Cross-interactions between CO and NO signaling in microvascular endothelium

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Carbon monoxide (CO) is synthesized endogenously by heme oxygenase (HO) which degrades heme to biliverdin-IXα. Like nitric oxide (NO), CO has a vasodilatory property in the liver where endogenous NO production appears low. However, in the brain producing relatively high NO, how these gaseous monoxides interact each other to control cerebrovascular tone is unknown. Here we show that CO derived from HO acts as a tonic regulator against NO-dependent vasodilatation in the adult rat brain. We found that suppressing endogenous CO caused an increase in arteriolar diameter that was accompanied by an increase in local NO generation. We examined further if such an inhibitory function of CO on NO generation is mediated through the ability of CO to bind the prosthetic heme of gas-producing enzymes. Using cultured porcine aortic endothelial cells (PAECs), we measured NO generation by an NO-specific fluorescent dye. Treatment with CO-releasing molecule attenuated the time-dependent elevation of NO generation. This effect of CO to suppress NO generation was cancelled by exposing PAECs to white light raising a possibility that CO directly targets The ferrous heme of NOS3 to inhibit the catalytic activity. Furthermore, immunohistochemical analyses of the rat brain showed that HO-2 occurred in neurons and arachnoid trabecullar cells where NOS1 resided, and also in vascular endothelium expressing NOS3, suggesting colocalization of CO-and NO-generating sites. Our results demonstrate an importance of spatial relationship among the gas-producing enzymes and their reception systems in evaluating roles of the two gases.