Somatostatin withdrawal as generator of pulsatile GH release in the dog: a possible tool to evaluate the endogenous GHRH tone?

Reportedly, somatostatin (SS) withdrawal is an effective generator of pulsatile GH release in mammals and it has been proposed that the amplitude of the GH bursts is related to the functional activity of GHRH-producing neurons. Our study was designed to test this hypothesis in the unanesthetized dog, under different conditions of endogenous GHRH function. First, we evaluated the ability of withdrawal of SS infusion to induce a GH secretory burst under basal conditions when GHRH function is thought to be enhanced, i.e. in young (2- to 3-year-old) dogs under sustained (30 days) caloric restriction (CR) or a 2-day fast. Secondly, we performed experiments in aged (11- to 17-year-old) dogs, in which hypothalamic GHRH secretion is thought to be reduced. Old dogs were evaluated under basal conditions, after a 2-day fast and after a 10-day administration of GHRH alone or followed by fasting. Both before and 14 h after the end of each experimental period, young and old dogs underwent a 3-hour (from 10.00 to 13.00 h) intravenous SS infusion (4 mu g . kg(-1). h(-1)). The secretory profile of GH was generated by 15-min sampling from 09.00 to 15.00 h. Under baseline conditions, SS withdrawal induced a significant burst of GH in young but not in old dogs. After CR, termination of SS infusion was followed in young dogs by a robust GH burst, significantly higher than that observed when dogs were fed ad Libitum. In this instance, reduction of plasma IGF-I concentrations was unlikely to be responsible for the higher GH burst; the same pattern was present in the young dogs after a 2-day fast, when circulating IGF-I was unaltered. In old dogs, SS withdrawal did not modify baseline GH levels even after fasting, but induced a significant GH increase after GHRH priming. When GHRH priming was followed by fasting, SS withdrawal resulted in a GH burst higher than that occurring after fasting or GHRH alone. Altogether, these data support the view that the rebound rise in GH induced by withdrawal of SS is related to the endogenous GHRH tone. It is suggested that extrapolation of these findings to humans might permit probing, albeit inferentially, the endogenous GHRH tone under different physiologic or pathologic conditions.