Failure to Demonstrate Transfer of Milk Prolactin into Blood of Milk-Fed Rats and Calves

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Abstract
Possible absorption of ingested prolactin into blood of milk-fed neonates was studied in 2 spp. by different experimental approaches. In calves secretion of endogenous prolactin was suppressed by administration of ergocryptine. Prolactin was measured in blood plasma at 20-min intervals before and after ingestion by the calves of bovine milk or colostrum which contained prolactin. Prolactin in plasma during the 2-h period after feeding was the same as that during the 1-h period before feeding. These data failed to demonstrate any absorption of ingested prolactin into the blood of neonatal calves. Previous research showed that bovine prolactin could be transferred into rat milk by injecting it into lactating rats. Blood serum was collected from rat pups that were suckling dams previously injected with bovine prolactin. Several dosages and times of administration were used in the experimental design, and blood serum also was collected from rat pups nursing control dams. All samples were immunoassayed for bovine prolactin with reagents that crossreacted only slightly with endogenous rat prolactin. Injection of lactating dams with bovine prolactin did not increase the amount of immunoreactive bovine prolactin in the sera of suckling rat pups. These results agree with those from the calf study and indicate that ingested bovine prolactin was not absorbed intact into the blood of neonatal rats.