Growth hormone and prolactin responses to bolus and sustained infusions of GRH-1-40-OH in man

To determine whether GRH stimulates PRL secretion we studied the effects of iv bolus injections and prolonged infusions of GRH 1-40-OH on PRL and GH serum levels in normal volunteers. Eight patients with acromegaly, two of whom had elevated basal levels of PRL, were also tested with single bolus injections. Six normal subjects given 3.3 μg/kg bolus injections of GRH showed a mean increment of GH of 22.0 ± 1.7 ng/ml (mean ± SE). A small rise in PRL was noted in 5 of the 6 subjects (mean peak level of 6.4 ± 1.9 ng/ml vs basal level of 3.3 ± 0.4 ng/ml, p < 0.05). During the continuous infusion of GRH (10 ng/kg/min), GH levels rose gradually from a mean baseline of 1.1 ± 0.1 ng/ml to a mean peak of 30.0 ± 7.2 ng/ml at about 2 h and then slowly declined to a nadir of 4.2 ± 0.4 ng/ml at 330 min. PRL levels did not rise significantly during the infusion. To determine whether the decline in GH levels in the face of continued infusion was due to loss of GH responsiveness, a 3.3 μg/kg bolus of GRH was given during the nadir at 330 min; this GH increment was significantly less than that obtained by the GRH bolus injection without the infusion (12.9 ± 3.5 ng/ml vs 22.0 ± 1.7 ng/ml, p < 0.05). The PRL response to the GRH bolus was the same during the infusion of GRH as before. In each of 8 acromegalic patients (including two who had initially elevated basal PRL levels) GRH led to an increase in both GH and PRL levels. PRL and GH levels spontaneously fluctuated in parallel in 4 acromegalic cases studied with repeated samples over 6 h during placebo administration. These experiments show that GRH has significant, though weak, PRF effect in normals and that it is a more potent PRF in acromegalic patients. Furthermore, the effects on GH and PRL of a sustained infusion of GRH for 5 1/2 h are both qualitatively and quantitatively different. These results suggest that the GRH effect is exerted either on different pituitary receptors for GH and PRL regulation, or that the releasable pools of the two hormones have different sizes and/or turnover times.