Thyrocyte release of asymmetric dimethylarginine does not account for human thyrocyte inhibition of endothelial cell cyclic GMP

Background: The thyroid gland produces and responds to the signalling molecule nitric oxide (NO). The activity of NO synthase (NOS) may be regulated by endogenous NOS inhibitors such as asymmetric dimethylarginine (ADMA). Objective: To investigate whether human thyrocytes are capable of regulating NOS activity via the production of ADMA. Design: Human thyrocytes were incubated with human umbilical vein endothelial cells (HUVEC) in order to determine the effect on HUVEC NOS activity. HUVEC cGMP production over a 3-h period was measured as an indicator of NOS activity in the absence and presence of thyrocytes. To determine thyrocyte production of ADMA, samples of conditioned media were analysed by HPLC. Results: The presence of primary human thyrocytes or immortalized human thyrocyte SGHTL-189 cells caused a significant inhibition of both basal (approximately 57% inhibition) and thrombinstimulated (approximately 42% inhibition) HUVEC cGMP production. Both primary human thyrocytes and SGHTL-189 cells released ADMA (approximately 0.28 mu g per 10(6) thyrocytes over a 3-day period). However, excess L-arginine, the natural substrate for NOS, was unable to overcome thyrocyte inhibition of HUVEC cGMP production. Conclusion: These data indicate that human thyrocytes potently reduce endothelial cell cGMP concentrations, and that thyrocytes produce the endogenous NOS inhibitor, ADMA. However, the inhibition of endothelial cGMP is not mediated via thyrocyte production of a competitive NOS inhibitor.