Possible involvement of L-arginine-nitric oxide pathway in modulating regional blood flow to brown adipose tissue of rats.

To evaluate whether the L-arginine-nitric oxide (NO) pathway is involved in the regulation of regional blood flow to brown adipose tissue (BAT), the effects of two specific NO synthase inhibitors, NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), on the blood flow to interscapular brown adipose tissue (IBAT) were studied in urethane-anesthetized rats. Regional blood flow in IBAT was measured with laser-Doppler flowmetry. An intravenous injection of L-NAME and L-NMMA, but not of either D-enantiomer, caused a transient and dose-dependent increase in IBAT blood flow. Dose-response curves for these NO synthase inhibitors showed that L-NAME was more potent than L-NMMA in increasing IBAT blood flow. We also observed a concomitant pressor effect accompanied by a slight decrease in heart rate following intravenous injection of L-NAME and L-NMMA. An elevation of IBAT blood flow and blood pressure induced by both L-NAME and L-NMMA was reversed by L-arginine in an enantiomerically specific manner. The increase in IBAT blood flow induced by NO synthase inhibitors was of shorter duration and less sensitive to L-arginine than the increase in blood pressure. Our results show that the IBAT blood flow is increased by inhibition of NO synthase and that the response of IBAT vasculature to NO synthase inhibitors is different from that of the resistance vessels which regulate blood pressure. The involvement of L-arginine-NO pathways in modulating microcirculation in IBAT is suggested.