Suppressibility of Plasma Growth Hormone Levels in Acromegaly with Dexamethasone and Phentolamine¹

Suppressibility of plasma GH levels by 2 GH suppressants, dexamethasone and phentolamine, was studied in a group of 17 patients of acromegaly. Dexamethasone, 9 mg daily for 2 days, suppressed the basal plasma GH levels by more than 30% in 10 out of 15 cases, while, phentolamine, 50 mg in 90 min, by more than 40% in 6 of 12 patients. The suppressibility in 8 cases examined with these 2 agents was significantly correlated (r = +0.82). This suppressibility also negatively correlated with the estimated size of the sella turcica within the group of responders (r = +0.91), but not in a whole group of acromegaly (r = +0.01). On the other hand, the changes of plasma GH level with oral loading of 100 g glucose did not correlate with those with dexamethasone or phentolamine. Sustained hyperglycemia induced by iv infusion of glucose solution decreased plasma GH level by 58% in one case whose plasma GH was not suppressed with oral glucose loading. The increase of plasma GH with insulin-induced hypoglycemia and the decrease with dexamethasone also significantly correlated (r = +0.62), though 2 exceptional cases were found. The responses to insulin-induced hypoglycemia were not suppressed with dexamethasone in acromegalic patients. These results further confirm the existence of 2 groups in acromegaly and the usefulness of suppression tests in differentiating them.