T-tubule profiles in Purkinje fibres of mammalian myocardium

Purkinje (P)-fibres are cardiac myocytes that are specialized for fast conduction of the electrical signal. P-fibres are usually defined as having the following identifying features: lack of T tubules; frequent lateral cell junctions; deep indentations at the intercalated discs level; the CX40 isoforms of gap junction proteins and, in large mammals, paucity of myofibrils and abundance of glycogen. We have examined the ultrastructure of P-fibres in free running P-strands from right and left ventricles of small (mouse and rat) intermediate (rabbit) and large (dog) size mammals focusing on presence and distribution of the T tubules. In contrast with previous studies, we find that P-fibres do have T tubules which form normal dyadic associations with the sarcoplasmic reticulum and that the frequency of tubules varies with the size of the animal. Profiles of T tubules and dyads are present over short segments of individual P-cells flanked by totally T tubule-free segments. It is thought that lack of T tubules in P-cells is necessary to reduce capacitance and thus accelerate action potential spread. This may not be as important in a small heart.