Abstract
The role of dopaminergic agents (DA) in the regulation of growth hormone (GH) secretion was investigated in patients with untreated acromegaly. TRH (0.5 mg iv), bromocriptine (Br) (2.5 mg orally) or L-Dopa (500 mg orally) loading tests were performed, and serum levels of TSH, GH and prolactin (PRL) were measured. Patients were defined as responders to TRH when peak TSH level after TRH test was higher than 5 microU/ml. Br or L-Dopa was considered to be effective when serum GH or PRL levels were suppressed more than 50% of the basal value. The patients were classified into large adenoma group with suprasellar extension or cisternal herniation (L group, n = 7) and intrasellar small adenoma group (S group, n = 11) which was further divided into TRH responder (Sr group, n = 4) and TRH non-responder with suppressed TSH (Ss group, n = 7). Br was effective in 7 or 100% of 7 patients in the Ss group but only in one or 25% of 4 patients in the Sr group. Br was also effective in 5 or 71% of 7 patients in the L group, although most of them were responders to TRH. Percent inhibition of serum GH levels by Br was significantly higher in the Ss group (82.3 +/- 12.3%, p less than 0.001) and in the L group (64.7 +/- 20.5%, p less than 0.05) compared with that in the Sr group (29.3 +/- 21.6%). Suppression of serum GH level by L-Dopa was also observed in the Ss group. In contrast to the difference in the response of GH, serum PRL level was equally suppressed by Br or L-Dopa in each group. Suppression of TSH by administration of exogenous T4 had no effect on the GH suppression effect of Br in the Sr group. Considering the dual effects of DA to enhance growth hormone-releasing hormone (GHRH) secretion in the hypothalamus and to suppress GH secretion in the pituitary gland, these findings suggest that the paradoxical effect of DA to suppress serum GH level is observed when the hypothalamo-pituitary axis is disturbed mechanically by large adenoma in the L group or functionally in the Ss group probably due to enhanced secretion of somatostatin which suppresses TSH secretion and impairs the effect of GHRH.
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