SOMATOSTATIN WITHDRAWAL ALONE IS AN INEFFECTIVE GENERATOR OF PULSATILE GROWTH-HORMONE RELEASE IN MAN

To assess the relative roles of growth hormone-releasing hormone (GHRH ) pulse and somatostatin withdrawal as potential generators of pulsati le growth hormone (GH) release in humans, we studied GH responses to i v bolus GHRH (1 mug/kg) and to termination of a 4 h iv somatostatin in fusion (7.2 mug.kg-1.h-1) in five normal young men, and in five men wi th previously diagnosed isolated GH deficiency. The patients were diag nosed 8-15 years previously on the basis of typical auxological and ho rmonal criteria, were treated with exogenous GH and were off GH therap y for 1.5-8.9 years prior to this study, Growth hormone rises to a bol us GHRH were similar between the controls and the patients (maximum GH 27.3 +/- 15.3 vs 8.0 +/- 4.0 mug/l). The controls exhibited only a sm all GH rise to somatostatin withdrawal (maximum GH 2.9 +/- 1.2 mug/l), while the patients did not (maximum GH 0.7 +/- 0.1 mug/l: p < 0.05). We conclude that somatostatin withdrawal by itself is an ineffective p romoter of GH pulsatility. Periodic quiescence of somatostatinergic ne urons must be associated with a concomitant GHRH pulse in order to res ult in a robust GH pulse.