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.
Di Maio
Alessandro
A
Department of Cell & Developmental Biology, University of Pennsylvania, School of Medicine, Anat/Chem Building, Philadelphia, PA 19104-6058, USA. dimaio@mail.med.upenn.edu
Ter Keurs
H E
HE
Franzini-Armstrong
Clara
C
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D049628:Body Size / Q000502:physiology; D020013:Calcium Signaling / Q000502:physiology; D002450:Cell Communication / Q000502:physiology; D004285:Dogs; D017629:Gap Junctions / Q000502:physiology / Q000648:ultrastructure; D006352:Heart Ventricles / Q000648:ultrastructure; D007365:Intercellular Junctions / Q000502:physiology / Q000648:ultrastructure; D051379:Mice; D046529:Microscopy, Electron, Transmission; D008870:Microtubules / Q000502:physiology / Q000648:ultrastructure*; D009119:Muscle Contraction / Q000502:physiology; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000648:ultrastructure*; D032383:Myocytes, Cardiac / Q000502:physiology / Q000648:ultrastructure; D011690:Purkinje Fibers / Q000502:physiology / Q000648:ultrastructure*; D011817:Rabbits; D051381:Rats; D012519:Sarcoplasmic Reticulum / Q000502:physiology / Q000648:ultrastructure*; D013045:Species Specificity