Thyrotropin-releasing hormone stimulates nitric oxide release from GH(3) cells

Constitutive nitric oxide synthase (NOS) is expressed in the rat adenohypop hysis but the mechanisms regulating its activity at the cellular level rema in to be elucidated. The effect of TRH on nitric oxide release from GH(3) c ells was studied by means of reverse-phase HPLC to measure NO2- and NO3- co ncentrations in the incubation medium, and by polarography using electrodes specific for NO. Medium NO2- concentrations in the incubation medium were dependent on the incubation time, and were further increased by sodium nitr oprusside (SNP) or high potassium. NO3- was detectable only in the presence of 100 mu M SNP. Addition of L-arginine increased medium NO2- concentratio ns. Diamino-hydroxypyrimidine decreased medium NO2- concentrations, which w ere restored by the addition of (6R)-5, 6, 7, 8-tetrahydro-L-biopterin (THB ). TRH elicited dose-related increases in medium NO2- concentrations and in nitric oxide-specific currents, which were abolished by N-omega-nitro-L-ar ginine methyl ester, TRH failed to increase medium NO2- concentrations in c ells loaded with an intracellular Ca2+-chelating agent. The findings sugges t that mobilization of intracellular Ca2+ by TRH stimulation activates Ca2-dependent NOS in GH(3) cells.