Hypoxic preconditioning in isolated rat hearts: non-involvement of activation of adenosine A1 receptor, Gi protein, and ATP-sensitive K+ channel

Cardiospecific Overexpression of the Prostaglandin EP 3 Receptor Attenuates Ischemia-Induced Myocardial Injury

Background— The generation of prostaglandin E 2 (PGE 2 ) is significantly increased in acute myocardial ischemia and reperfusion. PGE 2 , in addition to other prostaglandins, protects the reperfused ischemic myocardium. It has been hypothesized that this cardioprotection is mediated by E-type prostaglandin receptors of the G i -coupled EP 3 subtype.

Methods and Results— We tested this hypothesis by generating transgenic (tg) mice with cardiospecific overexpression of the EP 3 receptor. According to ligand binding, a 40-fold overexpression of the EP 3 receptor was achieved in membranes prepared from tg hearts compared with wild-type (wt) littermates. In isolated cardiomyocytes from tg mice, the forskolin-induced rise in cAMP was markedly attenuated, indicating coupling of the overexpressed EP 3 receptor to inhibitory G proteins (G i ) with constitutive receptor activity. There was no evidence for EP 3 receptor coupling to G q -mediated protein kinase C signaling. Isolated hearts from tg and wt mice were subjected to 60 minutes of no-flow ischemia and 45 minutes of reperfusion. In tg hearts, ischemic contracture was markedly delayed compared with wt hearts, and the ischemia-induced increase in left ventricular end-diastolic pressure was reduced by 55%. Creatine kinase and lactate dehydrogenase release was significantly decreased by 85% and 73%, respectively, compared with wt hearts.

Conclusions— Constitutive prostaglandin EP 3 receptor signaling exerts a protective effect on cardiomyocytes, which is probably G i mediated and results in a remarkable attenuation of myocardial injury during ischemia and reperfusion. Cardioprotective actions of E-type prostaglandins may be mediated by this receptor subtype.

Role of protein phosphatases in hypoxic preconditioning

To find a protein kinase C (PKC)-independent preconditioning mechanism, hypoxic preconditioning (HP; i.e., 10-min anoxia and 10-min reoxygenation) was applied to isolated rat hearts before 60-min global ischemia. HP led to improved recovery of developed pressure and reduced end-diastolic pressure in the left ventricle during reperfusion. Protection was unaffected by the PKC inhibitor bisindolylmaleimide (BIM; 1 micromol/l). It was abolished by the inhibitor of protein phosphatases 1 and 2A cantharidin (20 or 5 micromol/l) and partially enhanced by the inhibitor of protein phosphatase 2A okadaic acid (5 nmol/l). In adult rat cardiomyocytes treated with BIM and exposed to 60-min simulated ischemia (anoxia, extracellular pH 6.4), HP led to attenuation of anoxic Na(+)/Ca(2+) overload and of hypercontracture, which developed on reoxygenation. This protection was prevented by treatment with cantharidin but not with okadaic acid. In conclusion, HP exerts PKC-independent protection on ischemic-reperfused rat hearts and cardiomyocytes. Protein phosphatase 1 seems a mediator of this protective mechanism.

Myocardial function and effect of serum in isolated heart from hypertriglyceridemic and hypertensive rats

We evaluated the myocardial function of rats with sugar-induced hypertriglyceridemia (HTG) and hypertension, and the effect of serum on myocardial performance in the isolated heart preparation. Also, the response to reperfusion after 30 minutes of global ischemia was investigated. Hearts from HTG rats developed lower ventricular pressure (VP) and the conduction rate was higher than in hearts from control rats (CR). The recovery of VP after ischemia was significantly lower in HTG than in CR hearts (p < 0.05). The HTG sera produced a higher increase in the VP and in the perfusion pressure. During reperfusion, the incidence of premature beats, ventricular fibrillation and tachycardia in HTG hearts was increased so hypertriglyceridemia caused alterations in the mechanical and electrical conduction of the myocardium and exacerbated the injury produced by ischemia-reperfusion. Also a circulating factor in the HTG serum induced a vasoactive response of the heart which was reflected in its mechanical performance.

Inhibition of cardiac tumor necrosis factor-alpha production by calcitonin gene-related peptide-mediated ischemic preconditioning in isolated rat hearts

Previous investigations have demonstrated that calcitonin gene-related peptide (CGRP) plays an important role in the mediation of ischemic preconditioning in rats. In the present study, we examined signal transduction pathways of CGRP-mediated ischemic preconditioning. Thirty minutes of global ischemia and 40 min of reperfusion caused a dramatic decrease in myocardial function, and a significant increase in the release of cardiac creatine kinase in the coronary effluent and in the content of tumor necrosis factor-alpha (TNF-alpha) in myocardial tissues. However, ischemic preconditioning (three cycles of 5-min ischemia and 5-min reperfusion) or pretreatment with CGRP for 5 min dramatically improved the recovery of cardiac function, and reduced the release of cardiac creatine kinase and the TNF-alpha content. The effect of ischemic preconditioning was abolished by CGRP-(8-37), the selective CGRP receptor antagonist, and by capsaicin, which depletes sensory nerve neurotransmitter content, but was unaltered by treatment with glibenclamide, a blocker of the ATP-sensitive potassium (K(ATP)) channel. The protective effects of exogenous CGRP-induced preconditioning were also not blocked by glibenclamide. These results suggest that the cardioprotective effects afforded by CGRP-mediated ischemic preconditioning are related to inhibition of cardiac TNF-alpha production, but not to activation of the K(ATP) channel.

Involvement of adenosine receptor, potassium channel and protein kinase C in hypoxic preconditioning of isolated cardiomyocytes of adult rat

A possible mechanism for hypoxic preconditioning of adult rat cardiomyocytes was pharmacologically investigated. Isolated cardiomyocytes in all experimental groups were incubated for 120 min under hypoxic conditions followed by 15-min reoxygenation (sustained H/R). Sustained H/R decreased rod-shaped cells. Exposure of the cardiomyocytes to 20-min of hypoxia/30-min reoxygenation (hypoxic preconditioning) attenuated the sustained H/R-induced decrease in rod-shaped cells. The effects of hypoxic preconditioning were abolished by treatment with the protein kinase C (PKC) inhibitor polymyxin B, but abolished by neither the adenosine A1/A2-antagonist sulfophenyl theophylline (SPT) nor the ATP-sensitive potassium channel (K(ATP) channel) blocker glibenclamide. In another series of experiments, cardiomyocytes were incubated without hypoxic preconditioning in the presence of either the PKC activator PMA, adenosine or K(ATP)-channel opener nicorandil and then subjected to sustained H/R. Treatment of the cells with PMA, adenosine or nicorandil mimicked the effects of hypoxic preconditioning. The effects of treatment with adenosine and nicorandil were abolished by polymyxin B treatment. Combined treatment with both SPT and glibenclamide abolished the effects of hypoxic preconditioning, whereas it failed to abolish PMA-induced cytoprotection. These results suggest that the activation of PKC in hypoxic preconditioned cardiomyocytes coupled independently with stimulation of adenosine receptor or opening of K(ATP) channel, either of which is fully enough to exert the cytoprotective effects.

Beta-adrenoceptor stimulation-mediated preconditioning-like cardioprotection in perfused rat hearts

To determine whether adrenergic stimulation induces preconditioning-like cardioprotection, rat hearts were perfused for 2 min with either norepinephrine, phenylephrine, or isoproterenol followed by 10-min drug-free perfusion. Then the hearts were subjected to 40-min ischemia and 30-min reperfusion. Little recovery of left ventricular developed pressure (LVDP) and loss of the myocardial creatine kinase (CK) during reperfusion were observed in the drug-untreated heart. Preperfusion with norepinephrine (0.25 microM) or isoproterenol (0.25 microM), but not phenylephrine (10 microM), resulted in a better recovery of LVDP in the postischemic reperfused heart and a reduction in CK release during reperfusion. A similar improvement of postischemic cardiac contractile dysfunction and CK loss was seen in the heart subjected to 5-min ischemia followed by 5-min reperfusion (ischemic preconditioning) before the prolonged period of ischemia/reperfusion. Pretreatment with timolol, a beta-adrenoceptor blocker, abolished the protective effect of norepinephrine, whereas pretreatment with bunazosin, an alpha 1-adrenoceptor blocker, did not affect the protective effect of isoproterenol. The results suggest that a brief period of stimulation of cardiac beta-adrenoceptor exerts the preconditioning-mimetic protective effect against postischemic contractile dysfunction in perfused rat hearts.