Objective: Previous studies have suggested that post-irradiation GH insufficiency results from a loss of GHRH secretion, since many patients were able to release GH following exogenous GHRH stimulation. However, supramaximal doses of GHRH were used and the response may decline with time after radiotherapy. We re-evaluated the GHRH dose-response curve in patients post cranial irradiation and in controls.
Design: Randomized controlled study.
Methods: Five adult male long-term survivors of childhood brain tumours (median age 21.8 years (18.4-26.7); 13.7 years (11.4-15.7) post-radiotherapy, >30Gy) and five matched controls were studied. An intravenous bolus of GHRH(1-29)NH(2) was administered in doses at the lower (0.05 microg/kg) and upper (0.15 microg/kg) range of the dose-response curves for young males, as well as the standard supramaximal dose (1. 0 microg/kg). GH was measured before stimulation, every 2min for the first hour and every 5min for the second hour. All studies were conducted in a random fashion.
Results: Significantly lower peak and area under the curve (AUC) GH concentrations occurred in the irradiated group using 0.15 microg/kg (median peak Irradiated, 4. 5mU/l vs median Controls, 37.4mU/l; P<0.01) and 1.0 microg/kg (median peak Irradiated, 4.8mU/l vs median Controls, 15.2mU/l; P<0. 05) GHRH(1-29)NH(2). In irradiated subjects there was an incremental rise in GH output with increasing doses of GHRH(1-29)NH(2 )(median AUC: 122mU/l.min vs 179mU/l.min vs 268mU/l.min; P=0.007) reflecting altered pituitary sensitivity and reduced responsiveness.
Conclusion: The GH response to bolus GHRH(1-29)NH(2) is attenuated in adult long-term survivors of childhood brain tumours. This may reflect direct pituitary damage and/or the loss of the tropic effects of chronic GHRH deficiency.