Mitochondrial ROS-PKCepsilon signaling axis is uniquely involved in hypoxic increase in [Ca2+]i in pulmonary artery smooth muscle cells

Hypoxia sensing in the fetal chicken femoral artery is mediated by the mitochondrial electron transport chain

Vascular hypoxia sensing is transduced into vasoconstriction in the pulmonary circulation, whereas systemic arteries dilate. Mitochondrial electron transport chain (mETC), reactive O(2) species (ROS), and K(+) channels have been implicated in the sensing/signaling mechanisms of hypoxic relaxation in mammalian systemic arteries. We aimed to investigate their putative roles in hypoxia-induced relaxation in fetal chicken (19 days of incubation) femoral arteries mounted in a wire myograph. Acute hypoxia (Po(2) approximately 2.5 kPa) relaxed the contraction induced by norepinephrine (1 microM). Hypoxia-induced relaxation was abolished or significantly reduced by the mETC inhibitors rotenone (complex I), myxothiazol and antimycin A (complex III), and NaN(3) (complex IV). The complex II inhibitor 3-nitroproprionic acid enhanced the hypoxic relaxation. In contrast, the relaxations mediated by acetylcholine, sodium nitroprusside, or forskolin were not affected by the mETC blockers. Hypoxia induced a slight increase in ROS production (as measured by 2,7-dichlorofluorescein-fluorescence), but hypoxia-induced relaxation was not affected by scavenging of superoxide (polyethylene glycol-superoxide dismutase) or H(2)O(2) (polyethylene glycol-catalase) or by NADPH-oxidase inhibition (apocynin). Also, the K(+) channel inhibitors tetraethylammonium (nonselective), diphenyl phosphine oxide-1 (voltage-gated K(+) channel 1.5), glibenclamide (ATP-sensitive K(+) channel), iberiotoxin (large-conductance Ca(2+)-activated K(+) channel), and BaCl(2) (inward-rectifying K(+) channel), as well as ouabain (Na(+)-K(+)-ATPase inhibitor) did not affect hypoxia-induced relaxation. The relaxation was enhanced in the presence of the voltage-gated K(+) channel blocker 4-aminopyridine. In conclusion, our experiments suggest that the mETC plays a critical role in O(2) sensing in fetal chicken femoral arteries. In contrast, hypoxia-induced relaxation appears not to be mediated by ROS or K(+) channels.

Membrane depolarization causes a direct activation of G protein-coupled receptors leading to local Ca2+ release in smooth muscle

Membrane depolarization activates voltage-dependent Ca(2+) channels (VDCCs) inducing Ca(2+) release via ryanodine receptors (RyRs), which is obligatory for skeletal and cardiac muscle contraction and other physiological responses. However, depolarization-induced Ca(2+) release and its functional importance as well as underlying signaling mechanisms in smooth muscle cells (SMCs) are largely unknown. Here we report that membrane depolarization can induce RyR-mediated local Ca(2+) release, leading to a significant increase in the activity of Ca(2+) sparks and contraction in airway SMCs. The increased Ca(2+) sparks are independent of VDCCs and the associated extracellular Ca(2+) influx. This format of local Ca(2+) release results from a direct activation of G protein-coupled, M(3) muscarinic receptors in the absence of exogenous agonists, which causes activation of Gq proteins and phospholipase C, and generation of inositol 1,4,5-triphosphate (IP(3)), inducing initial Ca(2+) release through IP(3) receptors and then further Ca(2+) release via RyR2 due to a local Ca(2+)-induced Ca(2+) release process. These findings demonstrate an important mechanism for Ca(2+) signaling and attendant physiological function in SMCs.

Chronic hypoxia increases pressure-dependent myogenic tone of the uterine artery in pregnant sheep: role of ERK/PKC pathway

Chronic hypoxia during pregnancy has profound effects on uterine artery (UA) contractility and attenuates uterine blood flow. The present study tested the hypothesis that chronic hypoxia inhibits the pregnancy-induced reduction in pressure-dependent myogenic tone of resistance-sized UAs. UAs were isolated from nonpregnant ewes (NPUAs) and near-term pregnant ewes (PUAs) that had been maintained at sea level (approximately 300 m) or at high altitude (3,801 m) for 110 days. In normoxic animals, the pressure-dependent myogenic response was significantly attenuated in PUAs compared with NPUAs. Hypoxia significantly increased myogenic tone in PUAs and abolished its difference between PUAs and NPUAs. Consistently, there was a significant increase in PKC-mediated baseline Ca(2+) sensitivity of PUAs in hypoxic animals. Hypoxia significantly increased phorbol 12,13-dibutyrate (PDBu)-induced contractions in PUAs but not in NPUAs. Whereas the inhibition of ERK1/2 by PD-98059 potentiated PDBu-mediated contractions of PUAs in normoxic animals, it failed to do so in hypoxic animals. Hypoxia decreased ERK1/2 expression in PUAs. PDBu induced membrane translocation of PKC-alpha and PKC-epsilon. Whereas there were no significant differences in PKC-alpha translocation among all groups, the translocation of PKC-epsilon was significantly enhanced in NPUAs compared with PUAs in normoxic animals, and hypoxia significantly increased PKC-epsilon translocation in PUAs. In the presence of PD-98059, there were no significant differences in PDBu-induced PKC-epsilon translocation among all groups. Treatment of PUAs isolated from normoxic animals with 10.5% O(2) for 48 h ex vivo significantly increased PDBu-induced contractions and eliminated its difference between PUAs and NPUAs. The results suggest that hypoxia upregulates pressure-dependent myogenic tone through its direct effect in suppressing ERK1/2 activity and increasing the PKC signal pathway, leading to an increase in the Ca(2+) sensitivity of the myogenic mechanism in the UA during pregnancy.

Cellular localization of mitochondria contributes to Kv channel-mediated regulation of cellular excitability in pulmonary but not mesenteric circulation

Mitochondria are proposed to be a major oxygen sensor in hypoxic pulmonary vasoconstriction (HPV), a unique response of the pulmonary circulation to low oxygen tension. Mitochondrial factors including reactive oxygen species, cytochrome c, ATP, and magnesium are potent modulators of voltage-gated K(+) (K(v)) channels in the plasmalemmal membrane of pulmonary arterial (PA) smooth muscle cells (PASMCs). Mitochondria have also been found close to the plasmalemmal membrane in rabbit main PA smooth muscle sections. Therefore, we hypothesized that differences in mitochondria localization in rat PASMCs and systemic mesenteric arterial smooth muscle cells (MASMCs) may contribute to the divergent oxygen sensitivity in the two different circulations. Cellular localization of mitochondria was compared with immunofluorescent labeling, and differences in functional coupling between mitochondria and K(v) channels was evaluated with the patch-clamp technique and specific mitochondrial inhibitors antimycin A (acting at complex III of the mitochondrial electron transport chain) and oligomycin A (which inhibits the ATP synthase). It was found that mitochondria were located significantly closer to the plasmalemmal membrane in PASMCs compared with MASMCs. Consistent with these findings, the effects of the mitochondrial inhibitors on K(v) current (I(Kv)) were significantly more potent in PASMCs than in MASMCs. The cytoskeletal disruptor cytochalasin B (10 microM) also altered mitochondrial distribution in PASMCs and significantly attenuated the effect of antimycin A on the voltage-dependent parameters of I(Kv). These findings suggest a greater structural and functional coupling between mitochondria and K(v) channels specifically in PASMCs, which could contribute to the regulation of PA excitability in HPV.

Constriction of pulmonary artery by peroxide: role of Ca2+ release and PKC

Reactive oxygen species are implicated in pulmonary hypertension and hypoxic pulmonary vasoconstriction. We examined the effects of low concentrations of peroxide on intrapulmonary arteries (IPA). IPAs from Wistar rats were mounted on a myograph for recording tension and estimating intracellular Ca2+ using Fura-PE3. Ca2+ sensitization was examined in alpha-toxin-permeabilized IPAs, and phosphorylation of MYPT-1 and MLC(20) was assayed by Western blot. Peroxide (30 microM) induced a vasoconstriction with transient and sustained components and equivalent elevations of intracellular Ca2+. The transient constriction was strongly suppressed by indomethacin, the TP-receptor antagonist SQ-29584, and the Rho kinase inhibitor Y-27632, whereas sustained constriction was unaffected. Neither vasoconstriction nor elevation of intracellular Ca2+ was affected by removal of extracellular Ca2+, whereas dantrolene suppressed the former and ryanodine abolished the latter. Peroxide-induced constriction of permeabilized IPAs was unaffected by Y-27632 but abolished by PKC inhibitors; these also suppressed constriction in intact IPAs. Peroxide caused translocation of PKCalpha, but had no significant effect on MYPT-1 or MLC(20) phosphorylation. We conclude that in IPAs peroxide causes transient release of vasoconstrictor prostanoids, but sustained constriction is associated with release of Ca2+ from ryanodine-sensitive stores and a PKC-dependent but Rho kinase- and MLC(20)-independent constrictor mechanism.

Protein kinase C-epsilon regulates local calcium signaling in airway smooth muscle cells

Protein kinase C (PKC) is known to regulate ryanodine receptor (RyR)-mediated local Ca(2+) signaling (Ca(2+) spark) in airway and vascular smooth muscle cells (SMCs), but its specific molecular mechanisms and functions still remain elusive. In this study, we reveal that, in airway SMCs, specific PKCepsilon peptide inhibitor and gene deletion significantly increased the frequency of Ca(2+) sparks, and decreased the amplitude of Ca(2+) sparks in the presence of xestospogin-C to eliminate functional inositol 1,4,5-triphosphate receptors. PKCepsilon activation with phorbol-12-myristate-13-acetate significantly decreased Ca(2+) spark frequency and increased Ca(2+) spark amplitude. The effect of PKCepsilon inhibition or activation on Ca(2+) sparks was completely lost in PKCepsilon(-/-) cells. PKCepsilon inhibition or PKCepsilon activation was unable to affect Ca(2+) sparks in RyR1(-/-) and RyR1(+/-) cells. Modification of RyR2 activity by FK506-binding protein 12.6 homozygous or RyR2 heterozygous gene deletion did not prevent the effect of PKCepsilon inhibition or activation. RyR3 homogenous gene deletion did not block the effect of PKCepsilon inhibition and activation, either. PKCepsilon inhibition promotes agonist-induced airway muscle contraction, whereas PKCepsilon activation produces an opposite effect. Taken together, these results indicate that PKCepsilon regulates Ca(2+) sparks by specifically interacting with RyR1, which plays an important role in the control of contractile responses in airway SMCs.

Hypoxia activates NADPH oxidase to increase [ROS]i and [Ca2+]i through the mitochondrial ROS-PKCepsilon signaling axis in pulmonary artery smooth muscle cells

The importance of NADPH oxidase (Nox) in hypoxic responses in hypoxia-sensing cells, including pulmonary artery smooth muscle cells (PASMCs), remains uncertain. In this study, using Western blot analysis we found that the major Nox subunits Nox1, Nox4, p22(phox), p47(phox), and p67(phox) were equivalently expressed in mouse pulmonary and systemic (mesenteric) arteries. However, acute hypoxia significantly increased Nox activity and translocation of p47(phox) protein to the plasma membrane in pulmonary, but not mesenteric, arteries. The Nox inhibitor apocynin and p47(phox) gene deletion attenuated the hypoxic increase in intracellular concentrations of reactive oxygen species and Ca(2+) ([ROS](i) and [Ca(2+)](i)), as well as contractions in mouse PASMCs, and abolished the hypoxic activation of Nox in pulmonary arteries. The conventional/novel protein kinase C (PKC) inhibitor chelerythrine, specific PKCepsilon translocation peptide inhibitor, and PKCepsilon gene deletion, but not the conventional PKC inhibitor GO6976, prevented the hypoxic increase in Nox activity in pulmonary arteries and [ROS](i) in PASMCs. The PKC activator phorbol 12-myristate 13-acetate could increase Nox activity in pulmonary and mesenteric arteries. Inhibition of mitochondrial ROS generation with rotenone or myxothiazol prevented hypoxic activation of Nox. Glutathione peroxidase-1 (Gpx1) gene overexpression to enhance H(2)O(2) removal significantly inhibited the hypoxic activation of Nox, whereas Gpx1 gene deletion had the opposite effect. Exogenous H(2)O(2) increased Nox activity in pulmonary and mesenteric arteries. These findings suggest that acute hypoxia may distinctively activate Nox to increase [ROS](i) through the mitochondrial ROS-PKCepsilon signaling axis, providing a positive feedback mechanism to contribute to the hypoxic increase in [ROS](i) and [Ca(2+)](i) as well as contraction in PASMCs.

Heterogeneous gene expression and functional activity of ryanodine receptors in resistance and conduit pulmonary as well as mesenteric artery smooth muscle cells

BACKGROUND

Hypoxia causes heterogeneous contractile responses in resistance and conduit pulmonary as well as systemic (mesenteric) artery smooth muscle cells (RPASMCs, CPASMCs and MASMCs), but the underlying mechanisms are largely unknown. In this study, we aimed to investigate whether the gene expression and functional activity of ryanodine receptors (RyRs) would be different in these 3 cell types.

METHODS

RyR mRNA expression, Ca(2+) sparks and [Ca(2+)](i) were measured by real-time quantitative RT-PCR, laser scanning confocal microscopy and wide-field fluorescence microscopy, respectively.

RESULTS

All 3 RyR subtype (RyR1, RyR2 and RyR3) mRNAs are expressed in RPASMCs, CPASMCs and MASMCs, but their expression levels are different. Spontaneous Ca(2+) sparks (functional events of RyRs) show distinct frequency, amplitude, duration, size and kinetics in these 3 cell types. Similarly, activation of RyRs by caffeine, 4-chloro-m-cresol or high K(+) induces differential Ca(2+) release. Moreover, hypoxia-induced increase in [Ca(2+)](i) is largest in MASMCs relative to CPSAMCs and smallest in RPASMCs.

CONCLUSION

This study provides comprehensive evidence that RyRs are heterogeneous in gene expression and functional activity in RPASMCs, CPASMCs and MASMCs, which may contribute to the diversity of excitation-contraction coupling and hypoxic Ca(2+) responses in different vascular smooth muscle cells.

Identification of a PKCε-dependent regulation of myocardial contraction by epicatechin-3-gallate

In this study, the effects of tea catechins and tea theaflavins on myocardial contraction were examined in isolated rat hearts using a Langendorff-perfusion system. We found that both tea catechins and theaflavins had positive inotropic effects on the myocardium. Of the tested chemicals, epicatechin-3-gallate (ECG) and theaflavin-3,3′-digallate (TF4) appear to be the most effective tea catechin and theaflavin, respectively. Further studies of ECG-induced positive inotropy revealed the following insights. First, unlike digitalis drugs, ECG had no effect on intracellular Ca2+ level in cultured adult cardiac myocytes. Second, it activated PKCε, but not PKCα, in the isolated hearts as well as in cultured cells. Neither a phospholipase C (PLC) inhibitor ( U73122) nor the antioxidant N-acetyl cysteine (NAC) affected the ECG-induced activation of PKCε. Third, inhibition of PKCε by either chelerythrine chloride (CHE) or PKCε translocation inhibitor peptide (TIP) caused a partial reduction of ECG-induced increases in myocardial contraction. Moreover, NAC was also effective in reducing the effects of ECG on myocardial contraction. Finally, pretreatment of the heart with both CHE and NAC completely abolished ECG-induced inotropic effects on the heart. Together, these findings indicate that ECG can regulate myocardial contractility via a novel PKCε-dependent signaling pathway.

Protein kinase C as a stress sensor

While there are many reviews which examine the group of proteins known as protein kinase C (PKC), the focus of this article is to examine the cellular roles of two PKCs that are important for stress responses in neurological tissues (PKC gamma and epsilon) and in cardiac tissues (PKC epsilon). These two kinases, in particular, seem to have overlapping functions and interact with an identical target, connexin 43 (Cx43), a gap junction protein which is central to proper control of signals in both tissues. While PKC gamma and PKC epsilon both help protect neural tissue from ischemia, PKC epsilon is the primary PKC isoform responsible for responding to decreased oxygen, or ischemia, in the heart. Both do this through Cx43. It is clear that both PKC gamma and PKC epsilon are necessary for protection from ischemia. However, the importance of these kinases has been inferred from preconditioning experiments which demonstrate that brief periods of hypoxia protect neurological and cardiac tissues from future insults, and that this depends on the activation, translocation, or ability for PKC gamma and/or PKC epsilon to interact with distinct cellular targets, especially Cx43. This review summarizes the recent findings which define the roles of PKC gamma and PKC epsilon in cardiac and neurological functions and their relationships to ischemia/reperfusion injury. In addition, a biochemical comparison of PKC gamma and PKC epsilon and a proposed argument for why both forms are present in neurological tissue while only PKC epsilon is present in heart, are discussed. Finally, the biochemistry of PKCs and future directions for the field are discussed, in light of this new information.