Characteristics of single cells isolated from the atrioventricular node of the adult guinea-pig heart

Abstract

To date, data regarding the cellular electrophysiology of the atrioventricular node (AVN) have derived from AVN cells isolated from the rabbit heart. The aim of this study was to characterise for the first time the electrophysiological properties of single cells isolated from the AVN of the guinea-pig heart. Cells were isolated from the AVN region by a combination of enzymatic and mechanical dispersion. Mean (+/-SEM) cell dimensions were 110.8+/-2.3 microm in length by 11.4+/-1.3 micro m in width (n=76 cells). Electrophysiological recordings were made using the whole-cell patch-clamp technique at 37 degrees C. Mean cell capacitance was 25.1+/-0.9 pF (n=43) and mean cell input resistance was 1,377+/-178 Muomega (n=21). Spontaneously active cells exhibited action potentials characteristic of cardiac pacemaker tissue. Under whole-cell voltage clamp, the mean 'zero current' potential was -39.7+/-4.1 mV (n=21). Voltage commands of 500 ms duration to a range of test potentials from a holding potential of -40 mV revealed a number of distinct current components. At potentials positive to -40 mV an early inward current was observed that exhibited a bell-shaped current voltage (I-V) relation, typical of L-type calcium current. A delayed outward current that increased progressively with test potential magnitude was also observed. Analysis of the outward current 'tails' on repolarisation to -40 mV showed this to be comprised of two components with half-maximal activation voltages of -17.2+/-1.8 mV, and +27.1+/-3.6 mV (n=13). 'Transient outward' current appeared absent from guinea-pig AVN cells. Hyperpolarising test pulses activated net inward current, for which three components could be observed: a barium-sensitive (100 microM Ba(2+)) inwardly rectifying current evident at the start of the voltage command and prominent at potentials negative to the diastolic potential range; a time-dependent, hyperpolarisation-activated current, and a residual background current. On repolarisation to -40 mV, a large inward current spike typical of cardiac Na current was observed in some cells. Notably, the following electrophysiological characteristics of guinea-pig AVN cells are distinct from those characteristics previously reported for the rabbit AVN: (1) an absence of transient outward current, (2) the presence of two components of delayed outward current, and (3) the presence of a Ba(2+)-sensitive inwardly rectifying current at negative voltages. The guinea-pig AVN isolated cell preparation may be valuable in providing additional insights into the cellular electrophysiology of this important region of the heart.