Reduction in density of transverse tubules and L-type Ca2+ channels in canine tachycardia-induced heart failure

Objective: Persistent supraventricular tachycardia leads to the development of a dilated cardiomyopathy with impairment of excitation-contraction (EC) coupling. Since the initial trigger for EC coupling in ventricular muscle is the influx of Ca2+ through L-type Ca2+ channels (I-Ca) in the transverse tubules (T-tubules), we determined if the density of the T-tubule system a nd L-type Ca2+ channels change in canine tachycardia pacing-induced cardiom yopathy. Methods: Confocal imaging of isolated ventricular myocyles stained with the membrane dye Di-8-ANEPPS was used to image the T-tubule system, a nd standard whole-cell patch clamp techniques were used to measure I-Ca and intramembrane charge movement. Results: A complex staining pattern of inte rconnected tubules including prominent transverse components spaced every s imilar to1.6 mum was present in control ventricular myocytes, but failing c ells demonstrated a far less regular T-tubule system with a relative loss o f T-tubules. In confocal optical slices, the average % of the total cell ar ea staining for T-tubules decreased from 11.5+/-0.4 in control to 8.7+/-0.4 % in failing cells (P<0.001). Whole-cell patch clamp studies revealed that I-Ca density was unchanged. Since whole-cell I-Ca is due to both the number of channels as well as the functional properties of those channels, we mea sured intramembrane charge movement as an assay for changes in channel numb er. The saturating amount of charge that moves due to gating of L-type Ca2 channels, Q(on,max). was decreased from 6.5+/-0.6 in control to 2.8+/-0.3 fC/pF in failing myocytes,(P<0.001). Conclusions: Cellular remodeling in he art failure results in decreased density of T-tubules and L-type Ca2+ chann els, which contribute abnormal EC coupling. (C) 2001 Elsevier Science B.V. All rights reserved.