Although atrial tachycardia (AT) remodelling promotes agonist-independent, constitutively active, acetylcholine-regulated K⁺-current (I_K,ACh) that increases susceptibility to atrial fibrillation (AF), the underlying changes in I_K,Ach channel function are unknown. This study aimed to establish how AT remodelling affects I_K,ACh single-channel function.

I_K,ACh single-channel activity was studied via cell-attached patch-clamp in isolated left atrial cardiomyocytes of control and AT (7 days, 400 min⁻¹) dogs. Atrial tachycardia prolonged the mean duration of induced AF from 44 ± 22 to 413 ± 167 s, and reduced atrial effective refractory period at a 360 ms cycle length from 126 ± 3 to 74 ± 5 ms (n = 9/group, P < 0.001). In the absence of cholinergic stimulation, single-channel openings with typical I_K,ACh conductance and rectification properties were sparse under control conditions. Atrial tachycardia induced prominent agonist-independent I_K,ACh activity because of increased opening frequency (f_o) and open probability (P_o: approximately seven- and 10-fold, respectively, vs. control), but did not alter open time-constant, single-channel conductance, and membrane density. With maximum I_K,ACh activation (10 µmol/L carbachol), channel P_o was enhanced much more in control cells (∼42-fold) than in AT-remodelled myocytes (approximately five-fold). The selective Kir3 current blocker tertiapin-Q (100 nmol/L) reduced f_o and P_o at −100 mV by 48 and 51%, respectively (P < 0.05 for each), without altering other channel properties, confirming the identity of I_K,ACh. Atrial tachycardia had no significant effect on mRNA or protein expression of either of the subunits (Kir3.1, Kir3.4) underlying I_K,ACh.

Atrial tachycardia increases agonist-independent constitutive I_K,ACh single-channel activity by enhancing spontaneous channel opening, providing a molecular basis for AT effects on macroscopic I_K,ACh observed in previous studies, as well as associated refractoriness abbreviation and tertiapin-suppressible AF promotion. These results suggest an important role for constitutive I_K,Ach channel opening in AT remodelling and support its interest as a potential target for AF therapy.