Comparison of the T-tubule system in adult rat ventricular and atrial myocytes, and its role in excitation-contraction coupling and inotropic stimulation.
Narrow, tubular, inward projections of the sarcolemma ('T-tubules') are an established feature of adult mammalian ventricular myocytes that enables them to generate the whole-cell Ca2+ transients and produce coordinated contraction. Loss of T-tubules can occur during ageing and under pathological conditions, leading to altered cardiac excitation-contraction coupling. In contrast to adult ventricular cells, atrial myocytes do not generally express an extensive T-tubule system at any stage of development, and therefore rely on Ca2+ channels around their periphery for the induction of Ca2+ signalling and excitation-contraction coupling. Consequently, the characteristics of systolic Ca2+ signals in adult ventricular and atrial myocytes are temporally and spatially distinct. However, although atrial myocytes do not have the same regularly spaced convoluted T-tubule structures as adult ventricular cells, it has been suggested that a proportion of adult atrial cells have a more rudimentary tubule system. We examined the structure and function of these atrial tubules, and explored their impact on the initiation and recovery of Ca2+ signalling in electrically paced myocytes. The atrial responses were compared to those in adult ventricular cells that had intact T-tubules, or that had been chemically detubulated. We found that tubular structures were present in a significant minority of adult atrial myocytes, and were unlike the T-tubules in adult ventricular cells. In those cells where they were present, the atrial tubules significantly altered the on-set, amplitude, homogeneity and recovery of Ca2+ transients. The properties of adult atrial myocyte Ca2+ signals were different from those in adult ventricular cells, whether intact or detubulated. Excitation-contraction coupling in detubulated adult ventricular myocytes, therefore, does not approximate to atrial signalling, even though Ca2+ signals are initiated in the periphery of the cells in both of these situations. Furthermore, inotropic responses to endothelin-1 were entirely dependent on T-tubules in adult ventricular myocytes, but not in atrial cells. Our data reveal that that the T-tubules in atrial cells impart significant functional properties, but loss of these tubular membranes does not affect Ca2+ signalling as dramatically as detubulation in ventricular myocytes.
Smyrnias
Ioannis
I
Calcium Group, Laboratory of Molecular Signalling, The Babraham Institute, Babraham, Cambridge CB223AT, UK.
Mair
Waltraud
W
Harzheim
Dagmar
D
Walker
Simon A
SA
Roderick
H Llewelyn
HL
Bootman
Martin D
MD
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D020013:Calcium Signaling / Q000502:physiology*; D019332:Endothelin-1 / Q000378:metabolism / Q000494:pharmacology; D006325:Heart Atria / Q000378:metabolism / Q000648:ultrastructure*; D006352:Heart Ventricles / Q000378:metabolism / Q000648:ultrastructure*; D008297:Male; D008870:Microtubules / Q000502:physiology / Q000648:ultrastructure*; D009119:Muscle Contraction / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology; D032383:Myocytes, Cardiac / Q000378:metabolism / Q000648:ultrastructure*; D051381:Rats; D017208:Rats, Wistar; D012508:Sarcolemma / Q000502:physiology / Q000648:ultrastructure
D019332:Endothelin-1; D002118:Calcium