Time delays in propagation of cardiac action potential

Recent work has suggested that discrete time delays could occur between cardiac cells under physiological conditions. To investigate the existence of such time delays, monolayers of 10-day chick-embryo ventricular myocytes were grown in cell culture on arrays of extracellular microelectrodes. The most closely spaced recording electrodes were on 20-microns centers. Some pairs of electrodes recorded from different points on the same cell, whereas other pairs straddled intercellular junctions. The preparation was electrically paced to obtain a repeatable propagation pathway. Arrival times of activation at each microelectrode were measured. After control recordings, the preparation was exposed to medium containing 20 mM NH4Cl for 15 min and then returned to normal medium. This intervention produces transient intracellular acidification that decreases intercellular coupling. After acidification, the average (multicellular) conduction velocity decreased to about two-thirds of the control value. Propagation velocity measured between most electrode pairs decreased proportionately. However, disproportionately long propagation delays of up to 410 microseconds were observed between some pairs of electrodes in various experiments. The delays recovered as pH returned to normal. The localized long delays were thought to be due to decreased intercellular coupling at gap junctions.