Plasma growth hormone response to L-dopa in obese subjects

Abstract

The growth hormone response to oral administration of 500 or 1000 mg of L-dopa was investigated in nine obese subjects, none of whom showed evidence of glucose intolerance. Growth hormone and cortisol responsiveness to insulin hypoglycemia was also tested in these subjects. A significant growth hormone response following L-dopa administration was seen in only two subjects. Five subjects displayed normal growth hormone responsiveness following insulin-induced hypoglycemia, and normal increments of plasma cortisol levels following induction of such hypoglycemia were seen in six of the eight subjects in whom this was studied. Only one of the subjects with a normal growth hormone response following L-dopa administration exhibited a normal response to hypoglycemia. Increasing the dose of L-dopa in a given subject did not enhance growth hormone responsiveness. No consistent cortisol response to either dose of L-dopa was noted. L-dopa therefore, is still another stimulus, which, while effective in normal subjects, is ineffective with regard to eliciting growth hormone release in obese subjects. It appears to be less effective than hypoglycemia in this regard. The lack of responsiveness to L-dopa in these obese subjects would imply that their decreased sensitivity to other stimuli capable of provoking growth hormone release in normal subjects is not secondary to decreased levels of the hypothalamic neuro transmitter agent(s) believed to be involved in growth hormone regulation.