Reduced Vasorelaxant Effect of Carbon Monoxide in Diabetes and the Underlying Mechanisms

Abstract

Carbon monoxide (CO) is an endogenous gaseous factor that relaxes vascular tissues by acting on both the cGMP pathway and calcium-activated K + (K Ca ) channels. Whether the vascular effect of CO is altered in diabetes had been unknown. It was found that the CO-induced relaxation of tail artery tissues from streptozotocininduced diabetic rats was significantly decreased as compared with that of nondiabetic control rats. The blockade of the cGMP pathway with ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one) completely abolished the CO-induced relaxation of diabetic tissues but only partially inhibited the CO effect in normal tissues. Single-channel conductance of K Ca channels in diabetic smooth muscle cells (SMCs) was not different from that of normal SMCs. However, the sensitivity of K Ca channels to CO in diabetic SMCs was significantly reduced. CO (10 µmol/l) induced an 81 ± 24% increase in the mean open probability of single K Ca channels in normal SMCs but had no effect in diabetic SMCs. Longterm culture of normal vascular SMCs with 25 mmol/l glucose or 25 mmol/l 3-OMG (3-O-methylglucose) but not 25 mmol/l mannitol significantly reduced the sensitivity of K Ca channels to CO. On the other hand, the sensitivity of K Ca channels to CO was regained in diabetic SMCs that were cultured with 5 mmol/l glucose for a prolonged period. The decreased vasorelaxant effect of CO in diabetes represents a novel mechanism for the vascular complications of diabetes, which could be closely related to the glycation of K Ca channels in diabetic vascular SMCs. Diabetes 50: [166][167][168][169][170][171][172][173][174] 2001 T he vasorelaxant effects of carbon monoxide (CO) have been demonstrated (1-4), and heme oxygenase (HO) that cleaves the heme ring to form biliverdin and CO has been located in many different types of vascular smooth muscles (5,6). The production of CO from vascular tissues has also been directly measured (7). These studies emphasize the importance of CO as an endogenous vasorelaxant factor under physiological (8) or pathophysiological (9,10) conditions. The elevation of cellular cGMP levels and the opening of plasma membrane K + channels are the main mechanisms that have been proposed to explain the vascular effects of CO. CO may increase cGMP content via its stimulatory interaction with the heme in the regulatory subunit of guanylyl cyclase. Increased cGMP would consequently decrease the intracellular Ca 2+ concentration ([Ca 2+ ] i ) in smooth muscle cells (SMCs) through the inhibition of inositol triphosphate formation, the activation of Ca 2+ -ATPase, and the inhibition of Ca 2+ channels. The opening of K + channels leads to membrane hyperpolarization, which in turn inhibits the agonist-induced increase in inositol triphosphate, reduces Ca 2+ sensitivity and resting Ca 2+ level, and relaxes SMCs (11). Our studies have demonstrated that CO directly enhanced the activity of the big-conductance calcium-activated K + (K Ca ) channels in rat tail artery SMCs via a cGMP-independent mechanism (2,3). Whethe...

Carbon Monoxide Stimulates the Ca 2+ –Activated Big Conductance K Channels in Cultured Human Endothelial Cells

“…26 Also, it has been reported that inhibition of sGC attenuated the stimulatory effect of CO on BK channels in smooth muscle cells. 19 The finding that inhibition of sGC prevents the early effect of CO on BK channels suggests that the stimulatory effect of CO depends, at least in part, on the activation of sGC in endothelial cells. However, the observation that inhibition of sGC failed to abolish the stimulatory effect of CO on BK channels suggests that CO stimulates BK channels via a cGMP-independent mechanism.…”

“…Indeed, it has been shown that CO caused vasodilation by activation of BK channels and suppression of voltagedependent Ca 2ϩ channel activity in the smooth muscle. 14 This mechanism could play an important role in mediating CO-induced vasodilation in the rat-tail artery 19 and rat gracilis muscle arterioles. 13 There are several mechanisms by which CO activates BK channels: (1) CO could increase NO release, which, in turn, activates BK channels; (2) CO could increase the production of superoxide, alter redox signaling, and increase the production of superoxide anions; (3) CO stimulates sGC and increases cGMP formation in endothelial cells; and (4) CO activates BK channels by direct modification of BK channel protein.…”

“…34 HO converts heme to iron, biliverdin, and CO. 33 Although biliverdin is a strong antioxidant that affects redox signaling, CO has been reported to affect a variety of heme-containing proteins, including protein kinases 35 and K channels. It has been reported that CO activates BK activity in smooth muscle cells 14,19,36,37 and apical renal outer medullary K channel-related K channels in the thick ascending limb of the rat kidney. 38 Because BK channels are involved in generating the membrane potential in smooth muscle cells, CO-induced activation of BK channel is expected to hyperpolarize the membrane potential and to decrease Ca 2ϩ influx in the smooth muscle.…”

“…Briefly, the bath solution in a sealed glass tube was bubbled with pure CO gas for 15 minutes, and the final concentration of CO in the tube was estimated to be 1 mmol/L, as reported previously. 19 All of the other chemicals applied in the present study were purchased from Sigma-Aldrich. Data are presented as meanϮSEM, and significance was determined using Student t test or 1-way ANOVA, followed by Tukey's test.…”

Pharmacological and Clinical Aspects of Heme Oxygenase

“…Direct evidence for the role of CO in vascular response was presented when it was shown that a reduction in CO generation resulted in increased vascular resistance in rat liver (Suematsu et al, 1994). Subsequent studies have clearly shown that HO-1-derived CO and bilirubin result in a vasorelaxant effect not only via cGMP-dependent but also via cGMP-independent (Ollinger et al, 2007;Li et al, 2008) stimulation of certain K channels Wang et al, , 2001Dong et al, 2007) and an increase in adiponectin levels (L'Abbate et al., 2007). Furthermore, it is considered unlikely that CO and NO represent redundant messenger molecules, even though both are active in the vessel wall.…”