Thermal adaptation in fish is accompanied by morphological and electrophysiological changes in the myocardium. Little is known regarding changes of spatiotemporal organization of ventricular excitation and repolarization processes with acclimatization. We aimed to evaluate transmural and apicobasal heterogeneity of depolarization and repolarization characteristics in the in-situ heart of rainbow trout in seasonal acclimatization. Methods: The experiments were done in the summer-acclimatized

more »

... 18°C, n=8) and winter-acclimatized (WA, 3°C, n=8) rainbow trout. 24 unipolar electrograms were recorded with plunge needle electrodes (eight lead terminals each) impaled into the ventricular wall. Activation time (AT), end of repolarization time (RT), and activation-repolarization interval (ARI, a surrogate for action potential duration) were determined as dV/dt min during QRS-complex, dV/dt max during T-wave, and RT-AT difference, respectively. Results: The SA fish demonstrated relatively flat apicobasal and transmural AT and especially ARI profiles. In the WA animals, ATs and ARIs were longer as compared to SA animals (p≤0.001), ARIs were shorter in the compact layer than in the spongy layer (p≤0.050), and within the compact layer, the apical region had shorter ATs and longer ARIs as compared to the basal region (p≤0.050). In multiple linear regression analysis, ARI duration was associated with cardiac cycle duration and AT in SA and WA animals. The WA animals demonstrated additionally an independent association of ARIs with spatial localization across the ventricle. Conclusion: Adaptation to cold conditions in rainbow trout was associated with a spatial ventricular remodeling leading to the development of repolarization gradients typically observed in mammalian myocardium.