RELATIONSHIP OF PROLACTIN AND GROWTH HORMONE TO MAMMARY FUNCTION DURING PREGNANCY AND LACTATION IN THE C3H/ST MOUSE

SUMMARY The secretion of prolactin and growth hormone (GH) in C3H/St mice, a strain having a high incidence of spontaneous mammary tumours, was studied at intervals throughout pregnancy and lactation. Serum and pituitary concentrations of prolactin and GH were measured by specific, homologous radioimmunoassays. Crude extracts of mouse placentae did not cross-react in these assays, indicating that the assay systems measured prolactin and GH of hypophysial origin. Except for a small, brief rise on day 5, prolactin secretion was low throughout most of pregnancy; the serum concentration of prolactin increased appreciably only near term. During lactation, prolactin in serum averaged from 5 to 10 times basal levels (30–40 ng/ml) until day 15 of lactation, after which the concentration declined. In contrast, GH secretion increased early in pregnancy, the concentration rising both in serum and in the pituitary gland consistently after day 5. The serum concentrations reached 3–10 times basal levels (8–12 ng/ml) between days 10 and 20 of pregnancy. There were successive decreases in serum GH concentration after parturition; the concentration reached basal levels on day 5, but it increased thereafter and was maintained at over three times basal levels for the remainder of lactation. The levels of GH in serum were highly correlated with mammary DNA and RNA contents during pregnancy, while prolactin concentration was more strongly correlated with mammary function during lactation.