Correlation of growth hormone secretion during pregnancy with circulating prolactin in rats

Abstract

Growth hormone (GH) concentrations were measured throughout pregnancy in rats. The effects of surgical stress, ovariectomy, and treatments with the antiprogesterone mifepristone (RU 486) or the antioestrogen tamoxifen on serum GH, progesterone and prolactin were studied. GH concentrations were low during the first 18 days of pregnancy, except on the morning of day 5, and increased progressively from day 19 reaching peak values on the mornings of days 21 and 22. Thereafter GH concentrations decreased progressively, reaching very low values at 24.00 h on day 22, in parallel with a rise in serum prolactin concentrations. Surgical stress, performed at 12.00 h on day 20 of pregnancy, diminished serum GH concentrations 10 min later, but these returned to values similar to those of the non-operated rats 1-24 h later. Surgical stress did not modify serum prolactin concentrations at any time. Ovariectomy performed on the morning of day 19 produced the expected fall in serum progesterone and a rise in prolactin which lasted until the night of day 20. Serum GH concentrations were significantly diminished with respect to controls on day 20 and the morning of day 21 and then increased. Treatment with mifepristone on day 19 produced a simultaneous rise in serum prolactin and a fall in serum progesterone and GH by 08.00 h on day 21. Treatment with tamoxifen on days 3 and 4, or given daily from day 17 onwards did not modify prolactin concentrations but diminished serum GH concentrations at 08.00 on day 5 and on days 19-22, with the exception of a peak on day 22 (08.00 h). Tamoxifen also decreased serum progesterone concentrations. These results show that pregnant rats have a reduced capacity of response to stress in terms of changes in GH and prolactin secretion. There are high serum concentrations of GH at the end of pregnancy. The regulation of GH secretion at this time is different from that of prolactin and does not seem to depend on the fall in progesterone concentrations. However, serum GH concentrations seem to be inversely correlated with serum prolactin concentrations, as they tended to fall after increases in prolactin above basal concentrations. Oestrogen may also have a stimulatory role on GH since administration of an anti-oestrogen also resulted in a fall in GH concentrations in spite of reduced prolactin secretion.