Growth hormone (GH) secretion during continuous infusion of GH-releasing peptide: partial response attenuation

The synthetic hexapeptide GH-releasing peptide (GHRP; SK&F 110679) specifically stimulates GH release in man. To determine the effect of a continuous GHRP infusion and whether response attenuation occurs in man, we administered to six healthy subjects a 6-h infusion of saline and three doses of GHRP, each followed by a 1.0 micrograms/kg bolus injection. GH was measured every 10 min using an immunoradiometric assay. During the saline infusion, spontaneous GH peaks occurred at variable times in four of the six subjects. During the continuous GHRP infusion, a single burst of GH release occurred with the two lower doses (0.1 and 0.3 micrograms/kg.h). With the highest dose of 1.0 micrograms/kg.h, a primary burst of GH release was followed by sporadic secretory episodes of lesser magnitude during the infusion; the GH concentrations remained above baseline before administration of the iv GHRP bolus in all six subjects. The mass of GH secreted was indirectly determined using waveform-independent deconvolution analysis. Mean GH secretion rates (micrograms per L distribution volume/h) were calculated by dividing the GH mass by the time interval. The GH secretion rates during the infusion period (0900-1430 h) were 2.40 +/- 0.68, 2.47 +/- 0.61, 7.67 +/- 1.86, and 14.75 +/- 2.32 on the saline and GHRP (0.1, 0.3, and 1.0 micrograms/kg.h) infusion days, respectively (P less than 0.05, 1.0 micrograms/kg.h vs. saline). The GH secretion rates after the iv GHRP bolus were 18.28 +/- 3.81, 19.01 +/- 2.03, 11.70 +/- 2.55, and 7.86 +/- 0.80 on the saline and GHRP (0.1, 0.3, and 1.0 micrograms/kg.h) infusion days, respectively (P less than 0.05, 1.0 micrograms/kg.h vs. saline). Compared with the saline infusion, the GH response to GHRP infusions was dose dependent (r = 0.81; P less than 0.001). The GH response to the iv bolus was inversely related to the dose of the preceding 5.5-h continuous GHRP infusion (r = -0.58; P = 0.003), and the total amount of GH secreted (constant infusion plus the bolus infusion periods) was not different among the GHRP doses. Constant GHRP infusion stimulates GH release in man, and partial response attenuation occurs with a subsequent 1.0 micrograms/kg GHRP bolus. We hypothesize that GHRP is active at multiple sites and may act as a functional somatostatin antagonist. Further studies are needed to better determine the site(s) of GHRP action and its potential use as a diagnostic and therapeutic agent.