Changes in I-K,I-ACh single-channel activity with atrial tachycardia remodelling in canine atrial cardiomyocytes

Aims Although atrial tachycardia (AT) remodelling promotes agonist-independent, constitutively active, acetylcholine-regulated K+-current (I-K,I-ACh) that increases susceptibility to atrial fibrillation (AF), the underlying changes in I-K,I-Ach channel function are unknown. This study aimed to establish how AT remodelling affects I-K,I-ACh single-channel function. Methods and results I-K,I-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,I-ACh conductance and rectification properties were sparse under control conditions. Atrial tachycardia induced prominent agonist-independent I-K,I-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,I-ACh activation (10 mu mol/L carbachol), channel P-o was enhanced much more in control cells (similar to 42-fold) than in AT-remodelled myocytes (approximately five-fold). The selective Kir3 current blocker tertiapin-Q (100 nmol/L) reduced fo 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,I-ACh. Atrial tachycardia had no significant effect on mRNA or protein expression of either of the subunits (Kir3.1, Kir3.4) underlying I-K,I-ACh. Conclusion Atrial tachycardia increases agonist-independent constitutive I-K,I-ACh single-channel activity by enhancing spontaneous channel opening, providing a molecular basis for AT effects on macroscopic I-K,I-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,I-Ach channel opening in AT remodelling and support its interest as a potential target for AF therapy.