Reasons for the variability in growth hormone (GH) responses to GHRH challenge:the endogenous hypothalamic-somatotroph rhythm (HSR)

The aims of this study were: (1) to test the possibility that pre-GHRH plasma GH values could reflect the functional status of the hypothalamic-somatotroph rhythm (HSR) at testing, and thus explain if it is responsible for the marked variability in GH responsiveness to GHRH challenge and (2) to see if exogenous somatostatin (SS) could disrupt this endogenous HSR and thus make the GH responses homogeneous. (1) Two to 14 GHRH acute tests (GRF-29, 1 µg/kg, i.v. bolus) were performed in 12 normal men and 10 normal women at the same time (0830 h) at random intervals (2 to 60 days). Blood samples to measure plasma GH were drawn at 15 min intervals before and after GHRH challenge. Given that the increments in pre-GHRH plasma GH values (I = value at 0 min minus value at -15 min) were highly correlated with either GHRH-elicited peaks of GH (men, r = 0.81; women, r = 0.69; P