stored in the mammary alveoli that occurs immediately in response to oxytocin.…”