Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice

Abstract

. Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice. Am J Physiol Heart Circ Physiol 291: H223-H230, 2006. First published February 17, 2006 doi:10.1152/ajpheart.00058.2006 has been postulated to be a signaling molecule in many tissues, including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied and endogenously produced CO. The diameter of isolated, pressurized, and perfused rat middle cerebral arteries (MCAs) was not altered by authentic CO (10 Ϫ6 to 10 Ϫ4 M). Mouse MCAs, however, dilated by 21 Ϯ 10% at 10 Ϫ4 M CO. Authentic nitric oxide (NO⅐, 10 Ϫ10 to 10 Ϫ7 M) dilated both rat and mouse MCAs. At 10 Ϫ8 M NO⅐, rat vessels dilated by 84 Ϯ 4%, and at 10 Ϫ7 M NO⅐, mouse vessels dilated by 59 Ϯ 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor ␦-aminolevulinic acid (10 Ϫ10 to 10 Ϫ4 M) did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor chromium mesoporphyrin IX (CrMP) caused profound constriction of the rat MCA (44 Ϯ 2% at 3 ϫ 10 Ϫ5 M). Importantly, this constriction was unaltered by exogenous CO (10 Ϫ4 M) or CO plus 10 Ϫ5 M biliverdine (both HO products). In contrast, exogenous CO (10 Ϫ4 M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 Ϯ 1% in response to 10 Ϫ5 M CrMP. Magnesium protoporphyrin IX (10 Ϫ5 M), a weak HO inhibitor used to control for nonspecific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries and that metalloporphyrin HO inhibitors have nonspecific constrictor effects in rat cerebral arteries. cerebral arteries; chromium mesoporphyrin; endothelium-derived hyperpolarization factor; gracilis arteriole HEME OXYGENASE (HO) is the major enzyme responsible for degrading heme (29). Three isoforms of HO (HO-1, -2, and -3) catalyze the conversion of heme to carbon monoxide (CO), biliverdine IX␣, and iron (29,35,54). CO, in turn, has been postulated to be a signaling molecule in many tissues, including the vasculature (22, 29, 40 -42, 48, 54, 55, 58). HO-1 is not normally expressed in most tissues, although it may be upregulated by a wide variety of stimuli, including subarachnoid hemorrhage (45) and ischemia-reperfusion (38). HO-2 is constitutively expressed and is found in most tissues, including the brain, vascular smooth muscle, and endothelium (8,29,30,38,47,57). HO-3 may be constitutively expressed in the rat, although recent work suggests that it is a splice variant of HO-2 transcripts (16,31,54).CO binds to heme moieties such as those found in soluble guanylyl cyclase, nitric oxide (NO⅐) synthase, and cytochrome P-450 enzymes. Binding of CO to heme moieties can either increase or decrease enzymatic activity (7, 17, 22, 29, 40 -43, 46). Large-conductance calcium-dependent K ϩ channels can also be activated by CO (18,24,28,...

Cerebroprotective Functions of HO-2

“…In adult rats, topical CO caused dilation of rat pial arterioles, although maximal responses were smaller than in newborn piglets [11]. However, cerebral arteries isolated from mice, rats, and rabbits did not dilate to 10 -6 -10 -4 M CO [12,13]. Conversely, in another study, dog basilar artery segments did dilate to CO [14].…”

“…Although the measurements of extracellular concentration in CSF reflect the dynamics of overall CO production by the brain, they do not allow direct estimation of the intracellular level of CO. Brain CO production by the brain is rapidly activated (over 3-fold increase in [10][11][12][13][14][15][16][17][18][19][20] Overall, these experiments identify CO as a potent cerebral vasodilator endogenously produced by the brain and cerebral vasculature. Of particular significance is that immediate activation of brain HO is essential in maintaining blood flow to the brain during selected cerebrovascular insults, including epileptic seizures, hypoxia, and hypotension.…”

Inhibition of heme oxygenase augments tubular sodium reabsorption

“…HO-1 is induced by a variety of stimuli, such as ischemia, hypoxia, oxidative stress, inflammation, and hypertension, while HO-2 is constitutively present. These isoforms are present in the renal vasculature, as well as in other vascular beds (2,8,14,17,32). HO activity is partly regulated by substrate availability, thus both isoforms can increase endogenous CO levels (9, 11).…”

“…The renal vasculature and tubules express HO (1,5,10,18,31), and treatment of rats with HO inhibitors has been reported to decrease renal blood flow (RBF) (2,26). However, other investigators found that HO inhibition had no effect on basal renal hemodynamics (23).…”

“…It has also been reported that HO inhibition by zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) promotes regional differences in RBF by decreasing medullary blood flow with no significant changes in cortical blood flow (37). Furthermore, NO influences renal vascular responses to HO inhibition (2,26). Most of the studies examining the effects of HO blockade on renal hemodynamic function employed photosensitive inhibitors, such as stannious mesoporhyrin (SnMP) and ZnDPBG, that degrade in light (33).…”