Pituitary response of growth hormone release in administration of hpGRF-44 to pregnant rats.

To investigate the pituitary response to exogenous growth hormone releasing factor (GRF) during pregnancy, pregnant (from Day 7 to Day 16) and non-pregnant rats were cannulated through a femoral vein, then hpGRF-44 (1.0 microgram) or saline was administered intravenously under anaesthesia with pentobarbital sodium. An initial blood sample was drawn off before the administration and subsequent samples were taken at 5, 10, 15 and 20 min post injection for measurement of plasma growth hormone (GH) values by radioimmunoassay. In a group of pregnant rats (n = 9), a mean concentration of plasma GH was increased to a peak in 10 min, from 132.5 +/- 14.9 ng/ml (mean +/- S.E.) to 864.4 +/- 175.1 ng/ml after hpGRF-44 administration. The mean value for non-pregnant rats (n = 6) reached a peak in 5 min from 164.5 +/- 34.1 ng/ml to 921.2 +/- 257.2 ng/ml. The injection of saline produced no changes in plasma GH concentrations in pregnant (n = 4) or non-pregnant (n = 3) rats. Moreover no significant difference in plasma GH responses to hpGRF-44 was found between early (from Day 7 to Day 8, n = 5) and late (from Day 15 to Day 16, n = 4) pregnant rats. The data indicate that the plasma GH levels following the administration of hpGRF-44 were not different between pregnant and non-pregnant rats, though the pregnant pituitary responses to hpGRF-44 were delayed a little longer than those of the non-pregnant pituitary.