Ischemic preconditioning triggers phospholipase D signaling in rat heart

Lysophospholipid receptor activation of RhoA and lipid signaling pathways

The lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) signal through G-protein coupled receptors (GPCRs) which couple to multiple G-proteins and their effectors. These GPCRs are quite efficacious in coupling to the Gα(12/13) family of G-proteins, which stimulate guanine nucleotide exchange factors (GEFs) for RhoA. Activated RhoA subsequently regulates downstream enzymes that transduce signals which affect the actin cytoskeleton, gene expression, cell proliferation and cell survival. Remarkably many of the enzymes regulated downstream of RhoA either use phospholipids as substrates (e.g. phospholipase D, phospholipase C-epsilon, PTEN, PI3 kinase) or are regulated by phospholipid products (e.g. protein kinase D, Akt). Thus lysophospholipids signal from outside of the cell and control phospholipid signaling processes within the cell that they target. Here we review evidence suggesting an integrative role for RhoA in responding to lysophospholipids upregulated in the pathophysiological environment, and in transducing this signal to cellular responses through effects on phospholipid regulatory or phospholipid regulated enzymes. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Mitochondrial therapeutics for cardioprotection

Mitochondria represent approximately one-third of the mass of the heart and play a critical role in maintaining cellular function-however, they are also a potent source of free radicals and pro-apoptotic factors. As such, maintaining mitochondrial homeostasis is essential to cell survival. As the dominant source of ATP, continuous quality control is mandatory to ensure their ongoing optimal function. Mitochondrial quality control is accomplished by the dynamic interplay of fusion, fission, autophagy, and mitochondrial biogenesis. This review examines these processes in the heart and considers their role in the context of ischemia-reperfusion injury. Interventions that modulate mitochondrial turnover, including pharmacologic agents, exercise, and caloric restriction are discussed as a means to improve mitochondrial quality control, ameliorate cardiovascular dysfunction, and enhance longevity.

Revisited and revised: is RhoA always a villain in cardiac pathophysiology?

The neonatal rat ventricular myocyte model of hypertrophy has provided tremendous insight with regard to signaling pathways regulating cardiac growth and gene expression. Many mediators thus discovered have been successfully extrapolated to the in vivo setting, as assessed using genetically engineered mice and physiological interventions. Studies in neonatal rat ventricular myocytes demonstrated a role for the small G-protein RhoA and its downstream effector kinase, Rho-associated coiled-coil containing protein kinase (ROCK), in agonist-mediated hypertrophy. Transgenic expression of RhoA in the heart does not phenocopy this response, however, nor does genetic deletion of ROCK prevent hypertrophy. Pharmacologic inhibition of ROCK has effects most consistent with roles for RhoA signaling in the development of heart failure or responses to ischemic damage. Whether signals elicited downstream of RhoA promote cell death or survival and are deleterious or salutary is, however, context and cell-type dependent. The concepts discussed above are reviewed, and the hypothesis that RhoA might protect cardiomyocytes from ischemia and other insults is presented. Novel RhoA targets including phospholipid regulated and regulating enzymes (Akt, PI kinases, phospholipase C, protein kinases C and D) and serum response element-mediated transcriptional responses are considered as possible pathways through which RhoA could affect cardiomyocyte survival.

Cardioprotective effect of the Hibiscus rosa sinensis flowers in an oxidative stress model of myocardial ischemic reperfusion injury in rat

BACKGROUND

The present study investigates the cardioprotective effects of Hibiscus rosa sinensis in myocardial ischemic reperfusion injury, particularly in terms of its antioxidant effects.

METHODS

The medicinal values of the flowers of Hibiscus rosa sinensis (Chinese rose) have been mentioned in ancient literature as useful in disorders of the heart. Dried pulverized flower of Hibiscus rosa sinensis was administered orally to Wistar albino rats (150-200 gms) in three different doses [125, 250 and 500 mg/kg in 2% carboxy methyl cellulose (CMC)], 6 days per week for 4 weeks. Thereafter, rats were sacrificed; either for the determination of baseline changes in cardiac endogenous antioxidants [superoxide dismutase, reduced glutathione and catalase] or the hearts were subjected to isoproterenol induced myocardial necrosis.

RESULTS

There was significant increase in the baseline contents of thiobarbituric acid reactive substances (TBARS) [a measure of lipid per oxidation] with both doses of Hibiscus Rosa sinensis. In the 250 mg/kg treated group, there was significant increase in superoxide dismutase, reduced glutathione, and catalase levels but not in the 125 and 500 mg/kg treated groups. Significant rise in myocardial thiobarbituric acid reactive substances and loss of superoxide dismutase, catalase and reduced glutathione (suggestive of increased oxidative stress) occurred in the vehicle treated hearts subjected to in vivo myocardial ischemic reperfusion injury.

CONCLUSION

It may be concluded that flower of Hibiscus rosa sinensis (250 mg/kg) augments endogenous antioxidant compounds of rat heart and also prevents the myocardium from isoproterenol induced myocardial injury.

Expression of phospholipase D isozymes in scar and viable tissue in congestive heart failure due to myocardial infarction

The phospholipase D (PLD) associated with the cardiac sarcolemmal (SL) membrane hydrolyses phosphatidylcholine to produce phosphatidic acid, an important phospholipid signaling molecule known to influence cardiac function. The present study was undertaken to examine PLD isozyme mRNA expression, protein contents and activities in congestive heart failure (CHF) subsequent to myocardial infarction (MI). MI was induced in rats by occlusion of the left anterior descending coronary artery. At 8 weeks after the surgical procedure, hemodynamic assessment revealed that these experimental rats were at a moderate stage of CHF. Semi-quantitative reverse transcriptase-polymerase chain reaction revealed that PLD1 and PLD2 mRNA amounts were unchanged in viable left ventricular (LV) tissue of the failing heart. Furthermore, this technique demonstrated the presence of PLD1 and PLD2 mRNA in the scar tissue. While SL PLD1 and PLD2 protein contents were elevated in the viable LV tissue of the failing heart, SL PLD1 activity was significantly decreased, whereas SL PLD2 activity was significantly increased. On the other hand, although PLD1 protein was undetectable, PLD2 protein and activity were detected in the scar tissue. Our findings suggest that differential changes in PLD isozymes may contribute to the pathophysiology of CHF and may also be involved in the processes of scar remodeling.

KATP channels and myocardial preconditioning: an update

Ischemic or myocardial preconditioning (IPC) is a phenomenon whereby brief periods of ischemia have been shown to protect the myocardium against a more sustained ischemic insult. The result of IPC may be manifest as a marked reduction in infarct size, myocardial stunning, or incidence of cardiac arrhythmias. Whereas many endogenous neurotransmitters, peptides, and hormones have been proposed to play a role in the signal transduction pathways mediating the cardioprotective effect of IPC, nearly universal evidence indicates the involvement of the ATP-sensitive potassium (KATP) channel. Initial evidence suggested that the surface or sarcolemmal KATP (sarcKATP) channel triggered or mediated the cardioprotective effects of IPC; however, more recent findings have suggested a major role for a mitochondrial site or possibly a mitochondrial KATP channel (mitoKATP). This review presents evidence that supports a role for these two channels as a trigger and/or downstream mediator in the phenomenon of IPC or pharmacologically induced PC as well as recent evidence that suggests the involvement of a mitochondrial calcium-activated potassium (mitoKca) channel or the electron transport chain in mediating the beneficial effects of IPC or pharmacologically induced PC.

Adenosine and opioid receptor-mediated cardioprotection in the rat: evidence for cross-talk between receptors

The relative roles of free-radical production, mitochondrial ATP-sensitive K+ (mitoKATP) channels and possible receptor cross-talk in both opioid and adenosine A1 receptor (A1AR) mediated protection were assessed in a rat model of myocardial infarction. Sprague-Dawley rats were subjected to 30 min of occlusion and 90 min of reperfusion. The untreated rats exhibited an infarct of 58.8 +/- 2.9% [infarct size (IS)/area at risk (AAR), %] at the end of reperfusion. Pretreatment with either the nonselective opioid receptor agonist morphine or the selective A1AR agonist 2-chloro-cyclopentyladenosine (CCPA) dramatically reduced IS/AAR to 41.1 +/- 2.2% and 37.9 +/- 5.5%, respectively (P < 0.05). Protection afforded by either morphine or CCPA was abolished by the reactive oxygen species scavenger N-(2-mercaptopropionyl)glycine or the mitoKATP channel blocker 5-hydroxydecanoate. Both morphine- and CCPA-mediated protection were attenuated by the selective A1AR antagonist 1,3-dipropyl-8-cyclopentylxanthine and the selective delta1-opioid receptor (DOR) antagonist 7-benzylidenealtrexone. Simultaneous administration of morphine and CCPA failed to enhance the infarct-sparing effect of either agonist alone. These data suggest that both DOR and A1AR-mediated cardioprotection are mitoKATP and reactive oxygen species dependent. Furthermore, these data suggest that there are converging pathways and/or receptor cross-talk between A1AR- and DOR-mediated cardioprotection.

Sarcolemmal and mitochondrial K(ATP) channels and myocardial ischemic preconditioning

Ischemic preconditioning (IPC) is the phenomenon whereby brief periods of ischemia have been shown to protect the myocardium against a sustained ischemic insult. The result of IPC may be manifest as a marked reduction in infarct size, myocardial stunning, or incidence of arrhythmias. While many substances and pathways have been proposed to play a role in the signal transduction mediating the cardioprotective effect of IPC, overwhelming evidence indicates an intimate involvement of the ATP-sensitive potassium channel (K(ATP) channel) in this process. Initial hypotheses suggested that the surface or sarcolemmal K(ATP) (sarcK(ATP)) channel mediated the cardioprotective effects of IPC. However, much research has subsequently supported a major role for the mitochondrial K(ATP) channel (mitoK(ATP)) as the one involved in IPC-mediated cardioprotection. This review presents evidence to support a role for the sarcK(ATP) or the mitoK(ATP) channel as either triggers and/or downstream mediators in the phenomenon of IPC.

Chronic garlic administration protects rat heart against oxidative stress induced by ischemic reperfusion injury

BACKGROUND

Oxidative stress plays a major role in the biochemical and pathological changes associated with myocardial ischemic-reperfusion injury (IRI). The need to identify agents with a potential for preventing such damage has assumed great importance. Chronic oral administration of raw garlic has been previously reported to augment myocardial endogenous antioxidants. In the present study, the effect of chronic oral administration of raw garlic homogenate on oxidative stress induced by ischemic-reperfusion injury in isolated rat heart was investigated.

RESULTS

Raw garlic homogenate (125, 250 and 500 mg/kg once daily for 30 days) was administered orally in Wistar albino rats. Thereafter, hearts were isolated and subjected to IRI (9 min. of global ischemia, followed by 12 min of reperfusion; perfusion with K-H buffer solution; 37 degrees C, 60 mm Hg.). Significant myocyte injury and rise in myocardial TBARS along with reduction in myocardial SOD, catalase, GSH and GPx were observed following IRI. Depletion of myocardial endogenous antioxidants and rise in TBARS were significantly less in the garlic-treated rat hearts. Oxidative stress induced cellular damage as indicated by ultrastructural changes, like disruption of myofilament, Z-band architecture along with mitochondrial changes were significantly less.

CONCLUSIONS

The study strongly suggests that chronic garlic administration prevents oxidative stress and associated ultrastructural changes, induced by myocardial ischemic-reperfusion injury.

Role of oxidants in the signaling pathway of preconditioning

This review focuses on the possible role of reactive oxygen species in the pathogenesis of this phenomenon. Evidence in support of a role of oxidants in preconditioning has come from the observation that administration of oxygen radical scavengers during the reperfusion period following the initial "preconditioning" ischemia could prevent the phenomenon. In addition, a brief exposure to a low, nontoxic dose of oxygen radicals may reproduce the beneficial effects of ischemic preconditioning, thus suggesting that radicals can directly trigger the preconditioning pathway. To explain the effects of oxidants in this setting, it has been suggested that reperfusion after the initial, "preconditioning" ischemic episode results in the generation of relatively low amounts of oxygen radicals, which are insufficient to determine cell necrosis, but nevertheless could modify cellular activities that have been implicated as mediators of the preconditioning phenomenon. Recent evidence suggests that low levels of oxidants may have a modulatory role on several cell functions. Possible mechanisms of oxidant-mediated protection might be protein kinase C and other kinases, ATP-dependent potassium channels, or changes in sulfhydryl group redox state, while an effect on adenosine metabolism, or the induction of myocardial stunning presumably does not contribute to oxidant-mediated preconditioning. Finally, de novo protein synthesis and gene expression, and increased antioxidant defenses might be involved in the late phase of preconditioning. In summary, available data strongly suggest that oxygen radicals might be possible mediators of preconditioning. However, further investigation is required to clearly elucidate their exact role and mechanisms of action.

Altered cardiac Na(+)/H(+) exchange in phospholipase D-treated sarcolemmal vesicles

Cardiac sarcolemmal Na(+)/H(+) exchange is critical for the regulation of intracellular pH, and its activity contributes to ischemia-reperfusion injury. It has been suggested that the membrane phospholipid environment does not modulate Na(+)/H(+) exchange. The present study was carried out to determine the effects on Na(+)/H(+) exchange of modifying the endogenous membrane phospholipids through the addition of exogenous phospholipase D. Incubation of 0.825 U of phospholipase D with 1 mg of porcine cardiac sarcolemmal vesicles hydrolyzed 34 +/- 2% of the sarcolemmal phosphatidylcholine and increased phosphatidic acid 10.2 +/- 0.5-fold. Treatment of vesicles with phospholipase D resulted in a 46 +/- 2% inhibition of Na(+)/H(+) exchange. Na(+)/H(+) exchange was measured as a function of reaction time, extravesicular pH, and extravesicular Na(+). All of these parameters of Na(+)/H(+) exchange were inhibited following phospholipase D treatment compared with untreated controls. Passive efflux of Na(+) was unaffected. Treatment of sarcolemmal vesicles with phospholipase C had no effect on Na(+)/H(+) exchange. We conclude that phospholipase D-induced changes in the cardiac sarcolemmal membrane phospholipid environment alter Na(+)/H(+) exchange.

HPTLC analysis of sphingomylein, ceramide and sphingosine in ischemic/reperfused rat heart

Since the sphingomylein-ceramide-sphingosine pathway, especially ceramide, has been shown to induce programmed cell death (apoptosis), and since apoptosis may be involved with ischemic/reperfused injury in the heart, it became desirable to quantitate the three components in ischemic/reperfused rat heart. One group of rat hearts (n = 6) was isolated and perfused with Krebs-Henseleit buffer using the Langendorff non-recirculating mode. The hearts were perfused for 10 min, made ischemic for 30 min and reperfused for 120 min. Hearts were collected and stored at - 70 degrees C before ischemia, after ischemia and after 30, 60 and 120 min of reperfusion. The hearts were homogenized, and lipids were extracted using the Folch method. The lipids were then chromatographed on Whatman silica gel 60 A high-performance thin-layered chromatography (HPTLC) plates. The plates were developed with iodine, photographed using Photoshop software and quantitated using NIH Imaging software. The results show a 50% decrease of sphingomylein during reperfusion with a corresponding increase in ceramide with sphingosine showing a smaller decrease as compared with the ceramide increase.