Alterations in the Oxytocin-Induced Intramammary Pressure Response after Mechanical Stimulation of the Mammary Gland of the Anesthetized Lactating Rat*

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Abstract
Rhythmic, manual compression applied for 10–15 sec to two thoracic mammary glands of the urethane-anesthetized rat resulted in an immediate and significant reduction in amplitude of the intrammary pressure (IMP) response in abdominal glands induced by iv injected oxytocin. The period of reduced responsiveness to oxytocin lasted 13 min on the average; then the IMP responses gradually returned over the next few minutes to prestimulus heights. This invariably was followed by a phase of increased responsiveness to oxytocin lasting several minutes, during which the amplitude progressively increased to a maximum which was significantly above that seen before manual compression. A similar pattern of depression, recovery, and increased responsiveness of the IMP response to a given dose of oxytocin occurred if a single iv injection of 5 μg epinephrine was given instead of mammary compression. The IMP response was not reduced by manual compression of the thoracic glands if 1) the rat was pretreated with the Β-adrenergic blocking drug, propranolol; 2) the glands to be compressed were first locally anesthetized with carbocaine; or 3) manual compression was applied during the phase of increased responsiveness induced earlier either by manual compression or epinephrine. These results indicate that the motor-inhibitory system of the rat mammary gland, which is composed mainly of sympathetic components, can be reflexly activated by mechanical stimulation of the mammary gland area. This mechanism may be responsible for the failure of rat pups to obtain significant quantities of milk during the first 10–15 min of suckling in the conscious rat.