Sulfhydryl oxidation induces rapid and reversible closure of the ATP-regulated K+ channel in the pancreatic beta-cell

Expression of truncated Kir6.2 promotes insertion of functionally inverted ATP-sensitive K+ channels

ATP-sensitive K⁺ (K_ATP) channels couple cellular metabolism to electrical activity in many cell types. Wild-type K_ATP channels are comprised of four pore forming (Kir6.x) and four regulatory (sulfonylurea receptor, SURx) subunits that each contain RKR endoplasmic reticulum retention sequences that serve to properly translocate the channel to the plasma membrane. Truncated Kir6.x variants lacking RKR sequences facilitate plasma membrane expression of functional Kir6.x in the absence of SURx; however, the effects of channel truncation on plasma membrane orientation have not been explored. To investigate the role of truncation on plasma membrane orientation of ATP sensitive K⁺ channels, three truncated variants of Kir6.2 were used (Kir6.2ΔC26, 6xHis-Kir6.2ΔC26, and 6xHis-EGFP-Kir6.2ΔC26). Oocyte expression of Kir6.2ΔC26 shows the presence of a population of inverted inserted channels in the plasma membrane, which is not present when co-expressed with SUR1. Immunocytochemical staining of intact and permeabilized HEK293 cells revealed that the N-terminus of 6xHis-Kir6.2ΔC26 was accessible on both sides of the plasma membrane at roughly equivalent ratios, whereas the N-terminus of 6xHis-EGFP-Kir6.2Δ26 was only accessible on the intracellular face. In HEK293 cells, whole-cell electrophysiological recordings showed a ca. 50% reduction in K⁺ current upon addition of ATP to the extracellular solution for 6xHis-Kir6.2ΔC26, though sensitivity to extracellular ATP was not observed in 6xHis-EGFP-Kir6.2ΔC26. Importantly, the population of channels that is inverted exhibited similar function to properly inserted channels within the plasma membrane. Taken together, these data suggest that in the absence of SURx, inverted channels can be formed from truncated Kir6.x subunits that are functionally active which may provide a new model for testing pharmacological modulators of Kir6.x, but also indicates the need for added caution when using truncated Kir6.2 mutants.

Molecular Mechanisms Linking Oxidative Stress and Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is the most prevalent metabolic disorder characterized by chronic hyperglycemia and an inadequate response to circulatory insulin by peripheral tissues resulting in insulin resistance. Insulin resistance has a complex pathophysiology, and it is contributed to by multiple factors including oxidative stress. Oxidative stress refers to an imbalance between free radical production and the antioxidant system leading to a reduction of peripheral insulin sensitivity and contributing to the development of T2DM via several molecular mechanisms. In this review, we present the molecular mechanisms by which the oxidative milieu contributes to the pathophysiology of insulin resistance and diabetes mellitus.

Differential expression of islet glutaredoxin 1 and 5 with high reactive oxygen species production in a mouse model of diabesity

The onset and progression of diabetes mellitus type 2 is highly contingent on the amount of functional beta-cell mass. An underlying cause of beta-cell decay in diabetes is oxidative stress, which markedly affects the insulin producing pancreatic cells due to their poor antioxidant defence capacity. Consequently, disturbances of cellular redox signaling have been implicated to play a major role in beta-cell loss in diabetes mellitus type 2. There is evidence suggesting that the glutaredoxin (Grx) system exerts a protective role for pancreatic islets, but the exact mechanisms have not yet been elucidated. In this study, a mouse model for diabetes mellitus type 2 was used to gain further insight into the significance of Grx for the islets of Langerhans in the diabetic metabolism. We have observed distinct differences in the expression levels of Grx in pancreatic islets between obese, diabetic db mice and lean, non-diabetic controls. This finding is the first report about a decrease of Grx expression levels in pancreatic islets of diabetic mice which was accompanied by declining insulin secretion, increase of reactive oxygen species (ROS) production level, and cell cycle alterations. These data demonstrate the essential role of the Grx system for the beta-cell during metabolic stress which may provide a new target for diabetes mellitus type 2 treatment.

Redox Signal-mediated Enhancement of the Temperature Sensitivity of Transient Receptor Potential Melastatin 2 (TRPM2) Elevates Glucose-induced Insulin Secretion from Pancreatic Islets

Transient receptor potential melastatin 2 (TRPM2) is a thermosensitive Ca(2+)-permeable cation channel expressed by pancreatic β cells where channel function is constantly affected by body temperature. We focused on the physiological functions of redox signal-mediated TRPM2 activity at body temperature. H2O2, an important molecule in redox signaling, reduced the temperature threshold for TRPM2 activation in pancreatic β cells of WT mice but not in TRPM2KO cells. TRPM2-mediated [Ca(2+)]i increases were likely caused by Ca(2+) influx through the plasma membrane because the responses were abolished in the absence of extracellular Ca(2+). In addition, TRPM2 activation downstream from the redox signal plus glucose stimulation enhanced glucose-induced insulin secretion. H2O2 application at 37 °C induced [Ca(2+)]i increases not only in WT but also in TRPM2KO β cells. This was likely due to the effect of H2O2 on KATP channel activity. However, the N-acetylcysteine-sensitive fraction of insulin secretion by WT islets was increased by temperature elevation, and this temperature-dependent enhancement was diminished significantly in TRPM2KO islets. These data suggest that endogenous redox signals in pancreatic β cells elevate insulin secretion via TRPM2 sensitization and activity at body temperature. The results in this study could provide new therapeutic approaches for the regulation of diabetic conditions by focusing on the physiological function of TRPM2 and redox signals.

Role of K(ATP) channels in β-cell resistance to oxidative stress

The importance of K(ATP) channels in stimulus-secretion coupling of β-cells is well established, although they are not indispensable for the maintenance of glycaemic control. This review article depicts a new role for K(ATP) channels by showing that genetic or pharmacological ablation of these channels protects β-cells against oxidative stress. Increased production of oxidants is a crucial factor in the pathogenesis of type 2 diabetes mellitus (T2DM). T2DM develops when β-cells can no longer compensate for the high demand of insulin resulting from excess fuel intake. Instead β-cells start to secrete less insulin and β-cell mass is diminished by apoptosis. Both, reduction of insulin secretion and β-cell mass induced by oxidative stress, are prevented by deletion or inhibition of K(ATP) channels. These findings may open up new insights into the early treatment of T2DM.

Rotenone induces KATP channel opening in PC12 cells in association with the expression of tyrosine hydroxylase

The activation of ATP-sensitive potassium (KATP) channels in PC12 cells play a pivotal role in protection against the neurotoxic effect of rotenone. However, it remains unclear why rotenone seems to preferentially affect activation of KATP channels and if this could affect its physiological activity. In this study, we sought to determine how the different energy states caused by various doses of rotenone affect the KATP opening state and whether the KATP opening state influences the expression of tyrosine hydroxylase (TH) which is related with DA synthesis. With patch clamp technology, results showed that treatment of PC12 cells with rotenone (0.2-1 µg/ml) for 15 min can cause KATP channel opening with significantly increased intracellular ROS production. Treatment with rotenone (2-16 ng/ml) for 24 h also caused the channels to open with gently increased ROS. In order to study if the rather long-term action on KATP channel opening states could affect the specified function of PC12 cells, the KATP channel opener pinacidil and the inhibitor glibenclamide were used to treat cells for 24 h, and the expression of TH was detected. Our results showed that treatment of PC12 cells with glibenclamide for 24 h can notably promote TH expression and can also enhance the expression of TH which were reduced by rotenone. These data indicate that the energy states in PC12 induced by various doses of rotenone could significantly influence the opening states of KATP channels. However long-term energy stress may raise the opening rate and opening sensitivity of this channel. In addition, our results demonstrate for the first time that activation of plasma membrane KATP channels induced by rotenone inhibits TH expression which influences DA synthesis in PC12 cells.

Activation of cGMP-dependent protein kinase stimulates cardiac ATP-sensitive potassium channels via a ROS/calmodulin/CaMKII signaling cascade

Background

Cyclic GMP (cGMP)-dependent protein kinase (PKG) is recognized as an important signaling component in diverse cell types. PKG may influence the function of cardiac ATP-sensitive potassium (K_ATP) channels, an ion channel critical for stress adaptation in the heart; however, the underlying mechanism remains largely unknown. The present study was designed to address this issue.

Methods and Findings

Single-channel recordings of cardiac K_ATP channels were performed in both cell-attached and inside-out patch configurations using transfected human embryonic kidney (HEK)293 cells and rabbit ventricular cardiomyocytes. We found that Kir6.2/SUR2A (the cardiac-type K_ATP) channels were activated by cGMP-selective phosphodiesterase inhibitor zaprinast in a concentration-dependent manner in cell-attached patches obtained from HEK293 cells, an effect mimicked by the membrane-permeable cGMP analog 8-bromo-cGMP whereas abolished by selective PKG inhibitors. Intriguingly, direct application of PKG moderately reduced rather than augmented Kir6.2/SUR2A single-channel currents in excised, inside-out patches. Moreover, PKG stimulation of Kir6.2/SUR2A channels in intact cells was abrogated by ROS/H₂O₂ scavenging, antagonism of calmodulin, and blockade of calcium/calmodulin-dependent protein kinase II (CaMKII), respectively. Exogenous H₂O₂ also concentration-dependently stimulated Kir6.2/SUR2A channels in intact cells, and its effect was prevented by inhibition of calmodulin or CaMKII. PKG stimulation of K_ATP channels was confirmed in intact ventricular cardiomyocytes, which was ROS- and CaMKII-dependent. Kinetically, PKG appeared to stimulate these channels by destabilizing the longest closed state while stabilizing the long open state and facilitating opening transitions.

Conclusion

The present study provides novel evidence that PKG exerts dual regulation of cardiac K_ATP channels, including marked stimulation resulting from intracellular signaling mediated by ROS (H₂O₂ in particular), calmodulin and CaMKII, alongside of moderate channel suppression likely mediated by direct PKG phosphorylation of the channel or some closely associated proteins. The novel cGMP/PKG/ROS/calmodulin/CaMKII signaling pathway may regulate cardiomyocyte excitability by opening K_ATP channels and contribute to cardiac protection against ischemia-reperfusion injury.

Redox regulation of the mitochondrial K(ATP) channel in cardioprotection

The mitochondrial ATP-sensitive potassium channel (mK(ATP)) is important in the protective mechanism of ischemic preconditioning (IPC). The channel is reportedly sensitive to reactive oxygen and nitrogen species, and the aim of this study was to compare such species in parallel, to build a more comprehensive picture of mK(ATP) regulation. mK(ATP) activity was measured by both osmotic swelling and Tl(+) flux assays, in isolated rat heart mitochondria. An isolated adult rat cardiomyocyte model of ischemia-reperfusion (IR) injury was also used to determine the role of mK(ATP) in cardioprotection by nitroxyl. Key findings were as follows: (i) mK(ATP) was activated by O(2)(-) and H(2)O(2) but not other peroxides. (ii) mK(ATP) was inhibited by NADPH. (iii) mK(ATP) was activated by S-nitrosothiols, nitroxyl, and nitrolinoleate. The latter two species also inhibited mitochondrial complex II. (iv) Nitroxyl protected cardiomyocytes against IR injury in an mK(ATP)-dependent manner. Overall, these results suggest that the mK(ATP) channel is activated by specific reactive oxygen and nitrogen species, and inhibited by NADPH. The redox modulation of mK(ATP) may be an underlying mechanism for its regulation in the context of IPC. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection.

Oxidative stress and beta-cell dysfunction

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes. Because of their low antioxidant capacity, beta-cells are extremely sensitive towards oxidative stress. In beta-cells, important targets for an oxidant insult are cell metabolism and K(ATP) channels. The oxidant-evoked alterations of K(ATP) channel activity seem to be critical for oxidant-induced dysfunction because genetic ablation of K(ATP) channels attenuates the effects of oxidative stress on beta-cell function. Besides the effects on metabolism, interference of oxidants with mitochondria induces key events in apoptosis. Consequently, increasing antioxidant defence is a promising strategy to delay beta cell failure in (pre)-diabetic patients or during islet transplantation. Knock-out of K(ATP) channels has beneficial effects on oxidant-induced inhibition of insulin secretion and cell death. Interestingly, these effects can be mimicked by sulfonylureas that have been used in the treatment of T2DM for many years. Loss of functional K(ATP) channels leads to up-regulation of antioxidant enzymes, a process that depends on cytosolic Ca(2+). These observations are of great importance for clinical intervention because they show a possibility to protect beta-cells at an early stage before dramatic changes of the secretory capacity and loss of cell mass become manifest and lead to glucose intolerance or even overt diabetes.

Electrophysiology of islet cells

Stimulus-Secretion Coupling (SSC) of pancreatic islet cells comprises electrical activity. Changes of the membrane potential (V(m)) are regulated by metabolism-dependent alterations in ion channel activity. This coupling is best explored in beta-cells. The effect of glucose is directly linked to mitochondrial metabolism as the ATP/ADP ratio determines the open probability of ATP-sensitive K(+) channels (K(ATP) channels). Nucleotide sensitivity and concentration in the direct vicinity of the channels are controlled by several factors including phospholipids, fatty acids, and kinases, e.g., creatine and adenylate kinase. Closure of K(ATP) channels leads to depolarization of beta-cells via a yet unknown depolarizing current. Ca(2+) influx during action potentials (APs) results in an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](c)) that triggers exocytosis. APs are elicited by the opening of voltage-dependent Na(+) and/or Ca(2+) channels and repolarized by voltage- and/or Ca(2+)-dependent K(+) channels. At a constant stimulatory glucose concentration APs are clustered in bursts that are interrupted by hyperpolarized interburst phases. Bursting electrical activity induces parallel fluctuations in [Ca(2+)](c) and insulin secretion. Bursts are terminated by I(Kslow) consisting of currents through Ca(2+)-dependent K(+) channels and K(ATP) channels. This review focuses on structure, characteristics, physiological function, and regulation of ion channels in beta-cells. Information about pharmacological drugs acting on K(ATP) channels, K(ATP) channelopathies, and influence of oxidative stress on K(ATP) channel function is provided. One focus is the outstanding significance of L-type Ca(2+) channels for insulin secretion. The role of less well characterized beta-cell channels including voltage-dependent Na(+) channels, volume sensitive anion channels (VSACs), transient receptor potential (TRP)-related channels, and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is discussed. A model of beta-cell oscillations provides insight in the interplay of the different channels to induce and maintain electrical activity. Regulation of beta-cell electrical activity by hormones and the autonomous nervous system is discussed. alpha- and delta-cells are also equipped with K(ATP) channels, voltage-dependent Na(+), K(+), and Ca(2+) channels. Yet the SSC of these cells is less clear and is not necessarily dependent on K(ATP) channel closure. Different ion channels of alpha- and delta-cells are introduced and SSC in alpha-cells is described in special respect of paracrine effects of insulin and GABA secreted from beta-cells.

Stimulation of neuronal KATP channels by cGMP-dependent protein kinase: involvement of ROS and 5-hydroxydecanoate-sensitive factors in signal transduction

The ATP-sensitive potassium (K(ATP)) channel couples intracellular metabolic state to membrane excitability. Recently, we demonstrated that neuronal K(ATP) channels are functionally enhanced by activation of a nitric oxide (NO)/cGMP/cGMP-dependent protein kinase (PKG) signaling cascade. In this study, we further investigated the intracellular mechanism underlying PKG stimulation of neuronal K(ATP) channels. By performing single-channel recordings in transfected HEK293 and neuroblastoma SH-SY5Y cells, we found that the increase of Kir6.2/SUR1 (i.e., the neuronal-type K(ATP)) channel currents by PKG activation in cell-attached patches was diminished by 5-hydroxydecanoate (5-HD), an inhibitor of the putative mitochondrial K(ATP) channel; N-(2-mercaptopropionyl)glycine, a reactive oxygen species (ROS) scavenger, and catalase, a hydrogen peroxide (H(2)O(2))-decomposing enzyme. These reagents also ablated NO-induced K(ATP) channel stimulation and prevented the shifts in the single-channel open- and closed-time distributions resulting from PKG activation and NO induction. Bath application of H(2)O(2) reproduced PKG stimulation of Kir6.2/SUR1 but did not activate tetrameric Kir6.2LRKR368/369/370/371AAAA channels. Moreover, neither the PKG activator nor exogenous H(2)O(2) was able to enhance the function of K(ATP) channels in the presence of Ca(2+) chelators and calmodulin antagonists, whereas the stimulatory effect of H(2)O(2) was unaffected by 5-HD. Altogether, in this report we provide novel evidence that activation of PKG stimulates neuronal K(ATP) channels by modulating intrinsic channel gating via a 5-HD-sensitive factor(s)/ROS/Ca(2+)/calmodulin signaling pathway that requires the presence of the SUR1 subunit. This signaling pathway may contribute to neuroprotection against ischemic injury and regulation of neuronal excitability and neurotransmitter release by modulating the function of neuronal K(ATP) channels.

Modulation of K(ATP) currents in rat ventricular myocytes by hypoxia and a redox reaction

AIM

The present study investigated the possible regulatory mechanisms of redox agents and hypoxia on the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

METHODS

Single-channel and whole-cell patch-clamp techniques were used to record the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

RESULTS

Oxidized glutathione (GSSG, 1 mmol/L) increased the I(KATP), while reduced glutathione (GSH, 1 mmol/L) could reverse the increased I(KATP) during normoxia. To further corroborate the effect of the redox agent on the K(ATP) channel, we employed the redox couple DTT (1 mmol/L)/H2O2 (0.3, 0.6, and 1 mmol/L) and repeated the previous processes, which produced results similar to the previous redox couple GSH/GSSG during normoxia. H2O2 increased the I(KATP) in a concentration dependent manner, which was reversed by DTT (1 mmol/L). In addition, our results have shown that 15 min of hypoxia increased the I(KATP), while GSH (1 mmol/L) could reverse the increased I(KATP). Furthermore, in order to study the signaling pathways of the I(KATP) augmented by hypoxia and the redox agent, we applied a protein kinase C(PKC) inhibitor bisindolylmaleimide VI (BIM), a protein kinase G(PKG) inhibitor KT5823, a protein kinase A (PKA) inhibitor H-89, and Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitors KN-62 and KN-93. The results indicated that BIM, KT5823, KN-62, and KN-93, but not H-89, inhibited the I(KATP) augmented by hypoxia and GSSG; in addition, these results suggest that the effects of both GSSG and hypoxia on K(ATP) channels involve the activation of the PKC, PKG, and CaMK II pathways, but not the PKA pathway.

CONCLUSION

The present study provides electrophysiological evidence that hypoxia and the oxidizing reaction are closely related to the modulation of I(KATP).

Functional K(ATP) channels in the rat retinal microvasculature: topographical distribution, redox regulation, spermine modulation and diabetic alteration

The essential task of the circulatory system is to match blood flow to local metabolic demand. However, much remains to be learned about this process. To better understand how local perfusion is regulated, we focused on the functional organization of the retinal microvasculature, which is particularly well adapted for the local control of perfusion. Here, we assessed the distribution and regulation of functional K(ATP) channels whose activation mediates the hyperpolarization induced by adenosine. Using microvascular complexes freshly isolated from the rat retina, we found a topographical heterogeneity in the distribution of functional K(ATP) channels; capillaries generate most of the K(ATP) current. The initiation of K(ATP)-induced responses in the capillaries supports the concept that the regulation of retinal perfusion is highly decentralized. Additional study revealed that microvascular K(ATP) channels are redox sensitive, with oxidants increasing their activity. Furthermore, the oxidant-mediated activation of these channels is driven by the polyamine spermine, whose catabolism produces oxidants. In addition, our observation that spermine-dependent oxidation occurs predominately in the capillaries accounts for why they generate most of the K(ATP) current detected in retinal microvascular complexes. Here, we also analysed retinal microvessels of streptozotocin-injected rats. We found that soon after the onset of diabetes, an increase in spermine-dependent oxidation at proximal microvascular sites boosts their K(ATP) current and thereby virtually eliminates the topographical heterogeneity of functional K(ATP) channels. We conclude that spermine-dependent oxidation is a previously unrecognized mechanism by which this polyamine modulates ion channels; in addition to a physiological role, spermine-dependent oxidation may also contribute to microvascular dysfunction in the diabetic retina.

Nitric oxide activates ATP-sensitive potassium channels in mammalian sensory neurons: action by direct S-nitrosylation

Background

ATP-sensitive potassium (K_ATP) channels in neurons regulate excitability, neurotransmitter release and mediate protection from cell-death. Furthermore, activation of K_ATP channels is suppressed in DRG neurons after painful-like nerve injury. NO-dependent mechanisms modulate both K_ATP channels and participate in the pathophysiology and pharmacology of neuropathic pain. Therefore, we investigated NO modulation of K_ATP channels in control and axotomized DRG neurons.

Results

Cell-attached and cell-free recordings of K_ATP currents in large DRG neurons from control rats (sham surgery, SS) revealed activation of K_ATP channels by NO exogenously released by the NO donor SNAP, through decreased sensitivity to [ATP]i.

This NO-induced K_ATP channel activation was not altered in ganglia from animals that demonstrated sustained hyperalgesia-type response to nociceptive stimulation following spinal nerve ligation. However, baseline opening of K_ATP channels and their activation induced by metabolic inhibition was suppressed by axotomy. Failure to block the NO-mediated amplification of K_ATP currents with specific inhibitors of sGC and PKG indicated that the classical sGC/cGMP/PKG signaling pathway was not involved in the activation by SNAP. NO-induced activation of K_ATP channels remained intact in cell-free patches, was reversed by DTT, a thiol-reducing agent, and prevented by NEM, a thiol-alkylating agent. Other findings indicated that the mechanisms by which NO activates K_ATP channels involve direct S-nitrosylation of cysteine residues in the SUR1 subunit. Specifically, current through recombinant wild-type SUR1/Kir6.2 channels expressed in COS7 cells was activated by NO, but channels formed only from truncated isoform Kir6.2 subunits without SUR1 subunits were insensitive to NO. Further, mutagenesis of SUR1 indicated that NO-induced K_ATP channel activation involves interaction of NO with residues in the NBD1 of the SUR1 subunit.

Conclusion

NO activates K_ATP channels in large DRG neurons via direct S-nitrosylation of cysteine residues in the SUR1 subunit. The capacity of NO to activate K_ATP channels via this mechanism remains intact even after spinal nerve ligation, thus providing opportunities for selective pharmacological enhancement of K_ATP current even after decrease of this current by painful-like nerve injury.

Redox properties of the adenoside triphosphate-sensitive K+ channel in brain mitochondria

Brain mitochondrial ATP-sensitive K+ channel (mitoK(ATP)) opening by diazoxide protects against ischemic damage and excitotoxic cell death. Here we studied the redox properties of brain mitoK(ATP) . MitoK(ATP) activation during excitotoxicity in cultured cerebellar granule neurons prevented the accumulation of reactive oxygen species (ROS) and cell death. Furthermore, mitoK(ATP) activation in isolated brain mitochondria significantly prevented H2O2 release by these organelles but did not change Ca2+ accumulation capacity. Interestingly, the activity of mitoK(ATP) was highly dependent on redox state. The thiol reductant mercaptopropionylglycine prevented mitoK(ATP) activity, whereas exogenous ROS activated the channel. In addition, the use of mitochondrial substrates that led to higher levels of endogenous mitochondrial ROS release closely correlated with enhanced K+ transport activity through mitoK(ATP). Altogether, our results indicate that brain mitoK(ATP) is a redox-sensitive channel that controls mitochondrial ROS release.

Potential Benefits of Peroxynitrite

Peroxynitrite (PN) is generated by the reaction of nitric oxide (NO) and superoxide in one of the most rapid reactions in biology. Studies have reported that PN is a cytotoxic molecule that contributes to vascular injury in a number of disease states. However, it has become apparent that PN has beneficial effects including vasodilation, inhibition of platelet aggregation, inhibition of inflammatory cell adhesion, and protection against ischemia/reperfusion injury in the heart. It is our hypothesis that PN may serve to inactivate superoxide and prolong the actions of NO in the circulation. This manuscript reviews the beneficial effects of PN in the cardiovascular system.

L-beta,beta-dimethylcysteine attenuates the haemodynamic responses elicited by systemic injections of peroxynitrite in anaesthetized rats

1 There is direct chemical evidence that L-beta,beta-dimethylcysteine (L-penicillamine (L-PEN)) is a scavenger of peroxynitrite. The aim of this study was to determine whether L-PEN attenuates the haemodynamic responses elicited by peroxynitrite in pentobarbital-anaesthetized rats. 2 Peroxynitrite (1-20 micromol kg(-1), i.v.) elicited dose-dependent reductions in mean arterial blood pressure (MAP) and mesenteric and hindquarter vascular resistances. 3 L-PEN (2 mmol kg(-1), i.v.) elicited relatively minor but significant increases in MAP and vascular resistances. The initial reductions in MAP and vascular resistances elicited by peroxynitrite were not diminished after administration of L-PEN whereas they were much shorter in duration. As such, the total reductions in MAP and vascular resistances were markedly reduced by L-PEN. 4 The finding that L-PEN (2 mmol kg(-1), i.v.) did not affect the hypotensive or vasodilator responses elicited of the ATP-dependent potassium-channel agonist, cromakalim (3-18 microg kg(-1), i.v.), suggests that this dose of L-PEN is not a nonselective inhibitor of vasodilation. 5 These findings suggest that L-PEN may effectively scavenge peroxynitrite in vivo and/or interfere with the mechanisms by which peroxynitrite elicits its vasodilator responses.

Redox reaction modulates transient and persistent sodium current during hypoxia in guinea pig ventricular myocytes

Whole-cell and cell-attached patch clamp techniques were applied on isolated guinea pig ventricular myocytes to study the possible regulatory mechanisms of redox agent on persistent and transient sodium current related to hypoxia. The results showed that hypoxia for 15 min increased persistent sodium current (I (Na.P)) and decreased transient sodium current (I (Na.T)) at the same time, while 1 mmol/l of reduced glutathione (GSH) could reverse the increased I (Na.P) and the decreased I (Na.T) simultaneously. Both persistent and transient sodium channel activities could be reversed concurrently again by application of 1 mmol/l oxidized glutathione (GSSG). Hypoxia for 15 min decreased the action potential amplitude (APA) and shortened action potential duration at 90% repolarization (APD(90)) of ventricular papillary cells simultaneously, while 1 mmol/GSH could reverse the decreased APA and the shortened APD(90) at the same time; 1 mmol/l GSSG strengthened the decrease of APA induced by hypoxia and attenuated the decurtation of APD(90) induced by hypoxia compared with pure hypoxia. The correlation between I (Na.P) and I (Na.T) and the effects of GSH and GSSG on them suggested that during hypoxia, redox regulation played a tremendous part in sodium channel activity and that I (Na.P) and I (Na.T) might be charged by the same channel with different gating modes in guinea pig ventricular myocytes. Judging from their alterations during hypoxia and exposure to GSH and GSSG, we speculated that an interconversion might exist between I (Na.P) and I (Na.T). That was when one of them was increased, the other was decreased, and vice versa.

Loss of K+ATP-channel-mediated vasodilation after induction of tachyphylaxis to peroxynitrite

Systemic injections of peroxynitrite elicit pronounced vasodilator responses in rats by activation of ATP-dependent K+ channels (K+ATP-channels). The aim of this study was to determine whether development of tachyphylaxis to the vasodilator actions of peroxynitrite involves the loss of K+ATP-channel function. The falls in mean arterial blood pressure (MAP) and mesenteric and hindquarter vascular resistances produced by the K+ATP-channel agonist, cromakalim (3-18 microg/kg, iv), and the nitric oxide (NO) donor, sodium nitroprusside (SNP; 1-4 microg/kg, iv), were determined in pentobarbital-anesthetized rats before and after induction of tachyphylaxis to peroxynitrite induced by the administration of 10 injections of peroxynitrite (10 micromol/kg, iv). The first dose of peroxynitrite elicited pronounced falls in MAP and vascular resistances whereas the tenth injection elicited much smaller responses that were equivalent to those of decomposed peroxynitrite. Before induction of tachyphylaxis to peroxynitrite, cromakalim and SNP produced dose-dependent reductions in MAP and vascular resistances. The hemodynamic actions of cromakalim were markedly attenuated after induction of tachyphylaxis to peroxynitrite whereas the SNP-induced responses were only slightly attenuated. These results suggest that tachyphylaxis to the vasodilator actions of peroxynitrite involves the loss of K+ATP-channel function whereas tachyphylaxis to peroxynitrite minimally affects NO-mediated vasodilation. Taken together, these findings raise the possibility that peroxynitrite inhibits K+ATP-channel function by oxidation and/or nitration of amino acids in these channels.

Hydrogen sulfide protects HT22 neuronal cells from oxidative stress

Hydrogen sulfide (H2S) is a neuromodulator in the brain and a relaxant for smooth muscle. H2S protects primary cortical neurons from oxidative stress by increasing the intracellular concentrations of glutathione, the major antioxidant in cells. However, changes in glutathione alone are not sufficient to account for full protection in all types of nerve cells. H2S is here shown to protect an immortalized mouse hippocampal cell line from oxidative glutamate toxicity by activating ATP-dependent K+ (KATP) and Cl- channels, in addition to increasing the levels of glutathione. The present study therefore identifies a novel pathway for H2S protection from oxidative stress.

Peroxynitrite Elicits Dysfunction of Stereoselective S-Nitrosocysteine Recognition Sites

The aim of this study was to determine whether induction of tachyphylaxis to peroxynitrite (induced by giving 10 intravenous injections of a 10-micromol/kg dose) differentially affects the vasodilator responses elicited by systemic injections of the L- and D-isomers of S-nitrosocysteine (L-SNC and D-SNC), in pentobarbital-anesthetized rats. L- and D-SNC (12.5-200 nmol/kg, iv) elicited dose-dependent reductions in hindquarter, mesenteric, and renal vascular resistances. The L-SNC-induced vasodilator responses in the hindquarter and renal vascular beds were virtually abolished whereas the vasodilator responses in mesenteric bed were markedly diminished after administration of peroxynitrite. The D-SNC-induced vasodilator responses in the hindquarter and renal beds were slightly attenuated whereas the vasodilator responses in the mesenteric bed were not diminished after administration of peroxynitrite. The vasodilator responses elicited by the nitric oxide donor, MAHMA NONOate (5-50 nmol/kg, iv), were not attenuated by peroxynitrite. The finding that induction of tachyphylaxis to peroxynitrite diminishes the effects of L- and D-SNC but not MAHMA NONOate suggests that the stereoisomers exert their vasodilator effects by mechanisms other than their decomposition to nitric oxide. Moreover, the finding that induction of tachyphylaxis to peroxynitrite causes a more pronounced attenuation of the vasodilator effects of L- than D-SNC supports evidence that the stereoisomers differentially interact with stereoselective S-nitrosothiol recognition sites in the vasculature. Taken together, these novel results support the possibility that peroxynitrite diminishes the vasodilator potencies of L- and D-SNC by oxidation and/or nitration of amino acids in these recognition sites.

Activation of KATP channels by H2S in rat insulin-secreting cells and the underlying mechanisms

H2S is an important gasotransmitter, generated in mammalian cells from L-cysteine metabolism. As it stimulates K(ATP) channels in vascular smooth muscle cells, H2S may also function as an endogenous opener of K(ATP) channels in INS-1E cells, an insulin-secreting cell line. In the present study, K(ATP) channel currents in INS-1E cells were recorded using the whole-cell and single-channel recording configurations of the patch-clamp technique. K(ATP) channels in INS-1E cells have a single-channel conductance of 78 pS. These channels were activated by diazoxide and inhibited by gliclazide. ATP (3 mm) in the pipette solution inhibited K(ATP) channels in INS-1E cells. Significant amount of H2S was produced from INS-1E cells in which the expression of cystathinonie gamma-lyase (CSE) was confirmed. After INS-1E cells were transfected with CSE-targeted short interfering RNA (CSE-siRNA) or treated with DL-propargylglycine (PPG; 1-5 mm) to inhibit CSE, endogenous production of H2S was abolished. Increase in extracellular glucose concentration significantly decreased endogenous production of H2S in INS-1E cells, and increased insulin secretion. After transfection of INS-1E cells with adenovirus containing the CSE gene (Ad-CSE) to overexpress CSE, high glucose-stimulated insulin secretion was virtually abolished. Basal K(ATP) channel currents were significantly reduced after incubating INS-1E cells with a high glucose concentration (16 mm) or lowering endogenous H2S level by CSE-siRNA transfection. Under these conditions, exogenously applied H2S significantly increased whole-cell K(ATP) channel currents at concentrations equal to or lower than 100 microm. H2S (100 microm) markedly increased open probability by more than 2-fold of single K(ATP) channels (inside-out recording) in native INS-1E cells (n = 4, P < 0.05). Single-channel conductance and ATP sensitivity of K(ATP) channels were not changed by H2S. In conclusion, endogenous H2S production from INS-1E cells varies with in vivo conditions, which significantly affects insulin secretion from INS-1E cells. H2S stimulates K(ATP) channels in INS-1E cells, independent of activation of cytosolic second messengers, which may underlie H2S-inhibited insulin secretion from these cells. Interaction among H2S, glucose and the K(ATP) channel may constitute an important and novel mechanism for the fine control of insulin secretion from pancreatic beta-cells.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Hydrogen peroxide modulates K+ ion currents in cultured Aplysia sensory neurons

Hydrogen peroxide (H(2)O(2)) causes oxidative stress and is considered a mediator of cell death in various organisms. Our previous studies showed that prolonged (>6 h) treatment of Aplysia sensory neurons with 1 mM H(2)O(2) produced hyperpolarization of the resting membrane potential, followed by apoptotic morphological changes. In this study, we examined the effect of H(2)O(2) on the membrane conductance of Aplysia sensory neurons. Hyperpolarization was induced by 10 mM H(2)O(2) within 1 h, and this was attributed to increased membrane conductance. In addition, treatment with 10 mM H(2)O(2) for 3 min produced immediate depolarization, which was due to decreased membrane conductance. The H(2)O(2)-induced hyperpolarization and depolarization were completely blocked by dithiothreitol, a disulfide-reducing agent. The later increase of membrane conductance induced by H(2)O(2) was completely blocked by 100 mM TEA, a K(+) channel blocker, suggesting that H(2)O(2)-induced hyperpolarization is due to the activation of K(+) conductance. However, the inhibition of K(+) efflux by TEA did not protect against H(2)O(2)-induced cell death in cultured Aplysia sensory neurons, which indicates that the signal pathway leading to H(2)O(2)-induced cell death is more complicated than expected.

Redox signal transduction: reductive reasoning

The activation of receptors has been believed to be due to a conformational change that occurs when the agonist "locks" into the receptor. However, evidence suggests that several receptors are activated by redox reactions, which occur when an agonist binds with the receptor. The stereochemistry of the receptor likely provides specificity to the electron transfer by determining which agonist can bind to the receptor. The resulting signal, in some cases, may then be transferred across the membrane by G-proteins, which also are redox-coupled. This concept puts receptors into the large group of cell functions that are redox-regulated. Other systems for which evidence of redox regulation occurs include ion pumps and channels, as well as transcription factors.

An extracellular sulfhydryl group modulates background Na(+) conductance and cytosolic Ca(2+) in pituitary cells

Treatment of GH(3) pituitary cells with p-chloromercurybenzenesulfonate (PCMBS) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)). This effect was reversed by dithiothreitol and blocked by L-type Ca(2+) channel antagonists or Na(+) removal. PCMBS increased membrane conductance and depolarized the plasma membrane. Apart from minor effects on K(+) and Ca(2+) channels, PCMBS increased (6 times at -80 mV) an inward Na(+) current whose properties were similar to those of a background Na(+) conductance (BNC) described previously, necessary for generation of spontaneous electrical activity. In rat lactotropes and somatotropes in primary culture, PCMBS also produced a Na(+)-dependent [Ca(2+)](i) increase, whereas little or no effect was observed in thyrotropes, corticotropes, and gonadotropes. The Na(+) conductance elicited by PCMBS in somatotropes seemed to be the same as that stimulated by the hypothalamic growth hormone (GH)-releasing hormone, which regulates membrane excitability and GH secretion. The BNC studied here could play a physiological role, regulating excitability and spontaneous activity, and explains satisfactorily the [Ca(2+)](i)-increasing actions of the mercurials reported previously in several excitable tissues.

Homocysteine reduces smooth muscle [Ca2+]i and constrictor responses of isolated arterioles

Chronic elevation of plasma homocysteine concentration has been shown to be associated with impaired vascular function. The acute direct effect of homocysteine on the tone and vasoactive responses of arterioles and the possible underlying mechanisms, however, have not yet been elucidated. Thus arterioles were isolated from gracilis muscle of rats (d: approximately 130 microm) and their diameter was measured by videomicroscopy. Homocysteine (10(-6)-10(-4) M) elicited dose-dependent dilation of arterioles (maximum: 44+/-6% at 10(-4) M). The dilation was not affected by the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester or by removal of the endothelium, or the free radical scavenger catalase and superoxide dismutase, or the K+ channel inhibitors glibenclamide, 4-aminopyridine, or tetraethyl ammonium. Incubation of vessels with homocysteine (10(-4) M, 20 min) did not affect dilations to acetylcholine or sodium nitroprusside, whereas it significantly decreased constrictions to norepinephrine (at 10(-6) M; control: 57+/-7%, homocysteine: 21+/-5%) and to the thromboxane A2 analogue U46619 (at 10(-8) M: control: 44+/-3%, homocysteine: 20+/-4%). Homocysteine (10(-4) M), similar to the voltage-operated Ca2+ channel inhibitor nitrendipine (10(-8) M), significantly decreased the arteriolar smooth muscle [Ca2+]i as assessed by changes in the fura-2 ratiometric signal (R(Ca), -6+/-1% and -24+/-3%, respectively). These data suggest that in isolated arterioles homocysteine decreases pressure-induced tone and responses to vasoconstrictor agents, likely by altering Ca2+ signaling of arteriolar smooth muscle.

Diazoxide-induced cardioprotection requires signaling through a redox-sensitive mechanism

Diazoxide, a selective opener of the mitochondrial ATP-sensitive potassium channel, has been shown to elicit tolerance to ischemia in cardiac myocytes and in perfused heart. However, the mechanism of this cardioprotection is poorly understood. Because reactive oxygen species (ROS) are recognized as important intracellular signaling molecules and have been implicated in ischemic preconditioning, we examined diazoxide-induced ROS production in adult cardiomyocytes. Cells treated with 50 micromol/L diazoxide showed a 173% increase in ROS production relative to baseline. 5-Hydroxydecanoate was found to attenuate the diazoxide-induced increase in ROS generation. The diazoxide-induced increase in ROS also was abrogated by the addition of either the antioxidant N-acetylcysteine (NAC) or N-mercaptopropionylglycine. We also examined the ability of NAC to block the protective effects of diazoxide in the perfused rat heart. After 20 minutes of global ischemia and 20 minutes of reflow, hearts perfused with 100 micromol/L diazoxide before ischemia showed significantly improved postischemic contractile function relative to untreated hearts (84% versus 29% of initial left ventricular developed pressure, respectively). Hearts treated with diazoxide in the presence of 4 mmol/L NAC recovered 53% of initial left ventricular developed pressure, whereas hearts treated with NAC alone recovered 46% of preischemic function. Using (31)P NMR spectroscopy, we found that, similar to preconditioning, diazoxide significantly attenuated ischemia-induced intracellular acidification and enhanced post- ischemic recovery of phosphocreatine levels, both of which were blocked by cotreatment with NAC. These data suggest that the cardioprotective actions of diazoxide are mediated by generation of a pro-oxidant environment.

Triple-bonded unsaturated fatty acids are redox active compounds

Unsaturated fatty acids with triple bonds are used as inhibitors of unsaturated fatty acid metabolism or cytochrome P450 reactions because they are believed to be chemically inert. In this paper we use in vitro cytochrome C reduction to show that two commonly used triple-bonded unsaturated fatty acids are in fact potent electron transfer agents and could affect the multiple cellular systems that are redox-modulated.

Block of sodium channels by divalent mercury: role of specific cysteinyl residues in the P-loop region

Divalent mercury (Hg(2+)) blocked human skeletal Na(+) channels (hSkM1) in a stable dose-dependent manner (K(d) = 0.96 microM) in the absence of reducing agent. Dithiothreitol (DTT) significantly prevented Hg(2+) block of hSkM1, and Hg(2+) block was also readily reversed by DTT. Both thimerosal and 2,2'-dithiodipyridine had little effect on hSkM1; however, pretreatment with thimerosal attenuated Hg(2+) block of hSkM1. Y401C+E758C rat skeletal muscle Na(+) channels (mu1) that form a disulfide bond spontaneously between two cysteines at the 401 and 758 positions showed a significantly lower sensitivity to Hg(2+) (K(d) = 18 microM). However, Y401C+E758C mu1 after reduction with DTT had a significantly higher sensitivity to Hg(2+) (K(d) = 0.36 microM) than wild-type hSkM1. Mutants C753Amu1 (K(d) = 8.47 microM) or C1521A mu1 (K(d) = 8.63 microM) exhibited significantly lower sensitivity to Hg(2+) than did wild-type hSkM1, suggesting that these two conserved cysteinyl residues of the P-loop region may play an important role in the Hg(2+) block of the hSkM1 isoform. The heart Na(+) channel (hH1) was significantly more sensitive to low-dose Hg(2+) (K(d) = 0.43 microM) than was hSkM1. The C373Y hH1 mutant exhibited higher resistance (K(d) = 1.12 microM) to Hg(2+) than did wild-type hH1. In summary, Hg(2+) probably inhibits the muscle Na(+) channels at more than one cysteinyl residue in the Na(+) channel P-loop region. Hg(2+) exhibits a lower K(d) value (<1. 23 microM) for inhibition by forming a sulfur-Hg-sulfur bridge, as compared to reaction at a single cysteinyl residue with a higher K(d) value (>8.47 microM) by forming sulfur-Hg(+) covalently. The heart Na(+) channel isoform with more than two cysteinyl residues in the P-loop region exhibits an extremely high sensitivity (K(d) < 0. 43 microM) to Hg(+), accounting for heart-specific high sensitivity to the divalent mercury.

Dual effect of NO on K(+)(ATP) current of mouse pancreatic B-cells: stimulation by deenergizing mitochondria and inhibition by direct interaction with the channel

Nitric oxide (NO) is assumed to contribute to the impairment of B-cell function in type 1 diabetes mellitus (IDDM). In the present paper we show that in mouse B-cells with intact metabolism authentic NO (20 microM) led to a biphasic effect on the K(+)(ATP) current, namely a transient increase and a consecutive almost complete inhibition. This resembles closely the effect that we have observed previously with the NO donor S-nitrosocysteine (SNOC, 1 mM) suggesting that merely NO caused both phases of this effect. We now demonstrate that the rise in the current amplitude was accompanied by a depolarization of the mitochondrial membrane potential DeltaPsi and a concomitant reduction in the ATP/ADP ratio. Thus, it seems likely that the increase in current amplitude is due to the interference of NO with cell metabolism. The subsequent inhibition of the K(+)(ATP) current is assumed to be caused by a direct effect on the channel since K(+)(ATP) single channel current activity measured in excised patches was strongly reduced by authentic NO and SNOC. Our data reveal new insights into the mechanisms underlying the biphasic action of NO on K(+)(ATP) channels in pancreatic B-cells.

GTP (gammaS) and GDP (betaS) as electron donors: new wine in old bottles

G proteins are membrane-bound regulatory proteins which modulate the activity of ion channels and other effector systems. The GTP and GDP analogs GTP (gammaS) and GDP (betaS) have been used to study the role of G proteins in numerous physiologic systems. The prolonged effects of these analogs have been thought to be due to the fact that they are nonhydrolyzable. However, in this paper we show that the GTP (gammaS) and GDP (betaS) analogs are potent reducing agents at physiologic pH. This observation suggests that previous data obtained using these compounds may need to be reinterpreted.

Thimerosal: a versatile sulfhydryl reagent, calcium mobilizer, and cell function-modulating agent

An overview of the literature concerning the effects of thimerosal is presented. Because of its antibacterial effect, thimerosal is used for a variety of practical purposes such as antiseptic and preservative. In biomedical studies, thimerosal is used as a sulfhydryl reagent, and as a calcium-mobilizing agent. The ability of thimerosal to act as a sulfhydryl group is related to the presence of mercury. Relatively little study has been devoted to the mechanism of the reaction of thimerosal with the sulfhydryl group; the sulfhydryl reactive capacity is mostly concluded on the basis of inactivation of the effect by dithiothreitol (DTT). Thimersal causes a release of calcium from intracellular stores in many cells types; this is followed by an influx of extracellular calcium. Both InsP3- and ryanodine-sensitive calcium stores may be affected. Studies with permeabilized cells or organelles show that the effect of thimerosal on calcium is dependent on the concentration: low concentrations of thimerosal stimulate calcium release, high concentrations are inhibitory. This dependence is not found in intact cells. Thimerosal may activate or inhibit a number of cell functions. These are often related to the ability to release calcium or with the sulfhydryl reactivity. In platelets, thimerosal causes aggregation, increase of arachidonic acid metabolism, and exocytotic release of serotonin. In neutrophils, thimerosal causes, besides an increase of cytosolic free calcium, an increase of formyl-methionyl-leucyl-phenylalanine (fMLP)-activated leukotriene release, and a modulation of chemotactic migration and exocytosis. At low concentrations, thimerosal induces chemotactic migration of neutrophils, in the absence of other chemoattractants. The effect is also observed with thiosalicylic acid, indicating that the stimulation of migration was due to the thiosalicylic acid moiety of the thimerosal molecule. At higher concentrations, thimerosal causes inhibition of fMLP-activated migration. Low concentrations of thimerosal, but not of thiosalicylic acid, induced exocytotic enzyme release from neutrophils. High concentrations of thimerosal inhibited fMLP-activated exocytosis. The results point to an involvement of calcium mobilization and calcium influx of activation, and reaction with sulfhydryl groups for inhibition.

Regulation of mitochondrial KATP channel by redox agents

The ATP-dependent K+ channel (KATP) was purified from the inner mitochondrial membrane and reconstituted into lipid bilayer membranes. KATP activity was inhibited by high concentrations of ATP and ADP, but activated by low concentrations (up to 200 microM) of ADP. p-Diethylaminoethylbenzoate (DEB) acted as a KATP opener: at micromolar concentrations, it reversed inhibition by ATP and ADP and it also prevented KATP rundown. Pelargonidine, extracted from flowers of Pelargonium, reduced spontaneous activity of KATP channels and diminished their potentiation by DEB. Their opposite action on KATP corresponded with their opposite redox properties in reactions with free radicals: DEB behaved as an electron donor, whereas pelargonidine acted as an electron acceptor. We hypothesize that thiol groups on mitoKATP are targets for redox-active ligans.

Oxidation regulates cloned neuronal voltage-dependent Ca2+ channels expressed in Xenopus oocytes

Functional modifications of neuronal P/Q-type voltage-dependent Ca2+ channels expressed in Xenopus oocytes by oxidation were examined electrophysiologically. Oxidation by external H2O2 enhanced the whole-oocyte currents through the Ca2+ channels composed of the alpha1A, alpha2/delta, and beta3 subunits at negative voltages (<0 mV) without markedly affecting the currents at more positive voltages. Single-channel analysis showed that oxidation accelerates the overall channel opening process. The effect of H2O2 to enhance the Ca2+ channel activity did not require heterologous expression of the alpha2/delta subunit, and it was not mimicked by a cysteine-specific oxidizing agent. The results suggest that oxidative stress may regulate the activity of neuronal Ca2+ channels and that regulation by oxidation may be important in some clinical situations, such as in reperfusion injury after ischemic episodes.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Regulation of the human ether-a-gogo related gene (HERG) K+ channels by reactive oxygen species

Human ether-a-gogo related gene (HERG) K+ channels are key elements in the control of cell excitability in both the cardiovascular and the central nervous systems. For this reason, the possible modulation by reactive oxygen species (ROS) of HERG and other cloned K+ channels expressed in Xenopus oocytes has been explored in the present study. Exposure of Xenopus oocytes to an extracellular solution containing FeSO4 (25-100 microM) and ascorbic acid (50-200 microM) (Fe/Asc) increased both malondialdehyde content and 2',7'-dichlorofluorescin fluorescence, two indexes of ROS production. Oocyte perfusion with Fe/Asc caused a 50% increase of the outward K+ currents carried by HERG channels, whereas inward currents were not modified. This ROS-induced increase in HERG outward K+ currents was due to a depolarizing shift of the voltage-dependence of channel inactivation, with no change in channel activation. No effect of Fe/Asc was observed on the expressed K+ currents carried by other K+ channels such as bEAG, rDRK1, and mIRK1. Fe/Asc-induced stimulation of HERG outward currents was completely prevented by perfusion of the oocytes with a ROS scavenger mixture (containing 1,000 units/ml catalase, 200 ng/ml superoxide dismutase, and 2 mM mannitol). Furthermore, the scavenger mixture also was able to reduce HERG outward currents in resting conditions by 30%, an effect mimicked by catalase alone. In conclusion, the present results seem to suggest that changes in ROS production can specifically influence K+ currents carried by the HERG channels.

Redox modulation of hslo Ca2+-activated K+ channels

The modulation of ion channel proteins by cellular redox potential has emerged recently as a significant determinant of channel function. We have investigated the influence of sulfhydryl redox reagents on human brain Ca2+-activated K+ channels (hslo) expressed in both human embryonic kidney 293 cells and Xenopus oocytes using macropatch and single-channel analysis. Intracellular application of the reducing agent dithiothreitol (DTT): (1) shifts the voltage of half-maximal channel activation (V0.5) approximately 18 mV to more negative potentials without affecting the maximal conductance or the slope of the voltage dependence; (2) slows by approximately 10-fold a time-dependent right-shift in V0.5 values ("run-down"); (3) speeds macroscopic current activation kinetics by approximately 33%; and (4) increases the single-channel open probability without affecting the unitary conductance. In contrast to DTT treatment, oxidation with hydrogen peroxide shifts macropatch V0.5 values to more positive potentials, increases the rate of channel run-down, and decreases the single-channel open probability. KCa channels cloned from Drosophila differ from hslo channels in that they show very little run-down and are not modulated by the addition of DTT. These data indicate that hslo Ca2+-activated K+ channels may be modulated by changes in the cellular redox potential as well as by the transmembrane voltage and the cytoplasmic Ca2+ concentration.

Redox regulation of large conductance Ca(2+)-activated K+ channels in smooth muscle cells

The effects of sulfhydryl reduction/oxidation on the gating of large-conductance, Ca(2+)-activated K+ (maxi-K) channels were examined in excised patches from tracheal myocytes. Channel activity was modified by sulfhydryl redox agents applied to the cytosolic surface, but not the extracellular surface, of membrane patches. Sulfhydryl reducing agents dithiothreitol, beta-mercaptoethanol, and GSH augmented, whereas sulfhydryl oxidizing agents diamide, thimerosal, and 2,2'-dithiodipyridine inhibited, channel activity in a concentration-dependent manner. Channel stimulation by reduction and inhibition by oxidation persisted following washout of the compounds, but the effects of reduction were reversed by subsequent oxidation, and vice versa. The thiol-specific reagents N-ethylmaleimide and (2-aminoethyl)methanethiosulfonate inhibited channel activity and prevented the effect of subsequent sulfhydryl oxidation. Measurements of macroscopic currents in inside-out patches indicate that reduction only shifted the voltage/nP0 relationship without an effect on the maximum conductance of the patch, suggesting that the increase in nP0 following reduction did not result from recruitment of more functional channels but rather from changes of channel gating. We conclude that redox modulation of cysteine thiol groups, which probably involves thiol/disulfide exchange, alters maxi-K channel gating, and that this modulation likely affects channel activity under physiological conditions.

Thiol oxidation by 2,2'-dithiodipyridine causes a reversible increase in cytoplasmic free Ca2+ concentration in pancreatic beta-cells. Role for inositol 1,4,5-trisphosphate-sensitive Ca2+ stores

2,2'-Dithiodipyridine (2,2'-DTDP), a reactive disulphide that mobilizes Ca2+ from ryanodine-sensitive Ca2+ stores in muscle, induced a biphasic increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with fura 2. This increase consisted of an early transient followed by a second, slower, rise. The [Ca2+]i transient was dependent on extracellular Ca2+ and disappeared on treatment with nimodipine. The reactive disulphide caused plasma membrane depolarization, as studied by the perforated-patch configuration of the patch-clamp technique. Hence membrane depolarization and opening of the L-type voltage-gated Ca2+ channels were responsible for the first transient in [Ca2+]i. The second slower increase in [Ca2+]i was prolonged but readily reversed by the disulphide-reducing agent 1,4-dithiothreitol. This increase in [Ca2+]i was not decreased by nimodipine or by omission of extracellular Ca2+, but was eliminated when the Ins(1,4,5)P3-sensitive Ca2+ pool was first depleted by carbachol. Ryanodine or its beta-alanyl analogue did not release Ca2+ from intracellular stores, and a high concentration of ryanodine did not inhibit Ca2+ release by 2,2'-DTDP. The disulphide compound suppressed glucose metabolism and decreased the mitochondrial inner-membrane potential. We conclude that thiol oxidation by 2,2'-DTDP affects Ca2+ homeostasis in beta-cells by multiple mechanisms. However, unlike the situation in muscle, in beta-cells 2,2'-DTDP releases Ca2+ from intracellular pools by mechanisms that do not involve activation of ryanodine receptors. Instead, in these cells the Ins(1,4,5)P3-sensitive intracellular Ca2+ store comprises an alternative target for the Ca(2+)-mobilizing action of the reactive disulphide compound.

Oscillations in KATP channel activity promote oscillations in cytoplasmic free Ca2+ concentration in the pancreatic beta cell

Pancreatic beta cells exhibit oscillations in electrical activity, cytoplasmic free Ca2+ concentration ([Ca2+](i)), and insulin release upon glucose stimulation. The mechanism by which these oscillations are generated is not known. Here we demonstrate fluctuations in the activity of the ATP-dependent K+ channels (K(ATP) channels) in single beta cells subject to glucose stimulation or to stimulation with low concentrations of tolbutamide. During stimulation with glucose or low concentrations of tolbutamide, K(ATP) channel activity decreased and action potentials ensued. After 2-3 min, despite continuous stimulation, action potentials subsided and openings of K(ATP) channels could again be observed. Transient suppression of metabolism by azide in glucose-stimulated beta cells caused reversible termination of electrical activity, mimicking the spontaneous changes observed with continuous glucose stimulation. Thus, oscillations in K(ATP) channel activity during continuous glucose stimulation result in oscillations in electrical activity and [Ca2+](i).

Different modulation of Ca-activated K channels by the intracellular redox potential in pulmonary and ear arterial smooth muscle cells of the rabbit

We investigated the electrical responses of Ca-activated K (KCa) currents induced by hypoxia and reduction or oxidation of the channel protein in pulmonary (PASMC) and ear (EASMC) arterial smooth muscle cells using the patch-clamp technique. In cell-attached patches, in the presence of a high K solution (containing 0.316 microM Ca2+), the activity of KCa channels from PASMC was decreased (by 49 +/- 7% compared to control, pipette potential = -70 mV) by changing to a hypoxic solution (1 mM Na2S2O4, aeration with 100% N2 gas). EASMC channels did not respond to hypoxia. In order to investigate the possible mechanisms involved, using inside-out patches bathed symmetrically in 150 mM KCl, we applied redox couples to the intracellular side. Reducing agents, such as dithiothreitol (DDT, 5 mM), reduced glutathione, (GSH, 5 mM), and nicotinamide adenine dinucleotide reduced (NADH, 2 mM) decreased PASMC, but not EASMC, KCa channel activity. However, oxidizing agents such as 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB, 1 mM), oxidized glutathione (GSSG, 5 mM) and NAD (2 mM) increased KCa channel activity in both PASMC and EASMC. The increased activity due to oxidizing agents was restored by applying reducing agents. From these results, we could suggest that the basal redox state of the EASMC KCa channel is more reduced than that of the PASMC channel, since the response of KCa channels of the EASMC to intracellular reducing agents differs from that of the PASMC. This difference may be related to the different responses of PASMC and EASMC KCa channels to hypoxia.

Oxygen-derived free radical stress activates nonselective cation current in guinea pig ventricular myocytes. Role of sulfhydryl groups

Oxygen-derived free radicals (O-Rs) cause alterations in cardiac electrical activity, including sustained depolarization, which may contribute to arrhythmic activity in reperfusion after ischemia. The ionic current(s) and cellular mechanism(s) underlying the sustained depolarization are not well defined. We used the whole-cell variant of the patch-clamp technique to study sustained depolarization in guinea pig ventricular myocytes during the extracellular application of O-Rs (generating system: dihydroxyfumaric acid, 3 to 6 mmol/L; FeCl3/ADP, 0.05:0.5 mmol/L). Myocytes superfused with O-Rs (pipette EGTA, 0.1 mmol/L) showed (1) sustained depolarization to between -40 and -10 mV, (2) oscillations in membrane potential, and (3) triggered activity. The depolarization resulted from an increase in quasi-steady state difference current reversing at approximately -18 mV, and the oscillations were due to transient inward current. The latter were inhibited with ryanodine (10 mumol/L) or high pipette EGTA (5 mmol/L), but the steady state current was unaffected. Nonselective cation current (INSC) (recorded with Cs+, Li+, and Mg2+ replacing K+, Na+, and Ca2+, respectively; 20 mmol/L tetraethylammonium chloride [TEA] and 5 mmol/L BAPTA in the pipette solution and 10 mmol/L TEA, 10 mumol/L tetrodotoxin, and 10 mumol/L nicardipine in the bath solution) was activated by O-Rs; the increase in current was unaffected by preventing changes in [Ca2+]i but was inhibited with dithiothreitol. Oxidizing agents (diamide and thimerosal) or caffeine (pipette EGTA, 0.1 mmol/L) produced a similar increase in membrane conductance. INSC activated with O-Rs, oxidizing agents, or caffeine was sensitive to SK&F 96365. O-R treatment was without effect when INSC was already activated with caffeine. The data suggest that (1) extracellular O-Rs activate a Ca(2+)-sensitive INSC in the absence of changes in [Ca2+]i, (2) oxidative modification of extracellular sulfhydryl groups may be involved, and (3) this mechanism is different from the Ca(2+)-dependent activation of INSC by intracellular O-Rs, indicating that O-Rs may alter ion channel activity by differential mechanisms, depending on the compartment, extracellular or intracellular, in which they are present.

Effects of caffeine on cytoplasmic free Ca2+ concentration in pancreatic beta-cells are mediated by interaction with ATP-sensitive K+ channels and L-type voltage-gated Ca2+ channels but not the ryanodine receptor

In the pancreatic beta-cell, an increase in the cytoplasmic free Ca2+ concentration ([Ca2+]i) by caffeine is believed to indicate mobilization of Ca2+ from intracellular stores, through activation of a ryanodine receptor-like channel. It is not known whether other mechanisms, as well, underlie caffeine-induced changes in [Ca2+]i. We studied the effects of caffeine on [Ca2+]i by using dual-wavelength excitation microfluorimetry in fura-2-loaded beta-cells. In the presence of a non-stimulatory concentration of glucose, caffeine (10-50 mM) consistently increased [Ca2+]i. The effect was completely blocked by omission of extracellular Ca2+ and by blockers of the L-type voltage-gated Ca2+ channel, such as D-600 or nifedipine. Depletion of agonist-sensitive intracellular Ca2+ pools by thapsigargin did not inhibit the stimulatory effect of caffeine on [Ca2+]i. Moreover, this effect of caffeine was not due to an increase in cyclic AMP, since forskolin and 3-isobutyl-1-methylxanthine (IBMX) failed to raise [Ca2+]i in unstimulated beta-cells. In beta-cells, glucose and sulphonylureas increase [Ca2+]i by causing closure of ATP-sensitive K+ channels (KATP channels). Caffeine also caused inhibition of KATP channel activity, as measured in excised inside-out patches. Accordingly, caffeine (> 10 mM) induced insulin release from beta-cells in the presence of a non-stimulatory concentration of glucose (3 mM). Hence, membrane depolarization and opening of voltage-gated L-type Ca2+ channels were the underlying mechanisms whereby the xanthine drug increased [Ca2+]i and induced insulin release. Paradoxically, in glucose-stimulated beta-cells, caffeine (> 10 mM) lowered [Ca2+]i. This effect was due to the fact that caffeine reduced depolarization-induced whole-cell Ca2+ current through the L-type voltage-gated Ca2+ channel in a dose-dependent manner. Lower concentrations of caffeine (2.5-5.0 mM), when added after glucose-stimulated increase in [Ca2+]i, induced fast oscillations in [Ca2+]i. The latter effect was likely to be attributable to the cyclic AMP-elevating action of caffeine, leading to phosphorylation of voltage-gated Ca2+ channels. Hence, in beta-cells, caffeine-induced changes in [Ca2+]i are not due to any interaction with intracellular Ca2+ pools. In these cells, a direct interference with KATP channel- and L-type voltage-gated Ca(2+)-channel activity is the underlying mechanism by which caffeine increases or decreases [Ca2+]i.