Effects Of Repeated +Gz Exposures On Morphology And Connexin 43 Expression Of Ventricular Myocardia In Rats

Modern high performance aircraft is capable of producing acceleration as high as +9Gz sustained for 15– 45 s that may exceed human physiological tolerance.With the application of the high performance fighter, heart pathology in pilots, associated with exposure to high sustained +Gz, were reconsider. The cardiac event, defined as myocardial infarction, angina and sudden cardiac death, were happened with the average of 7.6 events per year during the years 1988-1992 in the U.S. Air Force (USAF) pilot population. There was a greater incidence of tricuspid regurgitation, pulmonic insufficiency and concurrent pulmonic insufficiency and tricuspid regurgitation in the high +Gz stress pilot group.Various arrhythmias can occur during simulated aerial combat maneuver (SACM) and high-G training, ranging from sinus arrhythmia and premature ventricular contraction to loss of automaticity (asystole). Most of these arrhythmias represent physiologic responses to high Gz stress, and rarely reduce pilots'tolerance of Gz stress, but some kind of arrhythmias, such as ventricular tachycardia, paroxysmal supraventricular tachycardia, and paroxysmal atrial fibrillation, had a significant proportion with cardiac anomalies.Gap junctions, that couple cardiac myocytes electrically and metabolically, play a fundamental role in cell-cell communication and impulse propagation. Connexin43 (Cx43) is the principal junctional protein in mammalian myocardium. The distribution and function of Connexin 43 are important determinants of the conduction properties of myocardium and undoubtedly play a role in intercellular communication crucial to normal cardiac function. The Connexin 43 expressed in decreased amounts and distributed in different ways have relation with the abnormal electrical remodeling in the heart and arrhythmogenesis in the setting of hypertrophy, ischemia, or infarction. In this study, we detected the change of ultrastructural changes of myocardium and pattern of Connexin43 expression in Gz stress, and then evaluated the relation between the change of Connexin43 expression and arrhythmias during a centrifuge test.Male rats, which were provided by the laboratory animal center of FMMU, were exposure by animal centrifuge. Hearts were rapidly excised for transmission electron microscope to ensure ultrastructural changes of myocardium, immunohistochemistry and Western blot analysis to detect the change of Connexin43 expression after Gz stress.The main findings are as follow:1. Ultrastructural changes of myocardium in rats after repeated +Gz exposuresWe observed the effects of +Gz exposures on morphological changes of myocardium in rats. The results showed that, ventricular muscle, stained with H&E, has similar appearances in +6Gz, +10Gz exposure groups and control group.We examined the ultrastructural changes of ventricular myocardium in rats using the transmission electron microscope. The results showed that, the ultrastructural changes observed in immediate after +6Gz exposure group were sarcoplasmic reticulum expansion, capillary dilating and stroma edema. Compared with myocytes from the heart of control group, the intercalated discs of +6Gz group were broaden but the structure of the intercalated disc still can be identified. There were no obviously differences between the immediate and one day after exposure in +6Gz group. The ultrastructural changes in immediate after +10Gz exposure group were edema of cardiac myocyte, heterochromatin margination, sarcoplasmic reticulum expansion, hyperplasia and pooling of mitochondrion, endothelium edema. Intercalated disc in the group of +10Gz were dissociated, vacuolization, and the characteristic of gap-junctional disorder at the ends of the disk. No obviously differences of intercalated disc was found between the immediate and one day after exposure in +10Gz group. It is suggested that slight reversible ultrastructural changes can be observed in myocardium after repeated+6Gz/3min exposure while obvious ultrastructural changes were found during repeated +10Gz/3min exposure.2. The expression and distribution of Cx43 in myocardium of rats after repeated +Gz exposuresWe examined the ventricular myocardium of rats from the three groups, to delineate distribution of Cx43, using the immunohistochemistry. The results showed that, control groups showed abundant expression of Cx43 in intense punctate labelling confined to the intercalated disks between cardiomyocytes. Most of the Cx43 clustered at the ends of left ventricular cardiac muscles regularly, only a few of them existed at sides. After +Gz exposure, there was a loss of the immunoreactivity which was consistent with Western blot results and the distribution of Cx43. It is obvious that side-side Cx43 increased and end-end Cx43 decreased. The quantity and distribution of Cx43 recover gradually with longer recovery time after exposure. These alterations of +10Gz groups in the immunostaining of Cx43 were similar with +6Gz groups except the slower recovery. It is suggested that the Cx43 distributed in a pattern characterized by a side-side Cx43 increased and end-end Cx43 decreased after +Gz exposure.We compare quantity of Cx43 in ventricular myocardium of rats, using the Western Blot analysis. Western analysis revealed that Cx43 pattern is modified in response to different exposure program and different recovery time. The quantitative changes of Cx43 were lower in 4 different time after exposure, either +6Gz or +10Gz compared with data from the control groups (P<0.001). Densitometry analysis of immunoblots revealed a total amount of Cx43 signals decreased obviously immediate after exposure and increased with the longer recovery time. After seven days recovery, the amounts of Cx43 of two exposure groups were still lower than the normal (P<0.001). The +10Gz groups seem to have serious decrease of Cx43 than the +6Gz groups. It is suggested +Gz exposure is associated with the decrease of Cx43 in the ventricle of rats which could be recovered.In summery, we investigated the effects of +Gz exposures on the change of Cx43 by means of animal centrifuge, morphology, transmission electron microscope, immunohistochemical staining and Western blot analysis. The present study has demonstrated that ultrastructural changes can be observed in myocardium after +Gz exposure repeated +Gz exposure is associated with the ultrastructural changes and the decrease and the distribution disturbance of Cx43 in the ventricle of rats which could be 11 recovered. The +10Gz groups seem to have serious change of the ultrastructure and expression of Cx43 in ventricular myocardium of rats compared with the +6Gz groups. These findings will facilitate our understanding of mechanisms of arrhythmias caused by +Gz acceleration and provide new protective measures.