New Insights into Myocardial Arrhythmogenesis: Distribution of Gap-Junctional Coupling in Normal, Ischaemic and Hypertrophied Human Hearts

Abstract

1. Ischaemic and hypertrophic heart diseases are associated with ventricular arrhythmias, in which abnormal cellular coupling is implicated as having a causative role. The aim of this series of studies was to characterize gap-junctional organization in normal human ventricular myocardium, and to investigate the hypothesis that alterations in the quantity and patterns of expression of myocardial gap junctions occur in ischaemic and hypertrophic myocardial disease. 2. An antibody raised against connexin43 was used for immunohistochemical labelling of myocardium examined by confocal laser scanning microscopy, permitting highly sensitive and quantifiable immunofluorescent imaging of gap junctions through volumes of intact cardiac tissue. 3. Connexin43 gap junctions in normal adult human ventricular myocardium are highly organized into clusters of fluorescent label confined to the intercalated disks as a peripheral ring of larger junctions, with smaller junctions centrally, and occupy a surface area of 0.005 μm2/μm3 myocyte volume. 4. Neonatal human myocardium has a punctate distribution of connexin43 over the entire surface of the ventricular myocytes, with a progressive polarization of the gap junctions towards the positions of the mature intercalated disks, reaching the adult pattern at about 6 years. 5. At the myocardial interface with the scar of a healed infarct, connexin43 gap junction distribution is grossly disturbed, being strewn in longitudinally orientated arrays along the lateral interfaces between degenerated but viable myocytes, which may be due to a redistribution of the pre-existing population of junctions. This altered distribution is present as early as 4 days after coronary occlusion in a canine model, in which it defines the location of circuits causing ventricular tachycardia. 6. Myocardium distant from infarction in patients with ischaemic heart disease has a normal pattern of connexin43 gap junction distribution, but has a 47% reduction in gap junction surface area per unit cell volume, and a 30% reduction per cell. 7. In hypertrophied myocardium from chronically pressure-loaded human left ventricles, connexin43 gap junction expression per myocyte is not significantly different from normal, but is reduced by 40% per unit volume of myocyte. 8. The early phases of the hypertrophic response of myocardium to renovascular hypertension in guinea pigs revealed a substantially increased connexin43 gap junction expression compared with controls, both when measured per cell (increased by 45%) and per unit volume of myocyte (increased by 30%), and therefore showed an alteration apparently contrary to that observed in chronically hypertrophied human ventricular myocardium. 9. In this series of studies, normal adult human ventricular myocardium and the post-natal developmental changes have been characterized with respect to connexin43 gap junction content, and the observed alterations of both distribution and quantity in ischaemic and hypertrophied hearts would be expected to influence myocardial conduction and the arrhythmogenic substrate.