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

Molecular and morphological modifications occurring in rat heart exposed to intermittent hypoxia: role for protein kinase C α

Exposure of rats to intermittent hypoxia determines different responses at tissue and cell level. Heart mainly undergoes the effects of hypoxic injury and its response is determined both by the relationship between oxygen supply and demand and by its functional state. Since molecular mechanisms mediate cells sensing and response to low O(2) concentration, here we explore the role played by Protein Kinase C alpha (PKC alpha) in the signal transduction mechanisms leading to the occurrence of morphological responses in rat neonatal, young and old heart subjected to intermittent hypoxia. Along with a key role for hypoxia inducible factor and vascular endothelial growth factor in the occurrence of continuous state of dynamic adaptation of vasculature, PKC alpha presumably phosphorylates IkBalpha in rat normoxic and hypoxic neonatal hearts, supporting the hypothesis of a rescue strategy carried out against hypoxia, together with an hypertrophic response. In hypoxic young heart PKC alpha activation, paralleled by sustained Bax homodimerization and caspase-3 activation, along with reduced p-IKBalpha and Inhibiting Apoptosis Protein (IAP) expression, suggests that the young early and deeply undergoes the effects of lowered oxygen tension. In addition, since no modifications concerning PKC alpha driven signalling system are evidenced in both the experimental conditions, we suggest an oxygen impaired sensing during ageing.

Hsp70 attenuates hypoxia/reoxygenation-induced activation of poly(ADP-ribose) synthetase in the nucleus of adult rat cardiomyocytes

Effects of heat shock protein 70 (Hsp70) translocated to nuclear fraction on hypoxia/reoxygenation injury was examined by using adult cardiomyocytes isolated from rats. Cardiomyocytes were exposed to heat shock at 42 degrees C for 15 min (HS group), and then incubated at 37 degrees C for 6-24 h. Hsp70 production increased and the protein translocated from cytosol to nucleus. The maximum level of Hsp70 in the nuclear fraction was observed 12 h after HS. When cardiomyocytes without exposure to HS (nHS group) were subjected to 120 min hypoxia/15 min reoxygenation (Hypo/Reoxy), post-hypoxic cell viability was approximately 25% of the pre-hypoxic value. A rise in poly(ADP-ribose) synthetase (PARS) activity in the nuclear fraction was observed in nHS group, associated with an increase in polyADP-ribosylated protein. In contrast, post-hypoxic cell viability of HS group was approximately 60% of the pre-hypoxic value. Hypo/reoxy-induced rise in PARS activity and increase in polyADP-ribosylated protein were attenuated in HS group. To confirm the relationship between an increase in cell viability after Hypo/Reoxy and attenuation of PARS activation, cardiomyocytes without exposure to HS were subjected to Hypo/Reoxy in the presence of 1 mM 3-aminobenzamide, an inhibitor of PARS. Treatment of cells with 3-aminobenzamide attenuated Hypo/ Reoxy-induced decrease in cell viability. These results suggest that Hsp70 translocated into nucleus after HS may attenuate PARS activation during Hypo/Reoxy, leading to the cytoprotection of cardiomyocytes against Hypo/Reoxy injury.

Role of adenosine in renal protection induced by a brief episode of ischemic preconditioning in rats

The protective effect of a brief episode of ischemic preconditioning was examined at an early phase of ischemic-reperfusion injury in the rat kidney. Rats were subjected to 50 min of left renal artery occlusion followed by 120 min of reperfusion. Ischemic preconditioned rats were subjected to preconditioning with two cycles of 3-min ischemia and 5-min reperfusion (IPC). Ischemic-reperfusion injury led to a low recovery of the glomerular filtration rate (GFR). Overt morphological changes, consisting of blood trapping and tubular collapse, were seen. IPC improved the recovery of GFR and renal morphology. The IPC effect was not blocked by 8-(p-sulfophenyl)-theophylline (SPT), a non-selective adenosine receptor antagonist, by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), a selective A1-receptor antagonist, or by 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A2-receptor antagonist. Intravenous infusion of adenosine (30 microg/min per rat, for 5 min) prior to the 50-min occlusion improved the recovery of GFR, and this protection of GFR was blocked by SPT. Thus, both IPC and exogenous adenosine attenuated ischemic-reperfusion injury of the kidney. However, because three adenosine receptor antagonists failed to abolish the protective effect of IPC, there is no evidence to indicate that activation of adenosine receptors contributes to the IPC effect in the kidney.

Cardioprotective effects of adenosine A1 and A3 receptor activation during hypoxia in isolated rat cardiac myocytes

Adenosine (ADO) is a well-known regulator of a variety of physiological functions in the heart. In stress conditions, like hypoxia or ischemia, the concentration of adenosine in the extracellular fluid rises dramatically, mainly through the breakdown of ATP. The degradation of adenosine in the ischemic myocytes induced damage in these cells, but it may simultaneously exert protective effects in the heart by activation of the adenosine receptors. The contribution of ADO to stimulation of protective effects was reported in human and animal hearts, but not in rat hearts. The aim of this study was to evaluate the role of adenosine A1 and A3 receptors (A1R and A3R), in protection of isolated cardiac myocytes of newborn rats from ischemic injury. The hypoxic conditions were simulated by exposure of cultured rat cardiomyocytes (4-5 days in vitro), to an atmosphere of a N2 (95%) and CO2 (5%) mixture, in glucose-free medium for 90 min. The cardiotoxic and cardioprotective effects of ADO ligands were measured by the release of lactate dehydrogenase (LDH) into the medium. Morphological investigation includes immunohistochemistry, image analysis of living and fixed cells and electron microscopy were executed. Pretreatment with the adenosine deaminase considerably increased the hypoxic damage in the cardiomyocytes indicating the importance of extracellular adenosine. Blocking adenosine receptors with selective A1 and A3 receptor antagonists abolished the protective effects of adenosine. A1R and A3R activation during the hypoxic insult delays onset of irreversible cell injury and collapse of mitochondrial membrane potential as assessed using DASPMI fluorochrom. Cardioprotection induced by the A1R agonist, CCPA, was abolished by an A1R antagonist, DPCPX, and was not affected by an A3R antagonist, MRS 1523. Cardioprotection caused by the A3R agonist, Cl-IB-MECA, was antagonized completely by MRS 1523 and only partially by DPCPX. Activation of both A1R and A3R together was more efficient in protection against hypoxia than by each one alone. Our study indicates that activation of either A1 or A3 adenosine receptors in the rat can attenuate myocyte injury during hypoxia. Highly selective A1R and A3R agonists may have potential as cardioprotective agents against ischemia or heart surgery.

Cytoprotective mechanism of heat shock protein 70 against hypoxia/reoxygenation injury

The role of heat shock protein 70 (HSP70) in the cytoprotection against hypoxia/reoxygenation injury was examined. Adult rat cardiomyocytes were isolated, subjected to hyperthermia at 42 degrees C for 15 min (heat shock treatment), and then incubated at 37 degrees C for 3 to 24 h (HSP production process). Heat shock treatment increased HSP70 production (80-260% increase); the peak increase was seen after 9 h of HSP production process. Thereafter, the cells were subjected to 120-min hypoxia and 15-min reoxygenation. Heat shock treatment increased the survival of the cells subjected to hypoxia/reoxygenation (1.5-2.5-fold); the maximal cytoprotection was observed after 12 h of HSP production process. Heat shock treatment increased HSP70 content in the nucleus when cells were subjected to 12 h of HSP production process. To examine the role of HSP70 accumulation in the nuclear fraction, the activity of poly(ADP-ribose) synthetase (PARS), which functions in the nucleus and consumes high-energy phosphates excessively in the reoxygenated state, were measured in the cells with heat shock and 12 h of HSP production process. Heat shock treatment attenuated the hypoxia/reoxygenation-induced increase in the PARS activity (50% decrease). Treatment of the cells with 3-aminobenzamide, an inhibitor of PARS, exerted the effects similar to those of heat shock treatment. These results suggest that attenuation of the PARS activity in the nucleus may play an important role in the cytoprotective effect of HSP70 on hypoxia/reoxygenation injury.