The activation sequence of the ventricular myocardium in ectotherms is a matter of debate. We studied the correlation between the ventricular activation sequence and the pattern of local stretches in 13 toads (Bufo bufo). Epicardial potential mapping was done with a 56-lead sock array. Activation times were determined as d VIdt (min) in each lead. Initial epicardial foci of activation were found on the left side of the ventricular base, whereas regions on the apex and the right side of the base demonstrated late activation. Video recordings (50 frames s(-1)) showed that the median presystolic stretch in left-side ventricular regions was greater than that in right-side regions [4.70% (interquartile range 3.25-8.85%) versus 1.45% (interquartile range 0.38-3.05%), P=0.028, respectively]. Intracardiac bolus injection elicited ventricular activation with a similar sequence and duration. Thus, ventricular areas of earliest activation were associated with greater presystolic stretch, implying the existence of a stretch-excitation relationship in ectotherm hearts.
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