Persistence of somatostatinergic tone in acromegaly

Growth hormone response to the hypothalamic somatostatinergic activity in acromegalic patients

Oral glucose administration suppresses the TRH-induced TSH response by enhancing the hypothalamic somatostatinergic activity (HSA). We assessed the HSA in 13 acromegalic patients by measuring glucose-induced suppression of TRH-stimulated TSH secretion. The HSA showed wide variation with up to 64% suppression. The mean HSA of the patients (25 +/- 6%) did not differ from that in normal young men (19 +/- 4%) in our previous study. Six patients had normal or low HSA, and the other 7 patients had high HSA. The mean TRH-stimulated TSH levels of the patients with normal or low HSA was significantly lower than that of the patients with high HSA (5.13 +/- 0.10 vs. 11.30 +/- 2.80 mU/L). The HSA did not relate to sex, age, tumor size, basal GH levels, the paradoxical responses to TRH and GnRH, octreotide response, or the gsp oncogene. In the combined glucose-TRH test, glucose pretreatment completely suppressed the paradoxical increase in GH level at 30 min in 4 patients. However, it could suppress the paradoxical GH response by only 6-51% in the other 5 patients who also showed the paradoxical response in TRH test. The tumor diameter of patients with good response to the HSA was significantly larger than that of the patients with poor response (31 +/- 5 vs. 14 +/- 2 mm) as was the tumor grade (3.3 +/- 0.3 vs. 1.7 +/- 0.2). This study supports the idea that a reduction of HSA is not a primary cause of acromegaly, and the HSA seems to increase to suppress the autonomous secretion of GH from the pituitary adenomas. HSA as well as the response of tumors to HSA do not determine tumor growth.