INDUCIBLE NITRIC-OXIDE SYNTHASE IN RAT BROWN ADIPOCYTES - IMPLICATIONS FOR BLOOD-FLOW TO BROWN ADIPOSE-TISSUE

Exposure of rat brown adipocytes differentiated in culture to norepine phrine (NE) results in the production of nitrites (NO2-), the breakdow n product of nitric oxide (NO). This production, which is blocked by a ctinomycin D, is directly related to the duration of exposure to and d ose of NE. Cytosol from NE-treated brown fat cells, but not from untre ated cultures, catalyzed the Ca2+-independent conversion of L-arginine to L-citrulline, which could be significantly blocked by the specific nitric oxide synthase (NOS) inhibitor N-G-nitro-L-arginine methyl est er. Reverse transcriptase-PCR demonstrates that the addition of NE; se lective beta(1)-, beta(2)-, or beta(2)-adrenergic receptor agonists; o r agents increasing cAMP production, such as forskolin, to brown adipo cytes stimulates inducible NOS (iNOS) messenger RNA, which is present within 4 h after exposure. That iNOS is synthesized in brown fat cells is confirmed by immunoblotting using an antibody to the iNOS of mouse macrophages. Finally, in both brown adipose tissue (BAT) and brown ad ipocyte preparations from animals exposed to low temperature, iNOS mes senger RNA and protein were expressed, and NOS activity was detectable ; these findings were unlikely for room temperature-acclimated rats. W e conclude that brown fat cells can express an inducible form of NOS s imilar to the iNOS of macrophages, and that its production is directly dependent on sympathetic activity in physiological conditions. NO gen erated by stimulation of iNOS in brown adipocytes may represent an imp ortant mechanism to modulate different BAT functions, among which is v asodilation of the BAT microcirculation.