Electrophysiologic properties differ in the ventricular endocardium and epicardium of the Japanese monkey

Epicardium Formation as a Sensor in Toxicology

Zebrafish (Danio rerio) are an excellent vertebrate model for studying heart development, regeneration and cardiotoxicity. Zebrafish embryos exposed during the temporal window of epicardium development to the aryl hydrocarbon receptor (AHR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exhibit severe heart malformations. TCDD exposure prevents both proepicardial organ (PE) and epicardium development. Exposure later in development, after the epicardium has formed, does not produce cardiac toxicity. It is not until the adult zebrafish heart is stimulated to regenerate does TCDD again cause detrimental effects. TCDD exposure prior to ventricular resection prevents cardiac regeneration. It is likely that TCDD-induced inhibition of epicardium development and cardiac regeneration occur via a common mechanism. Here, we describe experiments that focus on the epicardium as a target and sensor of zebrafish heart toxicity.

Heterogeneity of the properties of INa in epicardial and endocardial cells of rat ventricle

1. Ventricular I(Na) heterogeneity was investigated in adult rat hearts. Differences in transient outward potassium current (Ito) were used to confirm isolation of subepicardial and subendocardial cells. Mean peak Ito was 6.0 +/- 0.7 and 1.6 +/- 0.45 pA/pF in epicardial and endocardial cells, respectively (P < < 0.01). 2. Maximum sodium conductance was smaller in subendocardial cells compared with subepicardial cells (2.39 +/- 0.11 vs 2.78 +/- 0.12 nS/pF, respectively; n=17 for both; 0.01 < P < 0.05) and 50% activation occurred at a slightly more negative potential (-47.6 +/- 0.8 vs -44.9 +/- 0.9 mV, respectively; n=10 for both; 0.01 < P < 0.05). 3. The potential for 50% inactivation was not significantly different in subepicardial compared with subendocardial cells (72.2 +/- 1.0 vs 72.8 +/- 2.2 mV, respectively; n=17 for both; NS). 4. Persistent sodium current density appeared smaller in subendocardial (n =19) compared with subepicardial (n=11) cells (at a test potential of -25 mV current, density was 0.118 +/- 0.041 vs 0.144 +/- 0.085 pA/pF, respectively), although this was not statistically significant due to large variability between cells. 5. Mathematical modelling of the cardiac action potential indicated that the combined effects of differences in current density and voltage dependence of sodium currents are unlikely to contribute to ventricular action potential heterogeneity between epicardial and endocardial cells.

Inverse relation of body-surface activation-recovery interval and recovery time to activation time in normal subjects: stronger correlation and more heterogeneous distribution in activation-recovery interval than in recovery time

The activation-recovery interval (ARI), measured directly from the myocardium, has shown a good correlation with the action potential duration (APD) in experiments. APD has been reported to be inversely related to the activation time (AT). However, no studies have examined the correlation between the body-surface ARI and AT in normal subjects. Fifty normal subjects (25 men and 25 women) were studied to elucidate the relationship between the body-surface ARI and AT. The body-surface AT was defined as the duration between the QRS onset and the minimum dV/dt of the QRS wave, and ARI as the interval between the minimum dV/dt of the QRS wave and the maximum dV/dt of the T wave in each lead of an 87 unipolar lead system. We also measured the recovery time (RT) defined as the duration between the QRS onset and the maximum dV/dt of the T wave. ARI was inversely correlated with AT (r = -0.73). RT was also inversely correlated with AT (r = -0.61), however, RT had a less heterogeneous distribution than ARI (148 ms vs 159 ms). There were no differences between male and female subjects in the relation between ARI and RT or in the body-surface distribution of ARI and RT. These findings suggest that the body-surface ARI may reflect recovery properties over the cardiac surface and that APD may distribute inhomogeneously over the human cardiac surface with a longer RT over an area with a shorter AT. ARI calculated from body-surface ECG may be a useful noninvasive and repeatedly measurable estimate of APD.