Electrotonic spread of current in monolayer cultures of neonatal rat heart cells

The passive electrical properties of neonatal rat heart cells grown in monolayer cultures were determined. Hyperpolarizing current pulses were injected through one microelectrode via an active bridge circuit. Membrane voltage displacements caused by the injected current pulses were measured at various distances from the first with a second microelectrode. Using a modified least-squares method the experimental results were fitted to a Bessel function, which is the steady-state solution of the differential equation describing the relation between membrane voltage caused by current injection and interelectrode distance in a very large and very thin plane cell. Best fit was obtained with a space constant of 360 μm and an internal resistivity of 500 Ω cm. From these figures, specific membrane resistance was calculated to be 1,300 Ω cm(2), assuming all current to leave through the upper surface of the monolayer.The time constant of the membrane was measured from the time course of the current-induced membrane voltage displacements. From its value of 1.7 msec a membrane capacity of 1.3 μF/cm(2) was calculated.From these results and some literature data on nexus distribution (A. W. Spira,J. Ultrastruct. Res. 34:409, 1971) specific nexus resistance was calculated to range between 0.25 and 1.25 Ω cm(2), depending on the amount of folding of the intercalated discs. The results suggest that spread of activation in monolayer cultures of heart cells by means of local circuit currents is very likely.