Exogenous NO triggers preconditioning via a cGMP- and mitoKATP-dependent mechanism

Exogenous nitric oxide (NO) triggers a preconditioning-like effect in heart via a pathway that is dependent on reactive oxygen species. This study examined the signaling pathway by which the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 2 microM) triggers its anti-infarct effect. Isolated rabbit hearts experienced 30 min of regional ischemia and 120 min of subsequent reperfusion. Infarct size was determined by triphenyltetrazolium chloride staining. Infarct size was reduced from 30.5 +/- 3.0% of the risk zone in control hearts to 10.2 +/- 2.0% in SNAP-treated hearts. Bracketing the SNAP infusion with either the guanylyl cyclase blocker 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (2 microM) or the mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel blocker 5-hydroxydecanoate (200 microM) completely blocked the infarct-sparing effect of SNAP (34.3 +/- 3.8 and 32.2 +/- 1.6% infarction, respectively). Pretreatment of hearts with 8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (10 microM), which is a cell-permeable cGMP analog that activates protein kinase G, mimicked the preconditioning effect of SNAP by reducing infarct size to 7.5 +/- 1.1% of the risk zone. This salutary effect was abolished by either the free radical scavenger N-(2-mercaptopropionyl)glycine (1 mM) or 5-hydroxydecanoate (100 microM; 28.9 +/- 2.7 and 33.6 +/- 5.0% infarction of the risk zone, respectively). To confirm these functional data and the effect of SNAP on the guanylyl cyclase-protein kinase G signaling pathway, cGMP levels were measured. SNAP increased the level from 0.18 +/- 0.04 to 0.61 +/- 0.14 pmol/mg of protein (P < 0.05). These data suggest that exogenous NO triggers the preconditioning effect by initiating a cascade of events including stimulation of guanylyl cyclase to make cGMP, activation of protein kinase G, opening of mitoK(ATP) channels, and, finally, production of reactive oxygen species.

NECA and bradykinin at reperfusion reduce infarction in rabbit hearts by signaling through PI3K, ERK, and NO

The adenosine A1/A2 adenosine agonist 5'-(N-ethylcarboxamido) adenosine (NECA) and bradykinin both limit infarction when administered at reperfusion in rabbits. This study compares the signal transduction pathways responsible for their anti-infarct effect. Receptor agonists were administered to isolated rabbit hearts starting 25 min after the onset of a 30-min period of ischemia and continued into the 2-h reperfusion period. Infarct size was measured. Both NECA and bradykinin decreased infarction from 31.5 +/- 2.4% of the risk zone in untreated hearts to 11.8 +/- 2.0% and 15.4 +/- 2.4%, respectively (P<0.05). Protection from both agents was blocked by PD98059, wortmannin, and Nomega-nitro-L-arginine methyl ester (L-NAME), thus demonstrating dependence on activation of extracellular regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K) and stimulation of nitric oxide synthase (NOS). Both wortmannin and PD98059 prevented phosphorylation of ERK 1/2 in NECA-treated hearts, whereas only wortmannin and not PD98059 blocked Akt phosphorylation. These data suggest Akt is upstream of ERK 1/2. In addition, 8-(3-chlorostyryl) caffeine blocked NECA's protection indicating that A2 adenosine receptors trigger NECA's anti-infarct effect. Of note, both bradykinin and acetylcholine (ACh) administered before ischemia to trigger preconditioning's cardioprotection use PI3K and NOS in their signaling pathway. Curiously, however, ACh, unlike bradykinin, was not protective when administered at reperfusion. Hence, both NECA and bradykinin administered at reperfusion protect through a common signaling pathway that includes PI3K, NO, and ERK.

Augmented taurine release is not the mechanism of ischemic preconditioning?s cardioprotection

In ischemic preconditioning (IPC) a brief ischemic period protects the heart from a subsequent ischemic insult by an unknown mechanism. Osmotic swelling has been proposed to be a major cause of cell death when ischemic tissue is reperfused. The present study tests whether the preconditioned heart during reperfusion might release more taurine, an important osmolyte in the cardiac myocytes, to decrease cellular osmolarity, oppose swelling, and preserve viability. We collected the coronary effluent from isolated rabbit hearts for 10 min before and 10 min after preconditioning with 5 min of global ischemia. The heart then experienced 15 min of global ischemia and effluent was collected during reperfusion for 40 min. A control group was studied similarly but without the preconditioning ischemia. Fifteen min of ischemia was chosen to avoid any taurine release caused by ischemic cell death. Taurine was measured with HPLC. In the IPC group there was a postischemic release over baseline of 5.09 +/- 1.51 micromol (approx 3.3% of the total taurine pool), whereas in the control group the release was not significantly different, 5.72 +/- 1.67 micromol. The percent of the taurine pool lost from each heart during reperfusion was calculated based on an assumption of a total content of 20 microM taurine/gm wet weight. Since the amount of taurine released by the isolated rabbit heart following ischemia was not different in preconditioned and non-preconditioned hearts, we conclude that reduced swelling through taurine release is not the mechanism of the cardioprotective effects of IPC.

Bradykinin induces mitochondrial ROS generation via NO, cGMP, PKG, and mitoKATP channel opening and leads to cardioprotection

Bradykinin (BK) mimics ischemic preconditioning by generating reactive oxygen species (ROS). To identify intermediate steps that lead to ROS generation, rabbit cardiomyocytes were incubated in reduced MitoTracker Red stain, which becomes fluorescent after exposure to ROS. Fluorescence intensity in treated cells was expressed as a percentage of that in paired, untreated cells. BK (500 nM) caused a 51 +/- 16% increase in ROS generation (P < 0.001). Coincubation with either the BK B2-receptor blocker HOE-140 (5 microM) or the free radical scavenger N-(2-mercaptopropionyl)glycine (1 mM) prevented this increase, which confirms that the response was receptor mediated and ROS were actually being measured. Closing mitochondrial ATP-sensitive K+ (mitoKATP) channels with 5-hydroxydecanoate (5-HD, 1 mM) prevented increased ROS generation. BK-induced ROS generation was blocked by Nomega-nitro-m-arginine methyl ester (m-NAME, 200 microM), which implicates nitric oxide as an intermediate. Blockade of guanylyl cyclase with 1-H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ, 10 microM) aborted BK-induced ROS generation but not that from diazoxide, a direct opener of mitoKATP channels. The protein kinase G (PKG) blocker 8-bromoguanosine-3',5'-cyclic monophosphorothioate (25 microM) eliminated the effects of BK. Conversely, direct activation of PKG with 8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate (100 microM) increased ROS generation (39 +/- 15%; P < 0.004) similar to BK. This increase was blocked by 5-HD. Finally, the nitric oxide donor S-nitroso-N-acetylpenicillamine (1 microM) increased ROS by 34 +/- 6%. This increase was also blocked by 5-HD. In intact rabbit hearts, BK (400 nM) decreased infarction from 30.5 +/- 3.0 of the risk zone in control hearts to 11.9 +/- 1.4% (P < 0.01). This protection was aborted by either 200 microM m-NAME or 2 microM ODQ (35.4 +/- 5.7 and 30.4 +/- 3.0% infarction, respectively; P = not significant vs. control). Hence, BK preconditions through receptor-mediated production of nitric oxide, which activates guanylyl cyclase. The resulting cGMP activates PKG, which opens mitoKATP. Subsequent release of ROS triggers cardioprotection.

CGX-1051, A Peptide from Conus Snail Venom, Attenuates Infarction in Rabbit Hearts When Administered at Reperfusion

CGX-1051, isolated from the venom of the marine snail Conus purpurasens, was previously noted to interact with potassium channels. Since potassium channels play an important role in cardiac physiology, we assessed the effect of CGX-1051 on infarct size in a rabbit heart model of ischemia/reperfusion. A coronary branch was occluded for 30 minutes followed by 3 hours of reperfusion in in situ and 2 hours in in vitro preparations. Infarct size was measured with triphenyltetrazolium chloride staining and expressed as a percent of the risk zone. In in situ studies, a bolus intravenous injection of CGX-1051, either 10 or 100 microg/kg, administered 5 minutes before reperfusion, reduced infarct size from 40.4 +/- 2.8% of the risk zone in untreated animals to 19.8 +/- 3.8% and 15.0 +/- 1.9%, respectively. One microg/kg CGX-1051 was not protective. To see if the salvage was sustained, two groups of rabbits underwent 72 hours of reperfusion. The dose of 10 microg/kg infused 5 minutes before reperfusion reduced infarct size from 37.0 +/- 1.6% in untreated rabbits to 15.5 +/- 2.0%. When administered 10 minutes after reperfusion had begun, 100 microg/kg CGX-1051 had no effect. CGX-1051 also reduced infarct size in crystalloid-perfused, isolated rabbit hearts suggesting that protection did not depend on circulating leukocytes. The mitochondrial KATP inhibitors glibenclamide and 5-hydroxydecanoate and the MEK(1/2), ERK and hence, inhibitor PD 98059 aborted protection from CGX-1051. These data indicate that functionally active ERK and mitochondrial KATP channels are necessary for protection. CGX-1051 caused no hemodynamic alterations at any dose tested. We conclude that CGX-1051 has a powerful anti-infarct effect when given just before reperfusion.

Timing and Duration of Administration Are Crucial for Antiinfarct Effect of AMP 579 Infused at Reperfusion in Rabbit Heart

The adenosine A(1)/A(2) receptor agonist AMP 579 has been reported to protect the heart against infarction even when administered after the onset of ischemia. The present study explored both the timing and the duration of treatment required to limit infarct size in in situ rabbit hearts subjected to 30 min of regional ischemia and 3 hours of reperfusion. In groups 1 and 2, AMP 579 was infused from 10 min before reperfusion and continued for either 30 or 40 min. In group 3, AMP 579 was begun at the onset of reperfusion and continued for 70 min. In group 4, AMP 579 was also infused for 70 min but begun 10 min after reperfusion. In untreated hearts 36.4 +/- 3.1% of the risk zone infarcted. Protection was observed only in hearts having a 70-minute infusion of AMP 579 starting at reperfusion (13.0 +/- 1.9%, P < 0.05). Therefore, AMP 579 must be present at the moment of reperfusion and have a continued presence of more than 30 min thereafter to protect. Importantly, because AMP 579 can protect when administered up to the time of reperfusion, it likely prevents a reperfusion injury, and, therefore, has impressive clinical potential.

P1075 opens mitochondrial K(ATP) channels and generates reactive oxygen species resulting in cardioprotection of rabbit hearts

We have recently proposed that opening of mitochondrial K(ATP) channels (mitoK(ATP)) acts as a trigger for preconditioning (PC) by causing mitochondria to produce reactive oxygen species (ROS). Controversy exists as to whether the putative sarcolemma-selective K(ATP) channel opener P1075 also opens mitoK(ATP) channels and may be cardioprotective. We purified mitoK(ATP) channels from either rabbit heart, rat heart or rat brain and reconstituted the proteins into liposomes. mitoK(ATP) channels from each of these tissues were opened by P1075 with EC(50) values of 60-90 nM. We next tested whether P1075 causes rabbit cardiomyocytes to produce ROS in a K(ATP)-dependent fashion. Mitochondrial ROS production was monitored by the appearance of fluorescence as reduced MitoTracker Red was oxidized. P1075 (100 microM) led to a 44 +/- 9% increase in ROS generation (P < 0.001 vs. untreated cells), which was similar to the increase seen with 50 microM diazoxide, a selective mitoK(ATP) channel opener (49 +/- 9%, P < 0.001 vs. untreated cells). The effect of P1075 was equally potent at a concentration of 150 nM. The P1075-induced increase in ROS production was blocked by 50 microM glibenclamide (GLI), a non-selective K(ATP) blocker, and by 5-hydroxydecanoate (1 mM), a highly selective mitoK(ATP) blocker (-6 +/- 14% and +4 +/- 12%, respectively; P = n.s). In isolated rabbit hearts, P1075 (150 nM) markedly reduced infarct size compared to control animals (10.6 +/- 8.1% of the area at risk vs. 31.5 +/- 5.6%, P < 0.05). GLI (5 microM) as well as 5-hydroxydecanoate (200 microM) completely blocked P1075's anti-infarct effect (31.7 +/- 9.5% and 27.7 +/- 4.6% infarction, respectively; P = n.s. vs. untreated hearts). These data provide strong evidence that P1075 does open mitoK(ATP) channels and protects the ischemic rabbit heart in a mitoK(ATP)-dependent manner.

Acetylcholine-induced production of reactive oxygen species in adult rabbit ventricular myocytes is dependent on phosphatidylinositol 3- and Src-kinase activation and mitochondrial K(ATP) channel opening

Acetylcholine (ACh), like ischemic preconditioning (PC), protects against infarction and is dependent on generation of reactive oxygen species (ROS). To investigate the mechanism by which ACh causes ROS production, isolated adult rabbit cardiomyocytes underwent a timed incubation in reduced MitoTracker Red, which is oxidized to a fluorescent form after exposure to ROS. The mitochondrial ATP-sensitive potassium (mK(ATP)) channel opener diazoxide (50 microM) increased fluorescence by 47 +/- 9% (P = 0.007), indicating that opening of mK(ATP) leads to ROS generation, and that increase was blocked by the mK(ATP) blocker 5-hydroxydecanoate (5HD, 1 mM); 250 microM ACh caused a similar increase in ROS generation (+45 +/- 6% for all experiments, P < 0.001). ACh-induced ROS production was prevented by (1) blockade of muscarinic surface receptors with 100 microM atropine (-6 +/- 2%, P = n.s.) or 250 nM 4-DAMP (+5 +/- 13%, P = n.s.), indicating that ACh's effect was receptor mediated; (2) closing K(ATP) channels with either the non-selective channel closer glibenclamide (50 microM) (-1.2 +/- 17%, P = n.s.) or the selective mK(ATP) closer 5HD (-1.8 +/- 9%, P = n.s.), indicating that increased ROS production involved opening of mK(ATP); (3) blockade of mitochondrial electron transport chain with 200 nM myxothiazol (-4 +/- 9%, P = n.s.), indicating ROS came from the mitochondria; (4) addition of 100 nM wortmannin (-13 +/- 12%, P = n.s.), indicating that phosphatidylinositol 3-(PI3)-kinase was involved; and (5) blockade of Src-kinase with 1 microM PP2 (-2 +/- 5%, P = n.s.), indicating the involvement of an Src-kinase. These results support the hypothesis that occupation of muscarinic surface receptors by ACh causes activation of PI3- and Src-kinases that then open mK(ATP) resulting in mitochondrial ROS generation and triggering of the preconditioned state.

Activation of Akt is essential for acetylcholine to trigger generation of oxygen free radicals

OBJECTIVES

Acetylcholine (ACh) receptor activation in the heart causes mitochondrial production of reactive oxygen species (ROS) that is dependent on mitochondrial K(ATP) channel opening. Recent data show that Akt (also known as protein kinase B) is phosphorylated at its activation site following exposure to ACh. However, since no reliable Akt inhibitor is available, it has not been possible to determine whether Akt activation is an actual step in the protective pathway.

METHODS

Cultured rat vascular smooth muscle cells (A7r5) were transiently transfected with a dominant negative Akt (Akt-AAA), thus inhibiting the ability of ACh in these cells to phosphorylate Akt. Transfected cells were identified by co-transfection of enhanced green fluorescent protein (EGFP). ROS production was determined by incubating the cells for 15 min with 1 mM reduced MitoTracker Red which becomes fluorescent only after reacting with ROS. Cells were then triple-washed to remove any voltage-dependent pool of dye and single cell fluorescence was measured.

RESULTS

ACh exposure (2 mM) led to a 1.64+/-0.15-fold increase in the average fluorescence over that seen in untreated cells (P=0.002). A similar increase in ROS production occurred after treatment with either the K(ATP) channel opener diazoxide (DIAZ) or the potassium ionophore valinomycin (VAL). Akt-AAA transfection abolished ACh-induced ROS production, but not increased ROS production after treatment with either DIAZ or VAL.

CONCLUSION

Thus, at least in the smooth muscle cell model, Akt phosphorylation is an important step in the signal transduction pathway leading from ACh receptor activation to the generation of ROS. The experiments reveal that Akt is positioned between the receptor and the K(ATP) channel in this model.

Acetylcholine but not adenosine triggers preconditioning through PI3-kinase and a tyrosine kinase

Adenosine and acetylcholine (ACh) trigger preconditioning by different signaling pathways. The involvement of phosphatidylinositol 3-kinase (PI3-kinase), a protein tyrosine kinase, and Src family tyrosine kinase in preconditioning was evaluated in isolated rabbit hearts. Either wortmannin (PI3-kinase blocker), genistein (tyrosine kinase blocker), lavendustin A (tyrosine kinase blocker), or 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolol[3,4-d]pyrimidine (PP2; Src family tyrosine kinase blocker) was given for 15 min to bracket a 5-min infusion of either adenosine or ACh (trigger phase). The hearts then underwent 30 min of regional ischemia. Infarct size for ACh alone was 9.3 +/- 3.5% of the risk zone versus 34.3 +/- 4.1% in controls. All four inhibitors blocked ACh-induced protection. When wortmannin or PP2 was infused only during the 30-min ischemic period (mediator phase), ACh-induced protection was not affected (7.4 +/- 2.1% and 9.7 +/- 1.7% infarction, respectively). Adenosine-triggered protection was not blocked by any of the inhibitors. Therefore, PI3-kinase and at least one protein tyrosine kinase, probably Src kinase, are involved in the trigger phase of ACh-induced, but not adenosine-induced, preconditioning. Neither PI3-kinase nor Src kinase is a mediator of the protection of ACh.

ACh and adenosine activate PI3-kinase in rabbit hearts through transactivation of receptor tyrosine kinases

Adenosine and acetylcholine (ACh) trigger preconditioning through different signaling pathways. We tested whether either could activate myocardial phosphatidylinositol 3-kinase (PI3-kinase), a putative signaling protein in ischemic preconditioning. We used phosphorylation of Akt, a downstream target of PI3-kinase, as a reporter. Exposure of isolated rabbit hearts to ACh increased Akt phosphorylation 2.62 +/- 0.33 fold (P = 0.001), whereas adenosine caused a significantly smaller increase (1.52 +/- 0.08 fold). ACh-induced activation of Akt was abolished by the tyrosine kinase blocker genistein indicating at least one tyrosine kinase between the muscarinic receptor and Akt. ACh-induced Akt activation was blocked by the Src tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and by 4-(3-chloroanilino)-6,7-dimethoxyquinazoline (AG-1478), an epidermal growth factor receptor (EGFR) inhibitor, suggesting phosphorylation of a receptor tyrosine kinase in an Src tyrosine kinase-dependent manner. ACh caused tyrosine phosphorylation of the EGFR, which could be blocked by PP2, thus supporting this receptor hypothesis. AG-1478 failed to block the cardioprotection of ACh, however, suggesting that other receptor tyrosine kinases might be involved. Therefore, G(i) protein-coupled receptors can activate PI3-kinase/Akt through transactivation of receptor tyrosine kinases in an Src tyrosine kinase-dependent manner.

Treatment of orthostatic hypotension

Orthostatic hypotension (OH) is a fall in blood pressure after assuming an upright position. Whereas asymptomatic patients usually need no treatment, the majority of symptomatic patients can be cured by avoidance of trigger mechanisms and the use of physical countermaneuvers and non-pharmacological interventions. Several pharmacological therapies are available and generally fludrocortisone and midodrine are the drugs of first choice. Recently, highly individualized therapy with ambulatory norepinephrine therapy was able to mobilize otherwise immobile patients.

Protection from AMP 579 can be added to that from either cariporide or ischemic preconditioning in ischemic rabbit heart

AMP 579, an adenosine A /A receptor agonist, is cardioprotective when administered at reperfusion. Pretreatment with the Na /H exchanger inhibitor cariporide or ischemic preconditioning (PC) also limits infarct size. To gain insight into the mechanism of AMP 579 we investigated whether its protection could be added to that from either cariporide or PC. rabbit hearts were subjected to 45 min of regional ischemia followed by 3 h of reperfusion. Infarct size in the control group was 55.8 +/- 3.9% of the risk zone. PC significantly reduced infarct size to 26.0 +/- 6.7% (p<0.05). AMP 579 (30 micro g/kg) given just before reperfusion followed by 3 micro g/kg/min infusion for 70 min also limited infarct size (32.1 +/- 1.8%,) but the combination of AMP 579 and PC showed a significantly greater limitation of infarct size (5.5 +/- 2.7%, p < 0.05). Because cariporide pretreatment was so protective (8.5 +/- 3.7% infarction), we had to increase the ischemic insult to 60 min to test for any additive effect of the combination of AMP 579 + cariporide. Infarct size in the untreated group was 66.0 +/- 4.9% of the risk zone. Cariporide (0.5 mg/kg) 5 min prior to ischemia significantly reduced infarct size to 41.5 +/- 7.7%. When cariporide pre-treatment was combined with AMP 579 at reperfusion, infarction was further limited (14.2 +/- 4.5%). Because AMP 579's protection can be added to that of either cariporide or PC, AMP 579's mechanism of protection probably differs from either of them. The combination of AMP 579 + cariporide was particularly efficacious and could be useful in the surgical setting.

Dose-response relationships of the protective and antiprotective effects of acute ethanol exposure in isolated rabbit hearts

In the rabbit heart, acute ethanol exposure followed by washout before ischemia exerts a preconditioninglike effect. However, if alcohol is still present during ischemia, all preconditioning-related cardioprotection is abolished. The present follow-up study investigated the dose-response relationships of both the beneficial and detrimental effects of acute ethanol exposure. In the isolated rabbit heart either 2.5-, 5-, 10-, 20-, or 50-mmol/L ethanol was given as a 5-minute pulse followed by washout before 30 minutes of regional ischemia and 2 hours of reperfusion. Isolated rabbit hearts were also preconditioned with 5 minutes of global ischemia followed by 10 minutes of reperfusion (PC) before onset of 30 minutes of regional ischemia. This latter protocol was combined with a 35-minute infusion of ethanol at concentrations of either 5, 10, 20, or 50 mmol/L, starting 5 minutes before the onset of the 30-minute period of ischemia. Infarct size was determined with triphenyltetrazolium staining. No protection was seen with a 5-minute infusion of 2.5-mmol/L ethanol (29.9 +/- 1.6% of risk zone infarcted), and minimal protection was evident with the 5-mmol/L dose (25.4 +/- 3.4% infarction). In all other groups infarct size was significantly reduced (17.9 +/- 3.2, 18.4 +/- 3.5, and 16.8 +/- 3.4%, respectively, versus 33.0 +/- 3.0% in control group, P < 0.05). In the presence of 10-, 20-, or 50-mmol/L ethanol, infarct size following PC was not different from control (24.3 +/- 2.5, 28.4 +/- 4.3, and 39.0 +/- 4.0%, respectively, versus 28.5 +/- 2.5%). Thus the presence of alcohol during ischemia inhibited protection induced by preceding preconditioning ischemia. Only in the PC group exposed to 5-mmol/L ethanol was infarct size significantly smaller than in the control group (6.4 +/- 2.5%, P < 0.005). Thus both protective and antiprotective effects of alcohol were dose dependent with similarly low threshold doses in in vitro rabbit hearts. Since it might be impossible to find a dose of ethanol that would be protective if administered shortly before ischemia, ethanol should be removed before that ischemia to protect myocardium.

Opening of ATP-sensitive potassium channels causes generation of free radicals in vascular smooth muscle cells

Recent evidence suggests that opening of mitochondrial K(ATP) channels in cardiac muscle triggers the preconditioning phenomenon through free radical production. The present study tested the effects of K(ATP) channel openers in a vascular smooth muscle cell model using the fluorescent probe MitoTracker (MTR) Red trade mark for detection of reactive oxygen species (ROS). Rat aortic smooth muscle cells (A7r5) were incubated with 1 micro M reduced MTR (non-fluorescent) and the MTR oxidation product (fluorescent) was quantified. Thirty-minute pretreatment with either diazoxide (200 micro M) or pinacidil (100 micro M), both potent mitochondrial K(ATP) channel openers, increased fluorescent intensity (FI) to 149 and 162 % of control (p < 0.05 for both), respectively, and the K(ATP) channel inhibitor 5-hydroxydecanoate (5 HD) blocked it. Valinomycin, a potassium-selective ionophore, raised FI to 156 % of control (p <: 0.05). However, 5 HD did not affect the valinomycin-induced increase in FI. Inhibition of mitochondrial electron transport (myxothiazol) or uncoupling of oxidative phosphorylation (dinitrophenol) also blocked either valinomycin- or diazoxide-induced increase in FI, and free radical scavengers prevented any diazoxide-mediated increase in fluorescence. Finally the diazoxide-induced increase in fluorescence was not blocked by the PKC inhibitor chelerythrine, but was by HMR 1883, a putative surface K(ATP) channel blocker. Thus opening of K(ATP) channels increases generation of ROS via the mitochondrial electron transport chain in vascular smooth muscle cells. Furthermore, a potassium-selective ionophore can mimic the effect of putative mitochondrial KATP channel openers. We conclude that potassium movement through KATP directly leads to ROS production by the mitochondria.

The relative order of mK(ATP) channels, free radicals and p38 MAPK in preconditioning's protective pathway in rat heart

OBJECTIVES

Ischemic preconditioning (PC) reduces myocardial infarction by a mechanism that involves opening of mitochondrial ATP-dependent potassium channels (mK(ATP)), reactive oxygen species (ROS), and possibly activation of p38 mitogen-activated protein kinase (p38 MAPK). The actual order of these steps, however, is a matter of current debate. This study examined whether protection afforded by menadione, which protects by causing mitochondria to produce ROS, requires mK(ATP) opening. In addition, we tested whether protection from anisomycin, a p38 MAPK activator, is dependent on ROS production.

METHODS AND RESULTS

Isolated, buffer-perfused rat hearts were pretreated with menadione, and infarction was assessed after 30 min of regional ischemia and 120 min of reperfusion. Menadione reduced infarction in a dose-dependent manner with an EC(50) of 270 nM. Menadione's infarct-limiting effect was insensitive to 200 microM 5-hydroxydecanoate (5HD), an mK(ATP) channel blocker, whereas protection by diazoxide and PC were blocked by 5HD. Anisomycin caused hearts to resist infarction and this protective effect was abrogated by SB203580, a p38 MAPK inhibitor, and 2-mercaptopropionylglycine (MPG), a free radical scavenger.

CONCLUSIONS

These results indicate that mK(ATP) opening occurs upstream of mitochondrial ROS generation in the protective pathway. Furthermore, protection afforded by anisomycin was p38 MAPK- and ROS-dependent.

Acetylcholine leads to free radical production dependent on K(ATP) channels, G(i) proteins, phosphatidylinositol 3-kinase and tyrosine kinase

OBJECTIVE

Acetylcholine (ACh) mimics ischemic preconditioning (PC) and therefore protects the heart against lethal ischemia. Steps common to both ischemic and drug-induced PC are opening of mitochondrial K(ATP) channels (mito K(ATP)) and generation of reactive oxygen species (ROS). The aim of this study was to test whether ACh-induced ROS production could be seen in a vascular smooth muscle cell line, and, if so, to investigate the underlying signaling pathway.

METHODS

Mitochondrial ROS generation was quantified by measuring changes in fluorescence of ROS-sensitive intracellular markers in vascular smooth muscle cells (A7r5).

RESULTS

Fluorescence, and, therefore, ROS production, was increased to 197.5+/-8.5% of baseline after 45 min of exposure of cells to 2 mM ACh (P<0.001 vs. untreated controls). This effect was blocked by co-treatment with a muscarinic receptor antagonist (atropine 102.8+/-2.9%, 4-DAMP 92.6+/-7.4%) or by inhibition of G(i) with pertussis toxin (PTX) (90.5+/-4.4%), implicating a receptor-mediated rather than non-specific effect of ACh. The increased fluorescence induced by ACh was also abrogated by the free radical scavenger N-(2-mercaptopropionyl) glycine (104.2+/-10.1%), documenting that ROS were indeed the cause of the enhanced fluorescence. Both diazoxide, a K(ATP) channel opener, and valinomycin, a potassium ionophore, also significantly increased ROS production, and these effects were not blocked by PTX, while the K(ATP) channel closer 5-hydroxydecanoate blocked ACh-induced ROS production (92.3+/-3.8%). These results suggest ROS production is directly influenced by K(ATP) activity and K(+) movements in the cell. The tyrosine kinase inhibitor genistein (102.8+/-6.6%) and the phosphatidylinositol 3 (PI3)-kinase inhibitor wortmannin (90.7+/-4.1%) also inhibited the ability of ACh to increase ROS production.

CONCLUSION

The signaling pathway by which ACh leads to ROS generation in A7r5 cells involves a muscarinic surface receptor, a pertussis toxin-sensitive G protein, PI3-kinase, at least one tyrosine kinase, and a 5-hydroxydecanoate (5-HD)-dependent K(ATP) (presumably that in mitochondria).

Mitochondrial K(ATP) channels: role in cardioprotection

The role of the mitochondrial ATP-sensitive potassium channel (mK(ATP)) in ischemic preconditioning and cardioprotection is reviewed. A great deal of accumulated evidence implicatese opening of this channel as an important step in the anti-infarct effect of ischemic preconditioning. Recent studies, however, reveal that channel opening can actually serve as a signal transduction element. Data indicate that mK(ATP) opening causes mitochondria to generate reactive oxygen species (ROS) which then activate downstream kinases. Opening of mK(ATP) prior to ischemia can serve as a trigger since the critical time for its opening is prior to the onset of the lethal ischemic insult. Most G(i)-coupled receptors trigger protection through the mK(ATP)/ROS pathway except for the adenosine receptor which uses some other, as yet unidentified, pathway. Possible coupling schemes between the receptors and the mK(ATP) are discussed. Protection from preconditioning can also be aborted when a mK(ATP) blocker is present only during the lethal ischemic insult (mediator phase), but a much higher concentration of the blocker is required. Thus the mK(ATP) probably serves a dual role as both a trigger and a mediator. Possible end-effectors of preconditioning's protection are discussed including the mK(ATP) itself.

The protective and anti-protective effects of ethanol in a myocardial infarct model

Recent data indicate that acute alcohol exposure can have a preconditioning-like protective effect on the heart. We investigated the effect of ethanol exposure shortly before regional ischemia in an infarct model. Both in the open-chest rabbit and in the isolated rabbit heart, exposure of the heart to ethanol significantly reduced infarct size, but only if the alcohol were washed out or sufficiently metabolized before the onset of ischemia. If ethanol were still present during ischemia, it could not only prevent its own protective effect, but also abolish protection induced by ischemic preconditioning or the mitochondrial K(ATP) channel activator diazoxide. In the in vitro model, we tested for possible mediators of ethanol-induced protection and made comparisons to the signaling cascade of ischemic preconditioning. Neither adenosine receptor blockade with 8-(p-sulfophenyl) theophylline, scavenging of free radicals with N-2-mercaptopropionyl glycine, nor closure of K(ATP) channels with glibenclamide affected ethanol's protective effect. However, either a PKC inhibitor or a protein tyrosine kinase inhibitor could completely block ethanol-induced infarct size reduction. Both the protective and anti-protective effects of ethanol had a threshold of about 5 mM. Thus, ethanol-induced protection is mediated by protein kinase C and at least one protein tyrosine kinase, but, in contrast to ischemic preconditioning, is not triggered by either adenosine receptors, free radicals, or K(ATP) channels. Ethanol can only exert its protective effect if it is removed before the onset of ischemia. If still present during ischemia, ethanol has the opposite effect, and inhibits preconditioning by an as yet unidentified mechanism.