MEDIATION BY NITRIC-OXIDE OF TRH-STIMULATED, BUT NOT METOCLOPRAMIDE-STIMULATED TSH SECRETION IN HUMANS

IN order to establish whether nitric oxide (NO) participates in the regulation of thyroid stimulating hormone (TSH) secretion in humans, seven normal men were treated with a placebo (normal saline) or the NO synthase inhibitor L-NAME, given at doses (40 mu g kg(-1) injected plus 50 mu g kg(-1) infused i.v.) previously found to be unable to change blood pressure. Experiments were carried out either in basal conditions or during stimulation of TSH secretion with an i.v. injection of 200 mu g thyrotropin releasing hormone (TRH) or 10 mg of the dopaminergic antagonist metoclopramide (MCP). Administration of L-NAME did not change the basal secretion of TSH or the TSH response to MCP, but significantly reduced the TSH increase induced by TRH. These data fail to provide evidence of NO involvement in regulation of basal TSH secretion. NO also appears to be without effects on the dopaminergic control of TSH secretion. In contrast, the inhibitory effect of L-NAME on TRH-induced TSH secretion suggests the mediation by NO of the TSH-releasing action of TRH.