Impact of Loss of T-Tubules on Myocardial Contractile Force and Kinetics

Abstract

Action potential driven Ca2+ currents via the transverse tubular membrane synchronously trigger Ca2+release from the SR close to the myofibril rapidly activating contraction. Loss of T-tubules has been reported in disease including heart failure, but the effect of detubulation on muscle mechanics has never been investigated.We dissected thin (50-200μm) right ventricular and left atrial trabeculae from adult rats and recorded force and sarcomere length. To achieve acute detubulation, we exposed trabeculae to formamide 1.5M for 20 minutes. Return to normo-osmotic solution increases cell-volume suddenly and disconnects T-tubules from the sarcolemma as was directly verified using di-8-anepps staining and confocal microscopy.In seven ventricular trabeculae, detubulation prolonged the twitch, i.e. time to peak force (by 31±9%) and time to 50% relaxation (by 46±14%) at 1Hz. Detubulation reduced the inotropic effect of increased stimulus-rate (by 29±7%). Maximal twitch force after post-rest potentiation was unchanged. Detubulation increased the fraction of Ca2+ recirculating to the SR (by 17 ± 5%) measured by the decay of potentiation, suggesting an increased SERCA vs. NCX activity. None of these effects was seen in four formamide-treated atrial trabeculae, which constitutively lack T-tubules in rodents.T-tubular disruption from the membrane implies loss of 80% L-Type ICa and 60% INCX. Mathematical modeling shows that in myocytes with EC-Coupling via T tubules and SR alone the loss of the aforementioned currents is not sufficient to explain the differences between control and detubulated trabeculae. Such differences can be predicted assuming that EC-coupling is maintained by a fast Ca2+ rise near the sarcolemma but now followed by, Ca2+-diffusion mediated, propagated Ca2+ induced SR-Ca2+ release toward the core. Enhanced Ca2+_wave spread and recruitment of all myofibril layers can contribute to maintain maximal contractile force in the absence of T-tubules.