Abstract-We tested the hypothesis that the status of NO synthesis influences the renal heme-heme oxygenase system.Studies were conducted in untreated rats and rats treated with the NO synthesis inhibitor N G -nitro-L-arginine methyl ester for 2 days. Treated and untreated rats were contrasted in terms of renal expression of heme oxygenase-1 and -2, renal carbon monoxide (CO)-generating activity, and urinary CO concentration and excretion rate. Heme oxygenase-1 and -2 proteins were similarly expressed in the kidneys of untreated and treated rats. In contrast, the NADPH-dependent component of the CO-generating activity of renal homogenates incubated with heme (a measure of heme oxygenase activity) was higher (PϽ0.05) in kidneys from rats treated with the NO synthesis inhibitor relative to corresponding data in untreated rats (1015Ϯ95 versus 379Ϯ111 pmol CO/mg per hour). Similarly, relative to corresponding data in untreated rats, rats treated with the NO synthesis inhibitor displayed increased (PϽ0.05) urinary CO concentration (920Ϯ174 versus 2286Ϯ472 pmol/mL) and urinary CO excretion (4.7Ϯ0.4 versus 14.3Ϯ2.7 pmol/min). This study demonstrates that NO synthesis inhibition upregulates the urinary concentration and excretion rate of CO, and the HO-dependent generation of CO by renal homogenates, without affecting the expression of renal heme oxygenase isoforms. Our findings imply that endogenous NO is an inhibitory regulator of renal CO generation by HO. Key Words: kidney Ⅲ carbon monoxide Ⅲ nitric oxide Ⅲ heme oxygenase C arbon monoxide (CO), a product of heme metabolism by heme oxygenase isoforms (HO)-1 and -2, 1 has been linked to the regulation of arterial tone and/or reactivity. 2,3 Nitric oxide (NO), a product of L-arginine metabolism by NO synthase isoforms (NOS), is a major contributor to mechanisms of vasodilation in several vascular beds. 4 The heme-HO and the L-arginine-NOS pathways interact at multiple sites and influence each other's level of activity and function. 1,[5][6][7] On one hand, ex vivo studies indicate that CO inhibits NOS activity and attenuates the expression of vasodilatory mechanisms mediated by NO. 8 On the other hand, NO decreases the catalytic activity of HO, 9,10 promotes HO-1 protein expression 6,7,11 and cellular uptake of heme, 6 and interferes with the ability of CO to stimulate large conductance Ca ϩϩ -activated K ϩ channels in vascular smooth muscle cells 12 and produce vasodilation. 5,13 It is difficult to predict the impact of variations in NO synthesis on the activity of the heme-HO system, because NO downregulates the activity of constitutively-expressed HO-2 while upregulating HO-1 protein expression. 6,7,10,11 Information on this point is relevant to the notion that the status of NO synthesis conditions the vasomotor response to HO inhibition in gracilis muscle arterioles and renal interlobular arteries ex vivo, and in the rat kidney and hind limb in vivo. 5,14 For example, after NOS inhibition, an increase in HO product generation may help condition the associated intens...

Pulmonary intralobar arteries express heme oxygenase (HO)-1 and -2 and release carbon monoxide (CO) during incubation in Krebs buffer. Acute hypoxia elicits isometric tension development (0.77 +/- 0.06 mN/mm) in pulmonary vascular rings treated with 15 micromol/l chromium mesoporphyrin (CrMP), an inhibitor of HO-dependent CO synthesis, but has no effect in untreated vessels. Acute hypoxia also induces contraction of pulmonary vessels taken from rats injected with HO-2 antisense oligodeoxynucleotides (ODN), which decrease pulmonary HO-2 vascular expression and CO release. Hypoxia-induced contraction of vessels treated with CrMP is attenuated (P < 0.05) by endothelium removal, by CO (1-100 micromol/l) in the bathing buffer, and by endothelin-1 (ET-1) receptor blockade with L-754142 (10 micromol/l). CrMP increases ET-1 levels in pulmonary intralobar arteries, particularly during incubation in hypooxygenated media. CrMP also causes a leftward shift in the concentration-response curve to ET-1, which is offset by exogenous CO. In anesthetized rats, pretreatment with CrMP (40 micromol/kg iv) intensifies the elevation of pulmonary artery pressure elicited by breathing a hypoxic gas mixture. However, acute hypoxia does not elicit augmentation of pulmonary arterial pressure in rats pretreated concurrently with CrMP and the ET-1 receptor antagonist L-745142 (15 mg/kg iv). These data suggest that a product of HO activity, most likely CO, inhibits hypoxia-induced pulmonary vasoconstriction by reducing ET-1 vascular levels and sensitivity.

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