The T-tubule system in the specialized and general myocardium of the rat.

The T-tubule system in cardiac muscle cells has been investigated with the electron microscope in 10 adult rats after infiltration with horseradish peroxidase. All cardiac muscle cells possess a T-system, but its complexity varies according to the region of the heart. It is most extensive in the general ventricular myocardium where there are primary, secondary and tertiary transverse tubules as well as longitudinal elements, and there are numerous couplings between the T-system and the sarcoplasmic reticulum (SR). The T-system and associated SR couplings are less extensive in the atrium and in the atrioventricular conducting system. It is least well developed in the nodes, particularly in the sinuatrial node, where it is restricted to primary T-tubules. There are numerous SR couplings with the sarcolemma in all types of cardiac muscle cells. Where intercalated discs occur, SR couplings are associated with non-specialized parts of the disc. The possible significance of these ultrastructural features in regard to the speed of conduction by myocardial tissues, and in the excitation-contraction sequence is discussed.