An enzyme histochemical study of human sinus node, coronary sinus, and mitral valve muscle.

An enzyme histochemical study of the sinoatrial node, the coronary sinus, and the atrial muscle extending into the anterior mitral valve was performed on human hearts. Investigation of the activity and localization of the structurally bound enzymes was performed by conventional histochemical techniques. Determination of the activity of nonstructurally or weakly structurally bound enzymes was performed by histochemical techniques in which leakage of enzymes during the incubation period was reduced by the application of semipermeable membranes. The sinoatrial node is characterized by a high degree of anaerobic enzyme capacity and a relatively low degree of aerobic enzyme capacity. The discriminatory nature of these reactions allows examination of the structure of the sinoatrial node and its approaches. The presence of transitional cells was confirmed; isolated clusters of nodal cells were found in the atrial myocardium around the sinoatrial node, but no evidence of specialized tissue forming the beginning of an internodal pathway was found by this technique. The specific histochemical reactions that characterize the sinoatrial node also occur in the atrial muscle, extending into the anterior mitral valve, the anterior wall of the coronary sinus, and the atrial tissue near the orifice of the coronary sinus. These observations seem to corroborate the hypothesis that arrhythmic ectopic foci can arise in these regions.