Abstract 742: Atrial Tachycardia Remodeling of Muscarinic Acetylcholine Receptor Subtypes and Corresponding K+ Currents in Canine Atrial Myocytes
Background: Atrial tachycardia (AT) remodeling of ion channels and acetylcholine-sensitive K+-currents, both constitutely active IKACh and IKACh activated in response to vagal tone, are known to play an important role in atrial fibrillation (AF) pathophysiology. Recent studies indicate the presence in atrial cardiomyocytes of multiple muscarinic acetylcholine-receptor subtypes (mAChRs) that mediate kinetically and biophysically distinct K+-currents: IKM2 (classical IKACh) by the M2 receptor, IKM3 by the M3 receptor and IK4AP by the M4 receptor. The effect of AT on the expression of mAChR subtypes and their associated currents is unknown.
Method: Whole-cell patch clamp was used to study mAChR-coupled K+-currents in single cardiomyocytes from pulmonary veins (PVs) and left atria (LA) of 15 dogs after 7-day AT-pacing (400 bpm), as well as in 15 nonpaced control dogs. IKM2, IKM3 and IK4AP were induced by the agonists 1 μM acetylcholine, 10 mM choline and 1 mM 4-aminopyridine respectively, under appropriate conditions to isolate each current. Western blot was used to quantify mAChR proteins in LA-tissue.
Results: IKM2, IKM3 and IK4AP were all significantly reduced in AT-paced LA and PV cardiomyocytes. Overall results showed (PV and LA changes were equivalent and are therefore grouped for presentation): IKM2 at −100 mV: −9.6±1.8, mean±SEM, pA/pF, in 9 control cells versus −4.4±1.8 pA/pF* in AT-pacing, n=11 cells, *P<0.001; IKM3 at +60 mV: 10.6±0.8 pA/pF control, n=17 cells, versus 5.9±0.5 pA/pF* AT-pacing, n=21 cells; IK4AP at +60 mV: 4.6±0.3 pA/pF control, n=24 cells versus 2.8±0.3 pA/pF* AT-pacing, n=23 cells. Western blot analysis showed that receptor protein expression corresponding to M2, M3 and M4 subtypes (band intensity of target signals normalized to GAPDH bands) was decreased 50%, 49% and 53% respectively (all P<0.05) by AT-pacing. The agonist concentration-response relationships of both IKM3 and IK4AP were unchanged by AT-pacing.
Conclusions: AT produces congruent downregulation of all three mAChR-coupled K+-current subtypes in LA and PVs, along with correspondingly reduced mAChR expression. These changes in cholinergic currents likely contribute to the pathophysiology of AT-induced electrical remodeling.