ADAPTATIONS OF THE FROG MYOCARDIUM TO CONDITIONS OF NATURAL HIBERNATION - MORPHOFUNCTIONAL CHANGES

The present study describes morphological and functional changes in th e ventricular myocardium of Rana esculenta during hibernation, in comp arison with active life. Contractile performance was analyzed in vitro on isolated ventricular trabeculae. Morphological studies were carrie d out with light and electron microscopy. In hibernating frogs, the my ofibrillar apparatus appeared deteriorated and dispersed in an abundan t paraplasma mainly consisting of glycogen granules. The accumulation of glycogen was confirmed by the intense staining with the PAS reactio n. In accordance with morphological changes, tension development was r educed in trabeculae isolated from hibernating frogs. Whereas force-ve locity relations did not show any difference between trabeculae from h ibernating frogs and those from active frogs, significant differences were found in some parameters related to excitation-contraction coupli ng. In hibernating frog trabeculae, the relaxation phase was prolonged and the optimal interval of both transient and steady-state force-int erval relations was lengthened. These changes, which point to a differ ent calcium metabolism in cardiomyocytes from active and hibernating f rogs, were in agreement with the lower activity of the Ca2+-ATPase obs erved in hibernating frog myocardium.