Capacitive Membrane Activity in Isolated Cardiac Myocytes

Abstract

Cardiac myocytes are cells with complex membrane structure of unknown dynamics. A good physical measure of the state of the sarcolemma is membrane capacitance. High-resolution capacitance measurements were instrumental in disclosing secretory and endocytotic mechanisms in simpler cells. We used a new method of membrane capacitance recording based on deconvolution of current responses to square wave voltage stimuli (MAT-MECAS: https://sourceforge.net/projects/mat-mecas/) to study membrane dynamics of isolated cardiac myocytes; specifically, the spontaneous and stimulated membrane capacitance changes. We observed spontaneous discrete capacitance events of 1 − 20 fF that may correspond to fusion and fission of single vesicles of 180 − 800 nm in diameter. In addition, large spontaneous stepwise increases of membrane capacitance of 80 − 250 fF were also observed. These large capacitance increases were occasionally preceded by a transient opening of a fusion pore with a conductance of several hundreds of pS that lasted for a few hundreds of milliseconds. Such large stepwise changes of membrane capacitance might be related to the opening of the mouth of transversal tubules. Discrete capacitance increases were also observed after stimulation of myocytes by a train of calcium currents. In these experiments, the discrete capacitance increases of 50 − 160 fF occurred 3 to 20 seconds after stimulation. This might reflect calcium induced fusion of intracellular membrane bodies with the surface membrane. In conditions of permanently raised intracellular concentration of calcium ions, membrane capacitance showed increased activity of step-like capacitance events. These observations support the role of intracellular calcium in regulation of membrane dynamics in cardiac myocytes. Altogether, these findings provide the first evidence of vesicle fusion events related to physiologically relevant membrane activity in cardiac myocytes.Supported by: APVV-0721-10, VEGA 2/0147/14.