Role of IK and If in the pacemaker mechanisms of sino-atrial node myocytes

The role of I(K) (delayed rectifier current) and I(f) (hyperpolarization-activated current) in dominant and subsidiary pacemaker ranges was studied in single myocytes isolated from the guinea pig sino-atrial node by means of a perforated patch-clamp technique. In the dominant pacemaker range (approx. -55 to -40 mV), I(K) tails are present whereas I(f) is not activated. In the subsidiary pacemaker range (approx. -80 to -70 mV), I(f) is large whereas I(K) is minimal and reversing. The threshold for I(f) activation is more negative at short time intervals. Larger or longer depolarizations to -40 mV and +20 mV deactivate I(f) more and are followed by faster reactivation of I(f). Steps of 200-300 ms duration to +20 mV completely deactivate I(f). The slope conductance decreases during depolarizations at -40 and +20 mV and quickly re-increases after the steps. The I(f) deactivation range is between -70 and +10 mV, with a V(1/2) of -35 mV. Depolarizations from -80 to +20 mV at a rate of 120/min limit the subsequent I(f) reactivation owing to the short diastole. We conclude that I(K) plays a predominant role in the dominant pacemaker range and I(f) does so in the subsidiary pacemaker range. Either pacemaker mechanism is used by sino-atrial node cells depending on the diastolic potential range. A previous depolarization markedly increases the amplitude and rate of I(f) reactivation.