| A lot of researches have demonstrated that chronic intermittent hypobaric hypoxia (CIHH) confers significant cardioprotetive effects on adult heart, such as enchancing the resistance of heart against acute hypoxia/re-oxygen or acute ischemia/reperfusion injury, reducing infarct size, limiting cardiac ultrastruc- ture damage and anti-arrhythmia. The candidate mechanisms involved enhancement of antioxidation, increase of myocardium capillary desity and coronary flow, overexpression of heat shock protein, increase of NO production, anti-apoptosis, stabilization of mitochondria and function of handling calcium, prolongation of action potential duration and effective refractory period. However, it was not clear about the role of CIHH on postnatal developing hearts. The objective of the study was to explore the anti-arrhythmic effects of CIHH and underling electrophysiological mechanism on postnatal developing rat using functional and electro- physiological methods。Our study consists of four parts: (1) To make an animal model of CIHH cardioprotection through exposing the developing rat to simulated high altitude hypoxia. (2) To examine anti-arrhythmic effects of CIHH on developing rat via analysis of arrhythmia induced by ligation of coronary artery in vivo. (3) To examine effects of CIHH on action potential in papillary muscle from developing rat through intracellular recording, and to explore electrophysiological mechanism of CIHH against arrhythmias. (4) To examine of effects of CIHH on potassium currents in ventricular myocytes of developing rat by using whole-cell patch clamp techenique, and to explore the ionic mechanism of CIHH against arrhythmias. I Preparation of animal mode for cardioprotection of CIHH in developing ratObjective: to make animal model for cardioprotection of CIHH in developing rat by using different modes of CIHH.Methods: Age- and body weight- matched postnatal male rats were divided into three groups: 3000m chronic intermittent hypoxia (CIHH3000) group, 5000m chronic intermittent hypoxia (CIHH5000) group and control (CON) group. For CIHH groups, neonatal rats with the maternals were exposed to a hypobaric chamber, enduring CIHH mimicking 3000 m altitude (PB = 525 mmHg, PO2 = 108.8 mmHg, 5 hrs/day) or 5000 m altitude (PB = 404 mmHg, PO2 = 84 mmHg, 6 hrs/day) for 28, 42 and 56days, respectively. The isolated hearts were perfused in the langendorff apparatus, undergoing 30min global ischemia and 60min reperfusion. Parameters of cardiac function including left ventricular developing pressure (LVDP), left ventricular end-diastolic pressure (LVEDP), maximal velocity of left ventricular pressure (LVdp/dtmax), coronary flow (CF) and lactate dehydrogenase (LDH) activity were recorded respectively. The total heart weight, right ventricular weight and left ventricular plus inter-ventricular septum weight were measured at the end of the experiment.Results:(1) There was no difference of body weight gaining between CIHH3000 and CON rats. The gain of body weight in CIHH5000 rats was much lower than that in CIHH3000 and CON (P < 0.01).(2) There were no significant differences of cardiac function among 28-day, 42-day and 56-day of control rats under basic condition and during ischemia/reperfusion.(3) Compared with CON, CF in CIHH rats was increased under basic condition, the recovery of cardiac function in CIHH3000 rats was enhanced at 60 min after ischemia/reperfusion (P < 0.05), coronary flow was increased (P < 0.05), and LDH activity was decreased (P < 0.05), which means a cardioprotective effect occurred. There was no significant difference of heart weight between CIHH3000 and CON. In addition, cardiac function restored better in CIHH3000 rat after 42 days CIHH than that after 28 days CIHH (P < 0.05).(4) Compared with CON, the recovery of cardiac function in CIHH5000 rats was lower (P < 0.05), coronary flow was decreased (P < 0.05), and LDH activity was increased (P < 0.05). Right ventricular weight was higher in CIHH5000 than that in CON rats (P < 0.05).Conclusion: The result of the study suggests that moderate CIHH can protect developing rat heart against ischemia/reperfusion injury, which was affected by mode of CIHH exposure and related with CF increase.ⅡEffects of CIHH on ischemic arrhythmia in developing ratsObjective: To examine anti-arrhythmic effects of CIHH on developing rat via analysis of arrhythmia induced by ligation of coronary artery in vivo.Methods: In this part of experiment, age- and body weight-matched postnatal male rats were divided into four groups: 28-day CIHH group (CIHH28), 42-day CIHH group (CIHH42), 28-day control group (CON28) and 42-day control group (CON42). For CIHH group, neonatal rats with the maternals were exposed to a hypobaric chamber, enduring CIHH mimicking 3000 m altitude (PB = 525 mmHg, PO2 = 108.8 mmHg, 5 hrs/day) for 28days and 42days, respectively. Arterial blood pressure and electrocardiogram (ECG) were recorded under anesthetized state. Acute myocardial ischemia was produced by ligating left anterior descending coronary artery in anesthetized postnatal male rats. Ventricular extrasystole (VE), tachycardia (VT) and fibrillation (VF) were analyzed from ECG. Arrhythmia scores (AC) were used according to Johnston's Standards.Results:(1) The arrhythmia was mainly occurred between 3 and 8 min after coronary artery occlusion. There was no significant difference in ACs between CON28 (0.25±0.11) and CIHH28 (0.20±0.15) rats (P > 0.05).(2) There was a significant difference of AC between CON42 and CIHH42 rats. The AC was 2.13±0.74 in CON42 rats, much higher than 0.38±0.24 in CIHH42 rats (P < 0.05).(3) In CIHH group, there was no significant difference of AC between CIHH 28 (0.20±0.15) and CIHH42 (0.38±0.24) rats (P > 0.05).Conclusion: The result of the study demonstrates that CIHH has a significant anti-arrhythmic effect in developing rat, which is closely related with animals'age. The incidence of ischemic arrhythmia in developing rat increases along with rat growing in a certain age range.ⅢEffects of CIHH on action potential in ventricular papillary muscle of developing ratObjective: to investigate the effects of CIHH on action potential (AP) in ventricular papillary muscle of developing rats by using CIHH animal model and intracelluar recording method. Methods: In this part of experiment, age- and body weight-matched postnatal male rats were divided into four groups: 28-day CIHH group (CIHH28), 42-day CIHH group (CIHH42), 28-day control group (CON28) and 42-day control group (CON42). For CIHH group, neonatal rats with the maternals were exposed to a hypobaric chamber, enduring CIHH mimicking 3000 m altitude (PB = 525 mmHg, PO2 = 108.8 mmHg, 5 hrs/day) for 28days, 42days, respectively. The left vetricullar papillary muslces were perfused with Tyrodes solution and simulated ischemic solution. AP in papillary muscle was recorded before and during simulated ischemia by using intracellular microelectrode technique.Results:(1) There was no difference in any parameters of basic AP between CON and CIHH rats during Tyrodes solution perfusion (P > 0.05).(2) APA, OS, Vmax and APD90 of AP in papillary muscle were decreased significantly (P < 0.05) after 5minutes of simulated ischemia solution perfusion, but the decreasing of AP90 in CIHH42 rats was much smaller than than in CON42 rats (P < 0.05). There was no significant difference of AP decreasing during simulated ischemia between CON28 and CIHH28 rats (P > 0.05).(3) In CON groups, there was no difference of AP between CON28 and CON42 rats during Tyrodes solution perfusion (P > 0.05). The decreasing of APD90 of AP after 5minutes of simulated ischemia solution perfusion was more severe in CON 42 rats than that in CON28 rats (P < 0.05).(4) In CIHH groups, there was no difference of AP between CIHH28 and CIHH42 rats during Tyrodes solution perfusion (P > 0.05). Also there was no significant difference of AP decreasing after 5minutes of simulated ischemia solution perfusion between CIHH28 and CIHH42 rats (P > 0.05).Conclusion: The result of the study shows that CIHH does not affect basic AP in papillary muscle of developing rats. CIHH, however, can significantly alleviate the decreasing of APD90 induced by simulated ischemia, stabilizing the membrane repolarization during ischemia.ⅣEffect of CIHH on potassium currents in ventricular myocytes of developing ratObjective: to explore the effect of CIHH on cardiac potassium currents in developing rat heart by using whole-cell patch clamp technique, and the ionic mechanism of CIHH against arrhythmias.Methods: In this part of experiment, age- and body weight-matched postnatal male rats were divided into four groups: 28-day CIHH group (CIHH28), 42-day CIHH group (CIHH42), 28-day control group (CON28) and 42-day control group (CON42). For CIHH group, neonatal rats with the maternals were exposed to a hypobaric chamber, enduring CIHH mimicking 3000 m altitude (PB = 525 mmHg, PO2 = 108.8 mmHg, 5 hrs/day) for 28days, 42days, respectively. Isolated myocytes were obtained from ventricles by acute enzymic digestion method. Transient ourward potassium current (Ito), delayed rectifier potassium current (IK), inwardly rectifier potassium (IK1) and ATP-sensitive potassium current (IKATP) from myocytes were recorded before and during simulated ischemic solution perfusion. Results:(1) There was no difference of IK density between CON28 and CIHH28 rats before and during simulated ischemia. There was no difference of IK density between CON42 and CIHH42 rats before simulated ischemia, but IK density was increased significantly after 5minutes of simulated ischemia in CON42 rats compared with CIHH42 rats (P < 0.05).(2) There was no difference of IK1 density between CON28 and CIHH28 rats before and during simulated ischemia. There was no difference of IK1 density between CON42 and CIHH42 rats before simulated ischemia, but IK1 density was increased significantly after 5minutes of simulated ischemia in CON42 rats compared with CIHH42 rats (P < 0.05).(3) There was no difference of Ito density between CON and CIHH groups before or during simulated ischemia (P > 0.05). During simulated ischemia, Ito density was decreased, steady-state activation and inactivation curves of Ito were shifted negatively, and the time constant of recovery was prolonged (P < 0.05).Conclusion: The result of the study shows that CIHH has complicated effects on repolarization currents in cardiomyocytes of developing rats. CIHH has no effects on the basic IK, IK1 and Ito, but can significantly alliviate the increase of IK, IK1 during simulated ischemia.
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