BLOOD PROLACTIN CONCENTRATIONS AFFECT PROLACTIN TRANSFER INTO GOAT MILK - IMPLICATIONS FOR MAINTENANCE OF LACTATION

I-125-Labelled ovine prolactin was infused for 15 min into a pudic art ery supplying one mammary gland of lactating goats (n=17). Between 0 a nd 4.25 h significantly more total (P<0.01) and trichloroacetic acid ( TCA)-precipitable (P<0.001) radioactivity appeared in the milk of the infused compared with the non-infused gland. Gel chromatography and an tibody precipitation indicated the presence of undegraded I-125-labell ed prolactin in milk whey. Maximum transfer occurred 60-80 min after t he end of infusion suggesting passage via a transcellular route. High plasma prolactin concentrations, resulting from infusion of cold prola ctin with labelled prolactin in late lactation or from seasonally elev ated prolactin at peak lactation, reduced the specific activity of inf used prolactin and depressed the difference in secretion of I-125-labe lled prolactin into milk of infused and non-infused glands. This sugge sts the operation of a competitive and saturable mechanism. Together w ith the increase in the milk to blood ratio of prolactin in goats give n long-term (3 week) bromocriptine treatment, the results suggest that the goat mammary gland has a high avidity for prolactin especially wh en circulating prolactin is low. There was also evidence from TCA prec ipitation that prolactin may be protected from degradation in these ci rcumstances. These mechanisms may contribute to the resistance of rumi nant lactation to reduction in plasma prolactin and protect lactation from seasonal prolactin fluctuations.