Cardiac ultrastructure and electrocardiogram of the short-tailed shrew, Blarina brevicauda.

The smaller species of shrews have been of considerable interest to scientists because of their high rate of metabolism, structure-functional and behavioral adaptations to support their energy demands. The present data are the first detailed cardiac ultrastructural findings and electrocardiographic (ECG) data of adult and immature small short-tailed shrews, Blarina brevicauda. The heart is morphologically elongated and heart rates in excess of 900 b/min were observed, but ECG components and pattern are non-distinctive for this species. Ultrastructurally, the sarcomeres, tubular and sarcotubular systems and Purkinje cells resemble closely those observed in larger, less active mammals. Several distinctive features resembling those seen in some other shrews or hummingbirds exist, including reduced quantities of myocyte glycogen, irregularly shaped and tightly packed mitochondria, increased neural and vascular elements in the myocardium, and small size and unusual dispersion of atrial specific granules. These morphologic findings suggest that the remarkable physiologic performance of the heart of Blarina brevicauda is supported by a combination of macroscopic, histologic and cellular adaptations.