The Effect of l-Dopa on Plasma Growth Hormone, Insulin, and Thyroxine 

PRAKASH C. KANSAL, JOHN BUSE, O. RHETT TALBERT, MARIA G. BUSE

Abstract

The effect of l-dopa on glucose, insulin, growth hormone, and thyroxin in plasma was studied in 8 normal subjects and in 7 patients with Parkinson’s disease.

Growth hormone increased 60–110 min following a single oral dose of 500 mg of l-dopa. Stimulation of growth hormone release by l-dopa was inhibited by the iv infusion of phentolamine, an α-adrenergic blocking agent. The single dose of l-dopa had no effect on the concentration of glucose, insulin, or thyroxine.

In patients with Parkinson’s disease, treated with 2–3 g l-dopa daily for 2–3 weeks, fasting levels of glucose and growth hormone remained unchanged. The “k” of glucose disappearance and insulin levels in plasma after an iv glucose load were not significantly affected. Fasting plasma insulin and thyroxine increased during l-dopa therapy. Maximum plasma growth hormone levels after iv insulin were less during l-dopa treatment than during the control period.

Our results confirm that a single dose of l-dopa stimulates growth hormone secretion. This stimulatory effect appears to be mediated through adrenergic α-receptors.