Growth hormone refractory interval to levodopa stimulation.

The GH response to repeat l-dopa stimulation was studied in three phases. In phase I and phase II, 16 normal adults divided into four groups were restimulated at 3, 4, 5, and 6 hours with 500 mg l-dopa and 1000 mg l-dopa, respectively. No response could be elicited at 3 hours and only at 6 hours (500 mg l-dopa) and 5 hours (1000 mg l-dopa) did the repeat responses return in magnitude to that of the initial. The period of decreased responsiveness to GH was terned the "refractory interval". In phase III, addition of an inhibitor of peripheral L-aromatic amino acid decarboxylase (50 mg MK-486) to the restimulation dose shortened but did not eliminate the refractoriness. Our results suggest that the refractory interval is not due to a peripherally located mechanism.