Three‐micro‐electrode voltage clamp experiments in calf cardiac Purkinje fibres: is slow inward current adequately measured?

Abstract

The 3-micro-electrode voltage clamp method was adapted for study of regenerative inward currents in cardiac muscle. The adequacy of measurements of slow inward current in cardiac Purkinje fibers was assessed. Membrane current density is reported simultaneously by total applied current (IT), along with a longitudinal voltage difference signal (.DELTA.V), recorded between 2-intercellular micro-electrodes. Non-linear cable calculations show that .DELTA.V is a more reliable measure of membrane current density than IT as peak inward current increases. Quantitative agreement between .DELTA.V and IT only occurs when both signals report the membrane characteristics that were obtained with an ideal longitudinal space clamp. Agreement between .DELTA.V and IT is a useful criterion for satisfactory experimental measurements which were applied to the slow inward current. This component was elicited by depolarizing steps from a holding potential near -45 mV in the presence of tetrodotoxin. The IT signal was compared directly with .DELTA.V/R, where R is an effective experimentally determined longitudinal resistance. .DELTA.V/R and IT showed very good agreement in both peak amplitude and time course at all potentials studied. Radial non-uniformity during the measured peak slow inward current was estimated by calculations assuming clefts 200 .ANG. wide with a uniform distribution of ionic channels. The calculated voltage span from surface to center was always less than 5 mV, and the measured I-V [current-voltage] characteristics showed little distortion. I-V characteristics and slow response membrane action potentials were compared. The measured peak current showed good agreement with the product (total preparation capacitance) .times. (rate of rise). The experimental and theoretical analysis suggest that the measurements of slow inward current are a good approximation to in vivo membrane properties.