Different Responses of Growth Hormone Secretion to Guanfacine, Bromocriptine, and Thyrotropin-Releasing Hormone in Acromegalic Patients with Pure Growth Hormone (GH)-Containing and Mixed GH/Prolactin- Containing Pituitary Adenomas*

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Abstract
There is great variability in the GH secretory responses to different stimuli in patients with acromegaly. In the present study, we compared the effects on GH secretion of two compounds (bromocriptine and TRH), which presumably act directly at the pituitary level, with the effect of the centrally acting α-adrenergic agonist guanfacine in14 untreated acromegalic patients. These in vivo responses of GHrelease were correlated with the results of immunocytochemical studies of the pituitary adenomas. In nine patients with pure GH-containing adenomas, GH secretion was suppressedby bromocriptine by more than 50% in one patient, while TRH stimulated GH releaseby more than 100% in another patient. Guanfacine (2 mg, orally) did not elicit a change in circulating GH levels in any of these nine patients. In the group of five patients with mixed GH/PRL-containing adenomas, however, bromocriptine suppressed GH levels by more than 50% in all patients, and TRH stimulated GH release by more than 100% in four of them. Guanfacine stimulated GH secretion significantly in four of these five patients. Guanfacine inhibited GH secretion significantly in five other acromegalic patients who had been treated 5–10 yr previously by external pituitary irradiation. We conclude that in acromegaly, the presence of PRL within the GH-secreting pituitary adenoma makes GH secretion more sensitive to bromocriptine and TRH, while normal sensitivity to hypothalamus-mediated stimulation (α-adrenergic agonist) is retained to some extent. In contrast, pure GH-secreting tumors responded little or not at all to bromocriptine, TRH, or guanfacine.

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