Enhanced Propagation of Calcium-Induced Calcium-Release (CICR) from the Cell Periphery to the Core Increases Contractility of Detubulated Myocardium

Abstract

Mechanics and Excitation-Contraction-Coupling were studied in rat ventricular trabeculae and myocytes before and after formamide-induced detubulation. Detubulation was checked using FM4-64 membrane staining and dual-photon microscopy. Electrical uncoupling of t-tubules from sarcolemma was confirmed by simultaneous action potential recordings at surface and t-tubular membranes (Sacconi et al., this meeting). Global and regional (periphery/core) Ca2+-transients were measured either under confocal microscopy or in wide-field with a high-speed CCD-camera from Fluo4-loaded myocytes. At low [Ca2+]o (1mM,2Hz,30°C) twitch tension of detubulated trabeculae was reduced to ∼40% of controls and the time course of contraction was significantly slower. Under the same conditions, the global Ca2+-transients were depressed and slowed down in detubulated cells vs. controls. Regional Ca2+-transients were synchronous and homogeneous in control myocytes while in detubulated cells they were mostly restricted to a subsarcolemmal ring with the cell core displaying lower and delayed Ca2+-signals. The results are consistent with detubulation inducing a shift from synchronous CICR to a propagated form of CICR that is slower and may not recruit deep myofibril layers. At high [Ca2+]o (>8mM), or following post-rest potentiation, twitch tension of detubulated trabeculae was unchanged compared to controls, though contractions were prolonged. At high [Ca2+]o, or at resting stimulation frequency, the Ca2+ gradient between cell periphery and core was markedly reduced in detubulated myocytes while the time-lag between the two Ca2+-transients was still present. The results suggest that CICR propagation from cell periphery to the core can be enhanced and contractility improved in detubulated myocardium by increasing the Ca2+-trigger and the SR Ca2+-load. Interventions that increase RyR2 open probability may also enhance CICR propagation in detubulated myocytes. This has been confirmed by comparing the effects of caffeine (200µM) on control and detubulated myocytes.