Comparative morphometry of the mitochondria and activity of some enzymes in the myocardium of small mammals.

Heart rate in mammals is inversely related to body weight. In the etruscan shrew it exceeds 960 beats/min. Since the heart depends primarily on energy from aerobic sources, it was interesting to examine some morphological parameters of the mitochondria in some mammals with a high heart frequency. Volume fraction and surface to volume ratio of the mitochondria in the myocardium were determined in the white rat (221 g), white mouse (36 g), white-toothed shrew (8 g) and the etruscan shrew (2 g) using morphometric methods. The volume fraction and surface to volume ratio of the mitochondria increased progressively and significantly as body weight of the animal decreased. The increase was higher in the surface to volume ratio than in the volume fraction. It reached a value which was 62% higher in the etruscan shrew than that of the white rat, while in the volume fraction of the mitochondria the maximal increase was only 34%. In accordance with the morphometric data, enzymatic activity of succinic dehydrogenase in the myocardium was inversely related to body weight. Creatine phosphokinase activity revealed a similar but not statistically significant trend. Lactic dehydrogenase activity was about three-fold higher in the white rat than in the white toothed shrew. It is concluded that one of the adaptive responses of the heart in small sized mammals to increase ATP production is not only elevation of the volume fraction of the mitochondria, but also an increase of their surface to volume ratio to provide a higher rate of oxygen diffusion to them.