CORRELATION OF GROWTH-HORMONE SECRETION DURING PREGNANCY WITH CIRCULATING PROLACTIN IN RATS

Growth hormone (GH) concentrations were measured throughout pregnancy in rats. The effects of surgical stress, ovariectomy, and treatments w ith the antiprogesterone mifepristone (RU 486) or the antioestrogen ta moxifen on serum GH, progesterone and prolactin were studied. GH conce ntrations were low during the first 18 days of pregnancy, except on th e morning of day 5, and increased progressively from day 19 reaching p eak values on the mornings of days 21 and 22. Thereafter GH concentrat ions decreased progressively, reaching very low values at 24.00 h on d ay 22, in parallel with a rise in serum prolactin concentrations. Surg ical stress, performed at 12.00 h on day 20 of pregnancy, diminished s erum GH concentrations 10 min later, but these returned to values simi lar to those of the non-operated rats 1-24 h later. Surgical stress di d not modify serum prolactin concentrations at any time. Ovariectomy p erformed on the morning of day 19 produced the expected fall in serum progesterone and a rise in prolactin which lasted until the night of d ay 20. Serum GH concentrations were significantly diminished with resp ect to controls on day 20 and the morning of day 21 and then increased . Treatment with mifepristone on day 19 produced a simultaneous rise i n serum prolactin and a fall in serum progesterone and GH by 08.00 h o n day 21. Treatment with tamoxifen on days 3 and 4, or given daily fro m day 17 onwards did not modify prolactin concentrations but diminishe d serum GH concentrations at 08.00 on day 5 and on days 19-22, with th e exception of a peak on day 22 (08.00 h). Tamoxifen also decreased se rum 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 i s 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, a s they tended to fall after increases in prolactin above basal concent rations. Oestrogen may also have a stimulatory role on GH since admini stration of an anti-oestrogen also resulted in a fall in GH concentrat ions in spite of reduced prolactin secretion.