Electron microscopic cytochemistry of cytochrome oxidase activity in the conduction system of the canine heart.

Histochemistry and electron microscopic cytochemistry of cytochrome oxidase in the conduction system were studied with the 3,3'-diaminobenzidine (DAB) method in adult canine hearts. This enzyme was distinctly less active in the entire conduction system than in the working myocardium. By electron microscopy, enzymatic activity per cristal membrane was apparently similar in both specialized and working cardiocytes. However, the volume fraction of cell occupied by mitochondria and the density of cristal membranes in mitochondria were smaller in the specialized cells in the sinus node, atrioventricular (AV) node, His bundle, and Purkinje fibers. These observations define the nature of decreased histochemical activity of cytochrome oxidase in cells of the conduction system, which is caused entirely by the decrease in activity per unit of mitochondrial volume and unit of cell volume in the specialized cardiac tissues.