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 re gulation of thyroid stimulating hormone (TSH) secretion in humans, sev en normal men were treated with a placebo (normal saline) or the NO sy nthase 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 b lood 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 chan ge the basal secretion of TSH or the TSH response to MCP, but signific antly reduced the TSH increase induced by TRH. These data fail to prov ide evidence of NO involvement in regulation of basal TSH secretion. N O also appears to be without effects on the dopaminergic control of TS H secretion. In contrast, the inhibitory effect of L-NAME on TRH-induc ed TSH secretion suggests the mediation by NO of the TSH-releasing act ion of TRH.