PITUITARY LACTOGENIC HORMONE CONCENTRATION AND MILK SECRETION IN LACTATING RATS1

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Abstract
The role of the lactogenic hormone in maintaining lactation in the rat has been studied by comparing milk secretion with the change in concentration of pituitary lactogenic hormone resulting from nursing on each of clays 2, 6, 10, 14 and 21 postpartum. Milk yield per timed nursing, litter body weight and intensity of milk secretion were found to increase throughout lactation, though a slight lag in all occurred on day 14. Maternal body weights were rather uniform but maternal pituitary weight per 100 g. decreased steadily from beginning to end of lactation. Pituitary lactogenic hormone levels of lactating rats, which were isolated from their young for 10 hours to standardize the influence of nursing stimuli (prenursing level), and of rats allowed to nurse 30 minutes after isolation (postnursing level) was determined by the common pigeon micro-method of assay. The prenursing level of lactogenic hormone increased from the 2nd to the 6th day postpartum, then declined toward the end of lactation. The fall in hormone concentration as a result of 30 minutes nursing stimuli was the greatest on days 6 and 14 (95% and 90%, respectively). The smallest decline occurred on clay 21 (4%), the time of maximum milk secretion. The reason for this is unknown.