The milk ejection reflex in the pig

1. The milk ejection reflex in response to suckling was studied in conscious sows by continuous recording of intramammary pressure, radioimmunoassay of plasma concentrations of neurohypophysial hormones, and observation of the behaviour of the sows and piglets. 2. A regular pattern of nursing, suckling and milk ejection was observed. The mean duration of the suckling period was 6·3 min. Over 144 suckling periods, 113 milk ejections were recorded. Each milk ejection was characterized by a sudden rise in intramammary pressure reaching 20‐49 mmHg, and lasting 8‐41 sec. Milk ejections occurred only once per suckling period, at a mean interval of 44·3 min. 3. Each milk ejection occurred with a mean latency of 2·4 min from the onset of a period of initial massage of the udders by the piglets, and was coincident with a period of quiet suckling when the piglets were consuming milk. The onset of nursing was signalled by the sows grunting in a rhythmic manner. In most cases, the frequency of grunts, at first low, increased suddenly 23 sec before milk ejection. 4. During eighteen suckling periods leading to milk ejection, neurohypophysial hormone assays performed on serial blood samples showed an increase in plasma concentration of oxytocin up to 30 sec before milk ejection. The concentration of lysine—vasopressin did not rise above basal levels. 5. In 21·4% of the suckling periods, no rise in intramammary pressure was observed. In these ‘incomplete sucklings’, the sow usually failed to grunt rapidly, and the piglets obtained no milk. For three of these periods, hormone assay showed no increase in oxytocin or vasopressin concentrations in blood. 6. Oxytocin given intravenously produced variations in intramammary pressure which depended on the dose and the rate of injection. Rapid injections of 25‐50 m‐u. oxytocin, caused milk ejections similar to those induced by suckling. When oxytocin was administered at different rates, the faster the injection, the shorter the latency and the higher the amplitude of the response. Plasma concentrations of oxytocin after injection of 25 m‐u. were similar to those observed during reflex milk ejection. 7. Trains of electrical pulses were applied to the posterior pituitary of four anaesthetized sows. At frequencies of stimulation above 10 Hz, a rise in intramammary pressure and an increase in plasma oxytocin and vasopressin concentrations were observed. At frequencies of stimulation of 30‐50 Hz, the response of the mammary gland and the time course of the variations in oxytocin plasma concentrations were similar to those observed during natural reflex milk ejection. 8. It is concluded that reflex milk ejections during suckling in the pig are caused by the intermittent and spurt‐like release of about 25 m‐u. oxytocin, without concomitant vasopressin release. It is postulated that the release of oxytocin is probably precipitated by a brief and massive activation of oxytocin‐secreting neurones in the hypothalamus. Central mechanisms controlling the intermittent release of oxytocin are discussed.