Measuring Electrical Conductivity of the Cardiac T-Tubular System

Abstract

Cardiomyocytes are characterized by a complex network of membrane invagination (the transverse axial tubular system, TATS) that propagates the action potential to the cell core guarantying a synchronous and uniform cell contraction. Pathological settings are commonly associated with structural remodelling of TATS that may cause failures of action potential propagation at single T-tubular level. Here, we investigate whether structural alterations occurring in TATS are linked to overall changes in its electrical conductivity which in turn hinder the regular propagation of action potentials across the network. Exploiting the formal analogy between diffusion and electrical conductivity we linked the conductivity with the diffusion properties of TATS. Fluorescence recovery after photo-bleaching (FRAP) microscopy is used to probe the diffusion properties of the tubular system in isolated cardiomyocytes: the fluorescent dextran inside TATS lumen is photo-bleached and the diffusion of unbleached dextran from extracellular space to TATS is monitored. We designed a mathematical model that correlate the time constant of fluorescence recovery with the apparent diffusion coefficient of the dextran. Then, taken advantage of the analogy between diffusion and conductivity, the apparent diffusion is used to assess TATS conductivity. This value is used to evaluate the efficiency of the passive spread of voltage changes along TATS. To probe how TATS conductivity is influenced by geometrical features, we used a model of acutely detubulated cells. We found that the overall reduction of TATS halves tubular conductivity. Then we tested our method in a pathological setting characterized by structural alteration, i.e. the heart failure rat model. We found that the tubular conductivity is not changed as compared to control. Actually, this result should not be surprising since heart failure is characterized by compound alterations that potentially compensate each other in their individual effect on global conductivity.