Electrophysiological properties of myocytes isolated from the mouse atrioventricular node: Lâ•’type ICa, IKr, If, and Naâ•’Ca exchange

The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. This study investigated the electrophysiology of cells isolated from the AVN region of adult mouse hearts, and compared murine ionic current magnitude with that of cells from the more extensively studied rabbit AVN. Whole-cell patch-clamp recordings of ionic currents, and perforated-patch recordings of action potentials (APs), were made at 35–37°C. Hyperpolarizing voltage commands from 40 mV elicited a Ba-sensitive inward rectifier current that was small at diastolic potentials. Some cells (Type 1; 33.4 2.2 pF; n = 19) lacked the pacemaker current, If, whilst others (Type 2; 34.2 1.5 pF; n = 21) exhibited a clear If, which was larger than in rabbit AVN cells. On depolarization from 40 mV L-type Ca current, ICa,L, was elicited with a half maximal activation voltage (V0.5) of 7.6 1.2 mV (n = 24). ICa,L density was smaller than in rabbit AVN cells. Rapid delayed rectifier (IKr) tail currents sensitive to E-4031 (5 lmol/L) were observed on repolarization to 40 mV, with an activation V0.5 of 10.7 4.7 mV (n = 8). The IKr magnitude was similar in mouse and rabbit AVN. Under Na-Ca exchange selective conditions, mouse AVN cells exhibited 5 mmol/L Ni-sensitive exchange current that was inwardly directed negative to the holding potential ( 40 mV). Spontaneous APs (5.2 0.5 sec ; n = 6) exhibited an upstroke velocity of 37.7 16.2 V/s and ceased following inhibition of sarcoplasmic reticulum Ca release by 1 lmol/L ryanodine, implicating intracellular Ca cycling in murine AVN cell electrogenesis.