CARDIAC ULTRASTRUCTURE AND ELECTROCARDIOGRAM OF THE SHORT-TAILED SHREW, BLARINA-BREVICAUDA

The smaller species of shrews have been of considerable interest to sc ientists because of their high rate of metabolism, structure-functiona l and behavioral adaptations to support their energy demands. The pres ent data are the first detailed cardiac ultrastructural findings and e lectrocardiographic (ECG) data of adult and immature small short-taile d shrews, Blarina brevicauda. The heart is morphologically elongated a nd heart rates in excess of 900 b/min were observed, but ECG component s and pattern are non-distinctive for this species. Ultrastructurally, the sarcomeres, tubular and sarcotubular systems and Purkinje cells r esemble closely those observed in larger, less active mammals. Several distinctive features resembling those seen in some other shrews or hu mmingbirds exist, including reduced quantities of myocyte glycogen, ir regularly shaped and tightly packed mitochondria, increased neural and vascular elements in the myocardium, and small size and unusual dispe rsion of atrial specific granules. These morphologic findings suggest that the remarkable physiologic performance of the heart of Blarina br evicauda is supported by a combination of macroscopic, histologic and cellular adaptations.