Effect of antiserum to rat growth hormone (GH)-releasing factor on physiological GH secretion in the female rat.

To further study the physiological role of GH-releasing factor (GRF), we examined the effect of antiserum to rat hypothalamic GRF on spontaneous GH secretion in the normal female rat. Two groups of six conscious female rats were passively immunized with either nonimmune rabbit serum (NRS) or antirat GRF serum via a chronic indwelling atrial catheter. The secretory profiles of GH were observed by collecting blood samples at 15-min intervals for 1 h before and 4 h after administration. The NRS-treated rats showed a characteristic female pattern of spontaneous GH secretion. GH pulses were of low amplitude (mean +/- SEM, 26.8 +/- 2.4 ng/ml) and occurred irregularly at a frequency of 4.2 +/- 0.2/5 h, while interpeak through levels of GH were relatively high, with nadir values of 8.6 +/- 0.7 ng/ml. Synthetic rat GRF, given iv at a dose of 1 microgram/kg BW after the last blood sampling, stimulated GH release to a peak level of 153 +/- 37 ng/ml in the control rats. Administration of GRF antiserum caused a profound suppression of both pulse and trough components of GH secretion. This effect occurred rapidly, within 15 min after injection of antiserum, and GH secretion decreased uniformly to very low levels (3.4 +/- 0.1 ng/ml), with little or no fluctuation throughout the observation period. GRF antiserum also abolished the synthetic rat GRF-induced GH release, indicating sufficient potency of immunoneutralization. These results demonstrate that both GH pulses and troughs are dependent upon hypothalamic GRF in normal female rats, thereby substantiating earlier observations in male rats which demonstrated the physiological role of GRF in GH secretion.