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Ischemia and reperfusion affect multiple elements of cardiomyocyte electrophysiology, especially within the mitochondria. We previously showed that in cardiac monolayers, upon reperfusion after coverslip-induced ischemia, mitochondrial inner membrane potential (ΔΨ) unstably oscillates between polarized and depolarized states, and ΔΨ instability corresponds with arrhythmias. Here, through confocal microscopy of compartment-specific molecular probes, we investigate the mechanisms underlying the post-ischemic ΔΨ oscillations, focusing on the role of Ca

Oxidative stress arises from an imbalance in the production and scavenging rates of reactive oxygen species (ROS) and is a key factor in the pathophysiology of cardiovascular disease and aging. The presence of parallel pathways and multiple intracellular compartments, each having its own ROS sources and antioxidant enzymes, complicates the determination of the most important regulatory nodes of the redox network. Here we quantified ROS dynamics within specific intracellular compartments in the cytosol and mitochondria and determined which scavenging enzymes exert the most control over antioxidant fluxes in H9c2 cardiac myoblasts. We used novel targeted viral gene transfer vectors expressing redox-sensitive GFP fused to sensor domains to measure H2O2 or oxidized glutathione. Using genetic manipulation in heart-derived H9c2 cells, we explored the contribution of specific antioxidant enzymes to ROS scavenging and glutathione redox potential within each intracellular compartment. Our findings reveal that antioxidant flux is strongly dependent on mitochondrial substrate catabolism, with availability of NADPH as a major rate-controlling step. Moreover, ROS scavenging by mitochondria significantly contributes to cytoplasmic ROS handling. The findings provide fundamental information about the control of ROS scavenging by the redox network and suggest novel interventions for circumventing oxidative stress in cardiac cells.

Rationale: Activation of the mitochondrial ATP-sensitive potassium channel (mitoK ATP ) has been implicated in the mechanism of cardiac ischemic preconditioning, yet its molecular composition is unknown. Objective: To use an unbiased proteomic analysis of the mitochondrial inner membrane to identify the mitochondrial K + channel underlying mitoK ATP . Methods and Results: Mass spectrometric analysis was used to identify KCNJ1(ROMK) in purified bovine heart mitochondrial inner membrane and ROMK mRNA was confirmed to be present in neonatal rat ventricular myocytes and adult hearts. ROMK2, a short form of the channel, is shown to contain an N-terminal mitochondrial targeting signal, and a full-length epitope-tagged ROMK2 colocalizes with mitochondrial ATP synthase β. The high-affinity ROMK toxin, tertiapin Q, inhibits mitoK ATP activity in isolated mitochondria and in digitonin-permeabilized cells. Moreover, short hairpin RNA—mediated knockdown of ROMK inhibits the ATP-sensitive, diazoxide-activated component of mitochondrial thallium uptake. Finally, the heart-derived cell line, H9C2, is protected from cell death stimuli by stable ROMK2 overexpression, whereas knockdown of the native ROMK exacerbates cell death. Conclusions: The findings support ROMK as the pore-forming subunit of the cytoprotective mitoK ATP channel.

Insulin can alter myocardial contractility, in part through an effect on the cardiac sarcolemmal Na(+)/Ca(2+) exchanger (NCX), but little is known about its mechanism of action. The large cytoplasmic domain (f-loop) of NCX is required for regulation by various intracellular factors, and we have shown previously that residues 562-679 are determinants of NCX inhibition by exchanger inhibitory peptide (XIP). Here we show that the same f-loop deletion eliminates the enhancement of NCX current by insulin, and we examine the signal pathways involved in the insulin response. NCX current (I(NCX)) was measured in freshly isolated or cultured (up to 48 h) adult guinea pig myocytes and in myocytes expressing canine NCX1.1 with the 562-679 f-loop deletion (NCX-(Delta562-679)) via adenoviral gene transfer. I(NCX) was recorded by whole-cell patch clamp as the Ni(2+)-sensitive current at 37 degrees C with intracellular Ca(2+) buffered. Insulin (1 microm) increased I(NCX) (at +80 mV) by 110 and 83% in fresh and cultured myocytes, respectively, whereas in myocytes expressing NCX-(Delta562-679) the response was eliminated (with 100 microm XIP included to suppress any native guinea pig I(NCX)). The insulin effect on I(NCX) was not inhibited by wortmannin, a nitric-oxide synthase inhibitor, or disruption of caveolae but was blocked by chelerythrine, implicating protein kinase C, but not phosphatidylinositol-3-kinase, in the mechanism. The insulin effect was also not additive with phosphatidylinositol-4,5-bisphosphate-induced activation of I(NCX). The finding that the 562-670 f-loop domain is implicated in both XIP and receptor-mediated modulation of NCX highlights its important role in acute physiological or pathophysiological regulation of Ca(2+) balance in the heart.

The alpha(1c) subunit of the cardiac L-type Ca(2+) channel, which contains the channel pore, voltage- and Ca(2+)-dependent gating structures, and drug binding sites, has been well studied in heterologous expression systems, but many aspects of L-type Ca(2+) channel behavior in intact cardiomyocytes remain poorly characterized. Here, we develop adenoviral constructs with E1, E3 and fiber gene deletions, to allow incorporation of full-length alpha(1c) gene cassettes into the adenovirus backbone. Wild-type (alpha(1c-wt)) and mutant (alpha(1c-D-)) Ca(2+) channel adenoviruses were constructed. The alpha(1c-D-) contained four point substitutions at amino acid residues known to be critical for dihydropyridine binding. Both alpha(1c-wt) and alpha(1c-D-) expressed robustly in A549 cells (peak L-type Ca(2+) current (I(CaL)) at 0 mV: alpha(1c-wt) -9.94+/-1.00pA/pF, n=9; alpha(1c-D-) -10.30pA/pF, n=12). I(CaL) carried by alpha(1c-D-) was markedly less sensitive to nitrendipine (IC(50) 17.1 microM) than alpha(1c-wt) (IC(50) 88 nM); a feature exploited to discriminate between engineered and native currents in transduced guinea-pig myocytes. 10 microM nitrendipine blocked only 51+/-5% (n=9) of I(CaL) in alpha(1c-D-)-expressing myocytes, in comparison to 86+/-8% (n=9) of I(CaL) in control myocytes. Moreover, in 20 microM nitrendipine, calcium transients could still be evoked in alpha(1c-D-)-transduced cells, but were largely blocked in control myocytes, indicating that the engineered channels were coupled to sarcoplasmic reticular Ca(2+) release. These alpha(1c) adenoviruses provide an unprecedented tool for structure-function studies of cardiac excitation-contraction coupling and L-type Ca(2+) channel regulation in the native myocyte background.

The sarcolemmal Na + -Ca 2+ exchanger (NCX) is the main Ca 2+ extrusion mechanism in cardiac myocytes and is thus essential for the regulation of Ca 2+ homeostasis and contractile function. A cytosolic region (f-loop) of the protein mediates regulation of NCX function by intracellular factors including inhibition by exchanger inhibitory peptide (XIP), a 20 amino acid peptide matching the sequence of an autoinhibitory region involved in allosteric regulation of NCX by intracellular Na + , Ca 2+ , and phosphatidylinositol-4,5-biphosphate (PIP 2 ). Previous evidence indicates that the XIP interaction domain can be eliminated by large deletions of the f-loop that also remove activation of NCX by intracellular Ca 2+ . By whole-cell voltage clamping experiments, we demonstrate that deletion of residues 562–679, but not 440– 456, 498–510, or 680–685 of the f-loop selectively eliminates XIP-mediated inhibition of NCX expressed either heterologously (HEK293 and A549 cells) or in guinea pig cardiac myocytes. In contrast, by plotting I NCX against reverse-mode NCX-mediated Ca 2+ transients in myocytes, we demonstrate that Ca 2+ -dependent regulation of NCX is preserved in Δ562–679, but significantly reduced in the other three deletion mutants. The findings indicate that f-loop residues 562–679 may contain the regulatory site for endogenous XIP, but this site is distinct from the Ca 2+ -regulatory domains of the NCX. Because regulation of the NCX by Na + and PIP 2 involves the endogenous XIP region, the Δ562–679 mutant NCX may be a useful tool to investigate this regulation in the context of the whole cardiac myocyte.

Ion channels on the mitochondrial inner membrane influence cell function in specific ways that can be detrimental or beneficial to cell survival. At least one type of potassium (K + ) channel, the mitochondrial adenosine triphosphate–sensitive K + channel (mitoK ATP ), is an important effector of protection against necrotic and apoptotic cell injury after ischemia. Here another channel with properties similar to the surface membrane calcium-activated K + channel was found on the mitochondrial inner membrane (mitoK Ca ) of guinea pig ventricular cells. MitoK Ca significantly contributed to mitochondrial K + uptake of the myocyte, and an opener of mitoK Ca protected hearts against infarction.

Cyclic AMP is a major second messenger that inhibits the brush border Na+/H+ exchanger NHE3. We have previously shown that either of two related regulatory proteins, E3KARP or NHERF, is necessary for the cAMP-dependent inhibition of NHE3. In the present study, we characterized the interaction between NHE3 and E3KARP using in vitro binding assays. We found that NHE3 directly binds to E3KARP and that the entirety of the second PSD-95/Dlg/ZO-1 (PDZ) domain plus the carboxyl-terminal domain of E3KARP are required to bind NHE3. E3KARP binds an internal region within the NHE3 C-terminal cytoplasmic tail, defining a new mode of PDZ domain interaction. Analyses of cellular distribution of NHE3 and E3KARP expressed in PS120 fibroblasts show that NHE3 and E3KARP are co-localized on the plasma membrane, but not in a distinct juxtanuclear compartment in which NHE3 is predominantly expressed. The distributions of NHE3 and E3KARP were not affected by treatment with 8-bromo-cAMP. As shown earlier for the human homolog of NHERF, we also found that the cytoskeletal protein ezrin binds to the carboxyl-terminal domain of E3KARP. These results are consistent with the possibility that E3KARP and NHERF may function as scaffold proteins that bind to both NHE3 and ezrin. Since ezrin is a protein kinase A anchoring protein, we suggest that the scaffolding function of E3KARP binding to both ezrin and NHE3 localizes cAMP-dependent protein kinase in the vicinity of the cytoplasmic domain of NHE3, which is phosphorylated by elevated cAMP.