Transport of 3H-TRH from plasma to rat milk: accumulation and slow degradation in milk and presence of unaltered hormone in gastric content of pups.

10 microCi 3H-TRH was administered intravenously to lactating rats 14 days after delivery. The total radioactivity of plasma and mild equalized about 25 min after injection and increased continually in milk during the 2-hour study. The percentage of activity bound to specific TRH antibody was the same in extracts of plasma and mild (about 60% of standard binding) throughout the experiment. Besides milk, the radioactivity was also accumulated in pituitary, thyroid, kidney and liver; the tissue:plasma ratio being 2:4 2 h after injection. The presence of 3H-TRH in milk extracts was also confirmed by paper chromatography (butanol-acetic acid-water-ethylacetate, 1:1:1:1) and electrophoresis (in pyridine acetate buffer, pH 5.8). TRH-degrading activity, representing about one fifth of that in plasma, was found in rat milk in vitro by means of paper chromatography and electrophoresis. The gastric contents of suckling pups after injection of 3H-TRH were also analyzed. The total radioactivity increased from 30 to 60 min after the injection. The percentage reacting with the specific TRH antibody in extracts of gastric contents was the same as that in plasma and milk and did not change throughout the 2 h of experiment. Our results show one-way transport of TRH from plasma to milk, low TRH-degrading activity of milk and passage of unaltered neurohormone from plasma to milk and stomach of suckling. The gastric content does not have a deteriorating effect on TRH. We suppose that the presence of accumulated TRH in maternal milk could be some biological significance in suckling, in which the plasma TRH-degrading system is lacking.