Effect of thyrotropin-releasing hormone on dog thyroid in vitro

The in vitro action of thyrotropin-releasing hormone (TRH) on the cyclic AMP level and iodine metabolism in dog thyroid, has been studied. TRH inhibited cyclic AMP accumulation and subsequent secretion in slices stimulated by thyrotropic hormone (TSH), prostaglandin E₁, cholera toxin and to a lesser extent forskolin. The effect of TRH was suppressed in a medium deprived of calcium or in the presence of isobutylmethylxanthine. TRH also stimulated iodide binding to proteins, but not cyclic GMP accumulation. Although all these characteristics of TRH action on dog thyroid fit those of prostaglandin F_1α in this tissue, TRH effects were not relieved by indomethacine. The possibility of a TRH action through other known inhibitors of the cyclic AMP system in dog thyroid such as: acetylcholine, α-adrenergic agents, adenosine, iodide were checked and ruled out. The possible involvement of other neurotransmitters, such as ATP or vasoactive intestinal peptide were studied but could not be substantiated. Our data suggest the existence of a direct negative action of TRH on the thyroid itself besides its stimulatory role at the pituitary level. The great variability of the TRH effect was overcome by pretreatment of the dog by pyridostigmine, an acetylcholinesterase inhibitor.