T-tubule profiles in Purkinje fibres of mammalian myocardium
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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.
KeywordsTransverse (T) tubule Purkinje (P) strands Purkinje (P) cells Sarcoplasmic reticulum Action potential
We thank Ms. Nosta Glaser for her help during the experiments and photography, Dr. Didier Brochet for his help on the rabbit heart and Dr. Boyden for her very helpful suggestions. This work was supported by NIH grant HL48093.
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