Plasma growth hormone responses to sulpiride in patients with acromegaly

The role of endogenous dopamine (DA) on plasma GH secretion was studied in 12 patients with active acromegaly. After intramuscular administration of 100 mg sulpiride, 3 out of 12 patients showed distinct increases in their plasma GH (cases 1-3: sulpiride-responders). The GH increases far exceeded the ranges of physiological fluctuation in two cases studied (case 1 & 3). After domperidone administration, case 3 showed a quite similar GH increase as after sulpiride. Plasma GH responses to TRH and sulpiride were still observed in case 2 even after the basal GH levels were normalized by the transsphenoidal hypophysectomy. The three sulpiride-responders showed so-called paradoxical GH decreases following the administration of L-dopa (500 mg, p.o.) or DA (5 micrograms/kg/min over 90 min, i.v.). However, the basal plasma PRL levels and the responses to sulpiride in these 3 cases were not homogeneous. These results indicate that endogenous DA has a tonic inhibitory effect on GH secretion in some cases of acromegaly, and that tumorous somatotrophs rather than elevated GH levels are the critical factor for such inhibition. Further, the sensitivities of somatotrophs and lactotrophs to DA appeared to be heterogeneous in acromegalic patients.