Jackson-Weaver O, Osmond JM, Riddle MA, Naik JS, Gonzalez Bosc LV, Walker BR, Kanagy NL. Hydrogen sulfide dilates rat mesenteric arteries by activating endothelial large-conductance Ca 2ϩ -activated K ϩ channels and smooth muscle Ca 2ϩ sparks. Am J Physiol Heart Circ Physiol 304: H1446 -H1454, 2013. First published March 22, 2013 doi:10.1152/ajpheart.00506.2012We have previously shown that hydrogen sulfide (H2S) reduces myogenic tone and causes relaxation of phenylephrine (PE)-constricted mesenteric arteries. This effect of H2S to cause vasodilation and vascular smooth muscle cell (VSMC) hyperpolarization was mediated by large-conductance Ca 2ϩ -activated potassium channels (BKCa). Ca 2ϩ sparks are ryanodine receptor (RyR)-mediated Ca 2ϩ -release events that activate BKCa channels in VSMCs to cause membrane hyperpolarization and vasodilation. We hypothesized that H2S activates Ca 2ϩ sparks in small mesenteric arteries. Ca 2ϩ sparks were measured using confocal microscopy in rat mesenteric arteries loaded with the Ca 2ϩ indicator fluo-4. VSMC membrane potential (Em) was measured in isolated arteries using sharp microelectrodes. In PE-constricted arteries, the H2S donor NaHS caused vasodilation that was inhibited by ryanodine (RyR blocker), abluminal or luminal iberiotoxin (IbTx, BKCa blocker), endothelial cell (EC) disruption, and sulfaphenazole [cytochrome P-450 2C (Cyp2C) inhibitor]. The H2S donor NaHS (10 mol/l) increased Ca 2ϩ sparks but only in the presence of intact EC and this was blocked by sulfaphenazole or luminal IbTx. Inhibiting cystathionine ␥-lyase (CSE)-derived H2S with ␤-cyano-L-alanine (BCA) also reduced VSMC Ca 2ϩ spark frequency in mesenteric arteries, as did EC disruption. However, excess CSE substrate homocysteine did not affect spark activity. NaHS hyperpolarized VSMC E m in PE-depolarized mesenteric arteries with intact EC and also hyperpolarized EC Em in arteries cut open to expose the lumen. This hyperpolarization was prevented by ryanodine, sulfaphenazole, and abluminal or luminal IbTx. BCA reduced IbTx-sensitive K ϩ currents in freshly dispersed mesenteric ECs. These results suggest that H2S increases Ca 2ϩ spark activity in mesenteric artery VSMC through activation of endothelial BKCa channels and Cyp2C, a novel vasodilatory pathway for this emerging signaling molecule.cytochrome P-450 epoxygenase; sodium hydrosulfide; membrane potential HYDROGEN SULFIDE (H 2 S) is a recently described vasodilator produced in the vasculature by the enzymes cystathionine ␥-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3MST). H 2 S has been proposed to cause vasodilation through a variety of mechanisms (1,3,26,33) and genetic knockout of the CSE gene causes hypertension (32). We previously reported that inhibiting CSE or disrupting the endothelium enhances myogenic tone in small mesenteric arteries from rats and that exogenous H 2 S hyperpolarizes vascular smooth muscle cell (VSMC) membrane potential (E m ) and dilates arteries through activation of large-conductance Ca 2ϩ -activated K (4) and ...

The systemic vasculature exhibits attenuated vasoconstriction following hypobaric chronic hypoxia (CH) that is associated with endothelium-dependent vascular smooth muscle (VSM) cell hyperpolarization. We hypothesized that increased activity of endothelial cell (EC) large-conductance, calcium-activated potassium (BK(Ca)) channels contributes to this response. Gracilis resistance arteries from hypobaric CH (barometric pressure = 380 mmHg for 48 h) rats demonstrated reduced myogenic reactivity and hyperpolarized VSM membrane potential (E(m)) compared with controls under normoxic ex vivo conditions. These differences were eliminated by endothelial disruption. In the presence of cyclooxygenase and nitric oxide synthase inhibition, combined intraluminal administration of the intermediate and small-conductance, calcium-activated K(+) channel blockers TRAM-34 and apamin was without effect on myogenic responsiveness and VSM E(m) in both groups; however, these variables were normalized in CH arteries by intraluminal administration of the BK(Ca) inhibitor iberiotoxin (IBTX). Basal EC E(m) was hyperpolarized in arteries from CH rats compared with controls and was restored by IBTX, but not by TRAM-34/apamin. K(+) channel blockers were without effect on EC basal E(m) in controls. Similarly, IBTX blocked acetylcholine-induced dilation in arteries from CH rats, but was without effect in controls, whereas TRAM-34/apamin eliminated dilation in controls. Acetylcholine-induced EC hyperpolarization and calcium responses were inhibited by IBTX in CH arteries and by TRAM-34/apamin in controls. Patch-clamp experiments on freshly isolated ECs demonstrated greater K(+) current in cells from CH rats that was normalized by IBTX. IBTX was without effect on K(+) current in controls. We conclude that hypobaric CH induces increased endothelial BK(Ca) channel activity that contributes to reduced myogenic responsiveness and EC and VSM cell hyperpolarization.

A novel vasodilatory influence of endothelial cell (EC) large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels is present following in vivo exposure to chronic hypoxia (CH) and may exist in other pathological states. However, the mechanism of channel activation that results in altered vasoreactivity is unknown. We tested the hypothesis that CH removes an inhibitory effect of the scaffolding domain of caveolin-1 (Cav-1) on EC BK(Ca) channels to permit activation, thereby affecting vasoreactivity. Experiments were performed on gracilis resistance arteries and ECs from control and CH-exposed (380 mmHg barometric pressure for 48 h) rats. EC membrane potential was hyperpolarized in arteries from CH-exposed rats and arteries treated with the cholesterol-depleting agent methyl-β-cyclodextrin (MBCD) compared with controls. Hyperpolarization was reversed by the BK(Ca) channel antagonist iberiotoxin (IBTX) or by a scaffolding domain peptide of Cav-1 (AP-CAV). Patch-clamp experiments documented an IBTX-sensitive current in ECs from CH-exposed rats and in MBCD-treated cells that was not present in controls. This current was enhanced by the BK(Ca) channel activator NS-1619 and blocked by AP-CAV or cholesterol supplementation. EC BK(Ca) channels displayed similar unitary conductance but greater Ca(2+) sensitivity than BK(Ca) channels from vascular smooth muscle. Immunofluorescence imaging demonstrated greater association of BK(Ca) α-subunits with Cav-1 in control arteries than in arteries from CH-exposed rats, although fluorescence intensity for each protein did not differ between groups. Finally, AP-CAV restored myogenic and phenylephrine-induced constriction in arteries from CH-exposed rats without affecting controls. AP-CAV similarly restored diminished reactivity to phenylephrine in control arteries pretreated with MBCD. We conclude that CH unmasks EC BK(Ca) channel activity by removing an inhibitory action of the Cav-1 scaffolding domain that may depend on cellular cholesterol levels.

Agonist-induced Ca(2+) entry into the pulmonary endothelium depends on activation of both store-operated Ca(2+) (SOC) entry and receptor-operated Ca(2+) (ROC) entry. We previously reported that pulmonary endothelial cell SOC entry and ROC entry are reduced in chronic hypoxia (CH)-induced pulmonary hypertension. We hypothesized that diminished endothelial Ca(2+) entry following CH is due to derangement of caveolin-1 (cav-1) containing cholesterol-enriched membrane domains important in agonist-induced Ca(2+) entry. To test this hypothesis, we measured Ca(2+) influx by fura-2 fluorescence following application of ATP (20 μM) in freshly isolated endothelial cells pretreated with the caveolar-disrupting agent methyl-β-cyclodextrin (mβCD; 10 mM). Cholesterol depletion with mβCD attenuated agonist-induced Ca(2+) entry in control endothelial cells to the level of that from CH rats. Interestingly, endothelial membrane cholesterol was lower in cells isolated from CH rats compared with controls although the density of caveolae did not differ between groups. Cholesterol repletion with a cholesterol:mβCD mixture or the introduction of the cav-1 scaffolding peptide (AP-cav; 10 μM) rescued ATP-induced Ca(2+) entry in endothelia from CH arteries. Agonist-induced Ca(2+) entry assessed by Mn(2+) quenching of fura-2 fluorescence was also significantly elevated by luminal AP-cav in pressurized intrapulmonary arteries from CH rats to levels of controls. Similarly, patch-clamp experiments revealed diminished inward current in response to ATP in cells from CH rats compared with controls that was restored by AP-cav. These data suggest that CH-induced pulmonary hypertension leads to reduced membrane cholesterol that limits the activity of ion channels necessary for agonist-activated Ca(2+) entry.

A novel vasodilatory influence of endothelial cell (EC) large-conductance Ca(2+)-activated K(+) (BK) channels is present after in vivo exposure to chronic hypoxia (CH) and may exist in other pathological states. However, the mechanism of channel activation that results in altered vasoreactivity is unknown. Previously, we demonstrated that inhibition of either BK channels or heme oxygenase (HO) restores vasoconstrictor reactivity after CH. Additionally, administration of the scaffolding domain of caveolin (Cav)-1 inhibits EC BK activity and restores vasoconstrictor reactivity in this setting. These results led us to hypothesize that CH exposure results in a loss in Cav-1 inhibition of EC BK channels, resulting in their activation by HO-derived carbon monoxide (CO). Experiments were conducted on freshly dispersed aortic ECs from control and CH-exposed (barometric pressure: 380 mmHg for 48 h) rats. In electrophysiology experiments, outward currents were greater in cells from CH rats as well as from cells from control rats treated with the cholesterol-depleting agent methyl-β-cyclodextrin. These enhanced currents were returned to control by HO inhibition. Channel activity could be restored by the CO donor CO-releasing molecule (CORM)-2 during HO inhibition. Administration of the Cav-1 scaffolding domain eliminated BK currents in cells from CH rats, and current was not restored by the addition of CORM-2. Colocalization experiments in ECs from control and CH rats demonstrated an association between HO-2, Cav-1, and BK. We conclude that EC BK channel activity is HO dependent in the absence of the inhibitory effect of the Cav-1 scaffolding domain.

Chronic hypoxia (CH)-induced pulmonary hypertension is associated with decreased basal pulmonary artery endothelial cell (EC) Ca 2ϩ , which correlates with reduced storeoperated Ca 2ϩ (SOC) entry. Protein kinase C (PKC) attenuates SOC entry in ECs. Therefore, we hypothesized that PKC has a greater inhibitory effect on EC SOC and receptoroperated Ca 2ϩ entry after CH. To test this hypothesis, we assessed SOC in the presence or absence of the nonselec-in freshly isolated, Fura-2-loaded ECs obtained from intrapulmonary arteries of control and CH rats (4 weeks at 0.5 atm). We found that SOC entry and 1-oleoyl-2-acetyl-sn-glycerol (OAG)-and ATP-induced Ca 2ϩ influx were attenuated in ECs from CH rats versus controls, and GF109203X restored SOC and OAG responses to the level of controls. In contrast, nonselective PKC inhibition with GF109203X or the selective PKC inhibitor myristoylated V1-2 attenuated ATP-induced Ca 2ϩ entry in ECs from control but not CH pulmonary arteries. ATP-induced Ca 2ϩ entry was also attenuated by the T-type voltage-gated Ca 2ϩ channel (VGCC) inhibitor mibefradil in control cells. Consistent with the presence of endothelial T-type VGCC, we observed depolarization-induced Ca 2ϩ influx in control cells that was inhibited by mibefradil. This response was largely absent in ECs from CH arteries. We conclude that CH enhances PKC-dependent inhibition of SOC-and OAG-induced Ca 2ϩ entry. Furthermore, these data suggest that CH may reduce the ATP-dependent Ca 2ϩ entry that is mediated, in part, by PKC and mibefradil-sensitive Ca 2ϩ channels in control cells.

Chronic hypoxia (CH; 48 hr 0.5 atm) causes blunted arterial vasoconstriction in rats. Inhibition of endothelial cell (EC) heme oxygenase (HO) or large conductance Ca2+‐ activated K+ channels (BK) restores reactivity, but is without effect in controls. Further, BK currents in ECs from CH rats are inhibited by HO blockade, suggesting functional coupling of HO with BK channels. Since HO‐derived CO may enhance BK Ca2+ sensitivity, we hypothesized that channels from CH ECs are more sensitive to Ca2+ than BK channels from vascular smooth muscle (VSM) due to the tonic presence of CO. We performed patch clamp experiments on freshly dispersed aortic ECs in whole cell‐cell attached and inside‐out configurations to characterize BK. Small (SK) and intermediate (IK) conductance K+ channels were detected in cells from each group. In contrast, iberiotoxin (IBTX)‐sensitive channels with unitary conductance of 146±10 pS/222±13 pS (asymmetrical/symmetrical K+) were found only in ECs from CH rats. EC BK channels were more sensitive to Ca2+ than SK and IK and had markedly greater Ca2+ sensitivity than VSM BK. This enhanced Ca2+ sensitivity may contribute to the apparent tonic activity of EC BK channels following CH.

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