Transmural heterogeneity of action potentials andI to1 in myocytes isolated from the human right ventricle

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Abstract
Limited information is available about transmural heterogeneity in cardiac electrophysiology in man. The present study was designed to evaluate heterogeneity of cardiac action potential (AP), transient outward K+ current ( I to1) and inwardly rectifying K+ current ( I K1) in human right ventricle. AP and membrane currents were recorded using whole cell current- and voltage-clamp techniques in myocytes isolated from subepicardial, midmyocardial, and subendocardial layers of the right ventricle of explanted failing human hearts. AP morphology differed among the regional cell types. AP duration (APD) at 0.5–2 Hz was longer in midmyocardial cells (M cells) than in subepicardial and subendocardial cells. At room temperature, observed I to1, on step to +60 mV, was significantly greater in subepicardial (6.9 ± 0.8 pA/pF) and M cells (6.0 ± 1.1 pA/pF) than in subendocardial cells (2.2 ± 0.7 pA/pF, P < 0.01). Slower recovery of I to1 was observed in subendocardial cells. The half-inactivation voltage of I to1 was more negative in subendocardial cells than in M and subepicardial cells. At 36°C, the density of I to1 increased, the time-dependent inactivation and reactivation accelerated, and the frequency-dependent reduction attenuated in all regional cell types. No significant difference was observed in I K1 density among the regional cell types. The results indicate that M cells in humans, as in canines, show the greatest APD and that a gradient of I to1 density is present in the transmural ventricular wall. Therefore, the human right ventricle shows significant transmural heterogeneity in AP morphology and I to1properties.

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