Role of constitutively expressed heme oxygenase-2 in the regulation of guinea pig coronary artery tone

Abstract

Carbon monoxide (CO) is well known as a relaxing substance in the vasculature, where it is released during the heme oxygenase (HO) reaction. Little is known about the tissue-specific targets of CO in smooth muscles. To date the functional role of CO in the coronary artery remains unclear. The expression of HO-2, the constitutive isoform of HO, but not of HO-1 (inducible HO isoform) was demonstrated by immunohistochemical reaction. Contractile studies, performed under isometrical conditions, showed that CO, as well as hemin (given as a substrate for HO), relax de-endothelized coronary smooth muscle after the blockade of neuronal transmission. The action of hemin was antagonized by preliminary treatment of the vessel with SnPPIX--a competitive inhibitor of HO. The relaxatory effects of hemin were abolished in the presence of guanylyl-cyclase or protein kinase G antagonists. Patch-clamp studies revealed that hemin caused activation of iberiotoxin-blockable K outward current (I(K)) via guanylyl-cyclase and protein-kinase-G-dependent mechanisms. This activation coincided with hyperpolarization of the plasma membrane of single coronary smooth muscle cells by 8+/-3 mV, which was prevented by preliminary exposure of cells to 10 microM SnPPIX. The I(K)-augmenting effect of hemin was not affected by pretreatment of cells with cyclopiazonic acid and/or ryanodine, blockers of phospholipase C or heparin (applied via pipette), but was not observed when ATP was omitted from the dialyzing solution, or in the presence of Na-free, ATP-containing pipette solution. The omission of Ca(2+) from the bath or the replacement of Na with Li in both pipette and bath media also prevented the I(K)-activating effect of hemin. These results suggest that the constitutive HO-2 in coronary artery smooth muscle cells plays role in the modulation of tone. At the level of smooth muscle cells CO and its precursor hemin may cause hyperpolarization of the plasma membrane by activation of iberiotoxin-sensitive I(K) presumably via PKG-dependent activation of the Na/Ca exchanger. This activation is thought to increase the submembrane Ca(2+) concentration in the vicinity of large-conductance, Ca(2+)-sensitive K channels, thus causing voltage-dependent inhibition of Ca(2+) entry and subsequent relaxation of the vessel.

In vivo role of heme oxygenase in ischemic coronary vasodilation

“…Because CO is the active vasodilator produced by HO, we assume that this gaseous mediator is involved in dilation, although we did not measure CO production directly. A role for HO-2 in the regulation of coronary vasomotor reactions has been established in isolated coronary arteries (18). These investigators also found that CO-mediated dilation was mediated via cGMP and cGMP-dependent protein kinase modification of the K Ca channel.…”

“…CO is reported to modulate the activity of K Ca channels (47). In isolated coronary arterioles, CO produced dilation via activation of a pathway involving GC, cGMP-dependent protein kinase, and K Ca channels (18). Our protocols did not evaluate the contribution of K ϩ channels to CO-induced vasodilation in this setting, but such a contribution would have been eliminated by the KCl suffusion.…”

“…Thus CO dilates microvessels during myocardial ischemia via cGMP production. CO can also relax smooth muscle cells via activation of K Ca channels (18). Contribution of these two mechanisms, cGMP or K ϩ channel dependence, seems different between species and vascular preparations.…”

CO metabolism, sensing, and signaling

“…The BK Ca channel activity is subjected to modulation by a wide spectrum of biologically relevant factors such as Ser/Thr/Tyr phosphorylation, Cys/Met oxidation, steroid hormones, and diatomic gases (O 2 , NO, and CO) [111][112][113][114]. The PKG-dependent activation of the Na þ /Ca 2þ exchanger by CO has been proposed [12]: this may increase the submembrane Ca 2þ concentration in the vicinity of the BK Ca channels with their consequential opening [115]. An activated cGMP pathway may be permissive for BK Ca channels activation by CO [12].…”

Heme-arginate suppresses phospholipase C and oxidative stress in the mesenteric arterioles of mineralcorticoid-induced hypertensive rats

“…11,41 Consistently, the suppression of the PLC-IP 3 pathway observed in our study is in agreement with reports showing the attenuation of PLC activity in isolated guinea pig coronary smooth muscle cells in vitro. 46,47 The simultaneous reduction of PLC activity, ET-1 and NFkB by an upregulated HO system will attenuate tissue inflammation. 44,45,48 This notion is consistent with previous studies in which an upregulated HO system suppressed immune/inflammatory responses by abolishing macrophage infiltration and reducing mast-cell and basophils histamine release.…”