Characterization of a Hyperpolarization-activated Inward Current in Cultured Pacemaker Cells from the Sinoatrial Node

The perforated-patch, whole-cell voltage-clamp technique was used to characterize a hyperpolarization-activated inward current (if) in cultured pacemaker cells isolated from the rabbit sinoatrial (S-A) node. A modified Tyrode solution was used to block interfering currents. After correcting for uncompensated series resistance, leakage current and liquid–junction potentials between the pipette and bath, we obtained the following values for the maximum conductance and reversal potential of (if): 16.2±2.8 nS or 0.26±0.07 nS/pF (0.37±0.10 mS/cm2); and −27.7±1.2 mV (n=10). In the modified Tyrode solution, the threshold for activation of ifand the half-activation potential and slope factor for Boltzmann fits were −66±1 mV (n=53), −77.2±2.0 mV and 4.2±0.4 mV (n=16), respectively. The time course of ifwas best fit by a sum of two exponentials. Between −110 and −5 mV, time constants ranged from 0.06 to 0.42 s for the rapid component and from 0.65 to 4.39 s for the slow component. Using these characteristics of ifand a short segment of spontaneous firing as the membrane potential, we computed the expected time course of ifduring normal pacemaker activity. It reached a maximum of −3 pA during the diastolic depolarization and +4 pA during the action potential. We conclude that most of the characteristics of ifin cultured S-A node cells are similar to those measured previously in freshly isolated pacemaker cells.