Ionic currents during sustained pacemaker activity in rabbit sino‐atrial myocytes

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Abstract
The contribution of various ionic currents to diastolic depolarization (DD) in rabbit sino‐atrial myocytes was evaluated by the action potential clamp technique. Individual currents were identified, during sustained pacemaking activity reproduced under voltage clamp conditions, according to their sensitivity to supecific channel blockers. The current sensitive to dihydropyridines (DHPs), blockers of L‐type Ca2+ current (ICa,L), was small and outward during most of DD. Diastolic DHP‐sensitive current was affected by changes in the driving force for K+, but it was insensitive to E‐4031, which blocks the current termed IK,r; it was abolished by cell dialysis with a Ca2+ chelator. The current sensitive to 2 mm Cs+ (ICs), a blocker of hyperpolarization‐activated current (If), was inward during the whole DD and it was substantially larger than the net inward current flowing during this phase. However, diastolic IK,r, identified in the same cells as the current sensitive to the blocker E‐4031, exceeded ICs 2‐fold. These findings suggest that: (a) Ca2+ influx during the pacemaker cycle increases a K+ conductance, thus inverting the direction of the net current generated by L‐type Ca2+ channel activity during DD; (b) the magnitude of If would be adequate to account fully for DD; however, the coexistence of a larger IK,r suggests that other channels besides If contribute inward current during this phase.