Potency and Specificity of a Growth Hormone-Releasing Factor in a Primate and in Vitro*

The potency and specificity of the 44-amino acid human pancreatic tumor GRF were tested in six adult female rhesus monkeys and in a perifusion system containing a suspension of rat pituicytes. In vivo, plasma GH levels were elevated in a dose-dependent fashion, with an ED50 of approximately 5 µg/kg, a value of the same order of magnitude as other hypothalamic releasing hormones. The magnitude of the GH response after GRF treatment was similar to that observed during insulininduced hypoglycemia, with peak plasma GH concentrations occurring 5–15 min after GRF administration. High doses of GRF slightly stimulated PRL release, but had no effect on arterial blood pressure, heart rate, or plasma cortisol or glucose concentrations. In vitro, GRF released GH in a dose-dependent manner, but no PRL was released even at the highest GRF concentrations employed (100 nM). It thus appears that stimulation of PRL in vivo may be an indirect effect of GRF. Alternatively, there may be species differences in responsiveness to GRF.