A new approach to the determination of tubular membrane capacitance: passive membrane electrical properties under reduced electrical conductivity of the extracellular solution
Authors: Šimurda Jiří , Šimurdová Milena , Švecová Olga , Bébarová Markéta 
Published: November 2021

The tubular system of cardiomyocytes plays a key role in excitation-contraction coupling. To determine the area of the tubular membrane in relation to the area of the surface membrane, indirect measurements through the determination of membrane capacitances by electrophysiological measurements are currently used in addition to microscopic methods. Unlike existing electrophysiological methods based on an irreversible procedure (osmotic shock), the proposed approach uses a reversible short-term intermittent increase in the electrical resistance of the extracellular medium. The resulting increase in the lumen resistance of the tubular system makes it possible to determine separately capacitances of the tubular and surface membranes from altered capacitive current responses to subthreshold voltage-clamped rectangular pulses. Based on the analysis of the time course of capacitive current, computational relations were derived which allow to quantify elements of the electrical equivalent circuit of the measured cardiomyocyte including both capacitances. The exposition to isotonic low-conductivity sucrose solution is reversible which is the main advantage of the proposed approach allowing repetitive measurements on the same cell under control and sucrose solutions. In addition, it might be possible to identify changes in both surface and tubular membrane capacitances caused by various interventions. Preliminary experiments in rat ventricular cardiomyocytes (n = 10) resulted in values of the surface and tubular capacitances 72.3 ± 16.4 and 42.1 ± 14.7 pF, respectively, implying the fraction of tubular capacitance/area of 0.36 ± 0.08. We conclude that the newly proposed method provides results comparable to those reported in literature and, in contrast to the currently used methods, enables repetitive evaluation of parameters describing the surface and tubular membranes. It may be used to study alterations of the tubular system resulting from various interventions including associated cardiac pathologies.

Author summary

The cell membrane of cardiomyocytes is invaginated forming a net of tubules through the whole cell (the tubular system). These invaginations are essential for the coordinated contraction of cardiomyocytes. Various cardiac diseases may lead to an alteration of the tubular system and vice versa alteration of the tubular system may result in cardiac dysfunction. Therefore, it is desirable to investigate the tubular membrane separately. Unfortunately, it is not easy and methods currently used for this purpose provide diverse results. The widely used detubulation methods (separating the tubular and surface membrane by an osmotic shock) are irreversible. It makes impossible repeated measurements on the same cell, thus, disabling evaluation of parameters describing the surface and tubular membranes and particularly testing changes in the tubular system. We developed an alternative approach based on the electrical separation of the tubular and surface membranes induced by exposition of the cardiomyocyte to an isotonic sucrose solution with low electrical conductivity. It allows to quantify the approximate area of the surface and tubular membrane using evaluation of electrical capacitance of both membrane areas. Measurements can be repeated several times on the same cell.