Milk secretory response of the conscious lactating rat following intravenous injections of rat prolactin.

Mammary glands of rats on day 14 of lactation were emptied of milk by the pups after 8 h of nonsuckling with the aid of oxytocin injections to the mother. The glands subsequently refilled about 50% within 6 h and were completely refilled with mild within 16 h. Suckling for 5 min midway during the 8 h period of nonsuckling caused complete refilling of the glands within 6 h. Refilling within 6 h also occurred if, instead of suckling, rat prolactin was injected at 1 min intervals directly into the circulation of the conscious mother via an indwelling right atrial catheter. Five injections of 200 ng or three injections of 400 ng were without effect (in comparison with five injections of saline) upon mammary refilling whereas 10 injections of 200 ng or four injections of 400 ng simulated significant but submaximal refilling. Complete refilling resulted from five injections of 400 ng. Prolactin is secreted steadily over prolonged periods of suckling in the rat, but only that prolactin released during the first few minutes of suckling appears to be required for milk secretion.