INCORPORATION OF RAT PROLACTIN INTO RAT MILK IN VIVO AND IN VITRO

Abstract

SUMMARY Rat prolactin (11 i.u./mg) was continuously infused into the circulation of urethaneanaesthetized lactating rats for 35 min at doses of either 200 or 472 ng/min. The immunoreactive prolactin in both milk and plasma rose quickly during the first 20–25 min of infusion, then stabilized at similar levels over baseline (68 and 98 ng/ml for milk and plasma, respectively, with the 200 ng/min dose and 250 and 230 ng/ml, respectively, with the 472 ng/min dose). The concentration of prolactin in plasma fell after the infusion was stopped, whereas that in the milk either did not fall at all or fell slightly to a new stabilized level. There was a rapid and extensive loss in the immunoreactivity of prolactin added to milk when rat milk was incubated in vitro (37 °C for 1–120 min) with either 600 ng/ml of extracted pituitary prolactin (NIAMDD RP-1) or unit equivalent amounts of prolactin obtained from pituitary culture media (secreted prolactin, supplied by C. S. Nicoll). Significantly greater amounts of added RP-1 prolactin were lost when it was incubated with milk obtained after 4 h than after 18 h of non-suckling. There was, however, no drop in endogenous immunoreactive milk prolactin levels (350–400 ng/ml) when rat milk was incubated with saline for 30 min. This suggests that milk prolactin obtained as a result of plasma transfer is different chemically from the milk prolactin resulting from the addition of either RP-1 or secreted prolactin to milk in vitro. Approximately 90% of ¹³¹I-labelled rat prolactin appeared in the trichloroacetic acid precipitable fraction after incubation (37 °C for 120 min) with milk obtained after 4 h of non-suckling in either the presence or absence of thiouracil (added to prevent binding of ¹³¹I or ¹³¹I-labelled fragments to milk protein). The recovery was slightly less when ¹³¹I-labelled prolactin was incubated with milk obtained after 18 h of non-suckling. These data suggest that prolactin is quickly transferred from plasma into milk in direct relation to the plasma concentration. Once there, much of it appears to be retained by the milk perhaps chemically or physically bound; there is little, if any, degradation of the hormone. We conclude that the lactating rat mammary gland may function normally as an excretory organ for prolactin.