Paramdeep S. Dhillon

Relationship Between Gap-Junctional Conductance and Conduction Velocity in Mammalian Myocardium

Background—Gap junction resistivity, Rj, has been proposed as a key determinant of conduction velocity (CV). However, studies in connexin-gene knockout mice demonstrated significant CV slowing only with near-complete connexin deletion, and these findings led to the concept of a significant redundancy of myocardial gap junctions. We challenged this prevailing concept and addressed the hypothesis that there is a continuous relationship between Rj and CV, each independently measured in human and guinea-pig myocardium. Methods and Results—Rj and CV were directly measured by oil-gap impedance and microelectrode techniques in human left ventricular myocardium from patients with hypertrophic cardiomyopathy and in guinea-pig atrial and ventricular myocardium before and during pharmacological uncoupling with 20-µmol/L carbenoxolone. There was a continuous relationship between Rj and CV in human and guinea-pig myocardium, pre- and post-carbenoxolone (r2=0.946; P<0.01). In guinea-pig left ventricle, left atrium, and right atrium, carbenoxolone increased Rj by 28±9%, 26±16%, and 25±14% and slowed CV by 17±3%, 23±8%, and 11±4% respectively (all P<0.05 versus control). As a clinically accessible measure of local microscopic myocardial conduction slowing in vivo in the intact human heart, carbenoxolone prolonged electrogram duration in the right atrium (39.7±4.2 to 42.3±4.3 ms; P=0.01) and right ventricle (48.1±2.5 to 53.3±5.3 ms; P<0.01). Conclusions—There is a continuous relationship between Rj and CV that is consistent between cardiac chambers and across species, indicating that naturally occurring variations in cellular coupling can account for variations in CV, and that the concept that there is massive redundancy of coupling is not tenable.

Relationship Between Connexin Expression and Gap–Junction Resistivity in Human Atrial Myocardium

Background—The relative roles of the gap-junctional proteins connexin40 (Cx40) and connexin43 (Cx43) in determining human atrial myocardial resistivity is unknown. In addressing the hypothesis that changing relative expression of Cx40 and Cx43 underlies an increase in human atrial myocardial resistivity with age, this relationship was investigated by direct ex vivo measurement of gap-junctional resistivity and quantitative connexin immunoblotting and immunohistochemistry. Methods and Results—Oil-gap impedance measurements were performed to determine resistivity of the intracellular pathway (Ri), which correlated with total Cx40 quantification by Western blotting (rs=0.64, P<0.01, n=20). Specific gap-junctional resistivity (Rj) correlated not only with Western immunoquantification of Cx40 (rs=0.63, P=0.01, n=20), but also more specifically, with the Cx40 fraction localized to the intercalated disks on immunohistochemical quantification (rs=0.66, P=0.02, n=12). Although Cx43 expression showed no correlation with resistivity values, the proportional expression of the 2 connexins, (Cx40/[Cx40+Cx43]) correlated with Ri and Rj (rs=0.58, P<0.01 for Ri and rs=0.51, P=0.02 for Rj). Advancing age was associated with a rise in Ri (rs=0.77, P<0.0001), Rj (rs=0.65, P<0.001, n=23), Cx40 quantity (rs=0.54, P=0.01, n=20), and Cx40 gap–junction protein per unit area of en face disk (rs=0.61, P=0.02, n=12). Conclusions—Cx40 is associated with human right atrial gap-junctional resistivity such that increased total, gap-junctional, and proportional Cx40 expression increases gap-junctional resistivity. Accordingly, advancing age is associated with an increase in Cx40 expression and a corresponding increase in gap-junctional resistivity. These findings are the first to demonstrate this relationship and a mechanistic explanation for changing atrial conduction and age-related arrhythmic tendency.

Architectural Correlates of Myocardial Conduction Changes to the Topography of Cellular Coupling, Intracellular Conductance, and Action Potential Propagation with Hypertrophy in Guinea-Pig Ventricular Myocardium

Background—We tested the hypothesis that alterations to action potential conduction velocity (CV) and conduction anisotropy in left ventricular hypertrophy are associated with topographical changes to gap-junction coupling and intracellular conductance by measuring these variables in the same preparations. Methods and Results—Left ventricular papillary muscles were excised from aortic-banded or sham-operated guinea-pig hearts. With intracellular stimulating and recording microelectrodes, CV was measured in 3 dimensions with simultaneous conductance mapping with subthreshold stimuli and correlated with quantitative histomorphometry of myocardial architecture and connexin 43 distribution. In hypertrophied myocardium, CV in the longitudinal axis was smaller and transverse velocity was greater compared with control; associated with similar differences of intracellular conductance, consistent with more cell contacts per cell (5.7±0.2 versus 8.1±0.5; control versus hypertrophy), and more intercalated disks mediating side-to-side coupling (8.2±0.2 versus 10.2±0.4 per cell). Intercalated disk morphology and connexin 43 immunolabelling were not different in hypertrophy. Hypertrophied preparations showed local submillimeter (≈250 &mgr;m) regions with slow conduction and low intracellular conductance, which, although not affecting CV on the millimeter scale, were consistent with discontinuities from increased microscopical connective tissue content. Conclusions—With myocardial hypertrophy, altered longitudinal and transverse CV, and greater nonuniformity of CV anisotropy correspond to changes of intracellular conductance. These are associated with alteration of myocardial architecture, specifically the topography of cell–cell coupling and gap-junction connectivity.