Abstract 1511: Molecular Mechanisms of Tachycardia-dependent Atrial Remodeling Probed in a Novel in Vitro Model
Atrial tachycardia causes atrial electrical remodeling that increases vulnerability to AF. Our understanding of underlying molecular mechanisms is presently limited by the interventions that can practically be applied in vivo.
Objective and Methods: To probe the mechanisms of atrial tachycardia remodeling in an in vitro model, we paced isolated canine left atrial cardiomyocytes for 24 hrs at different frequencies (0, 1 or 3 Hz) in primary culture. Action potentials and ionic currents were then recorded with whole cell patch clamp.
Results : In vitro tachypacing accelerated repolarization (APD to 90% repolarization, APD90, recorded at 1 Hz: 1 Hz cells 173±57ms; 3 Hz cells 127±21 ms, P<0.05 vs 1 Hz cells). L-type Ca2+-current (ICaL ) and transient-outward current (Ito) were reduced by tachystimulation (eg ICaL at +10 mV: 3 Hz cells −1.9±0.4 pA/pF, 40% of 1 Hz cell value of −4.8±1.6 pA/pF, P<0.001; Ito at +40 mV: 3 Hz cells 3.5±1.6 pA/pF, 46% of 1 Hz cell value of 7.6±2.6 pA/pF, P<0.001). ICaL and Ito activation and inactivation voltage-dependence and recovery kinetics were unchanged in 3 Hz cells. T-type Ca2+-current, delayed-rectifier (IKr, IKs) and inward-rectifier (IK1) currents were unaffected by 3 Hz pacing. In the presence of reduced Ca2+ load with nimodipine (1 μM) to block ICaL during tachystimulation, remodeling effects on ICaL and Ito were prevented (3 Hz cell values 89±33%, 105±11% of 1 Hz cells respectively, P=NS for both). Similarly, 3 Hz effects on ICaL were prevented by the CaMKII blocker KN93 (3 Hz values 102.2±40% of 1 Hz cells) and the calcineurin inhibitor FK506 (3 Hz values 112.7±28.6% of 1 Hz cells), and attenuated by the protein phosphatase inhibitor okadaic acid (3 Hz value 70.5±18.4% of 1 Hz cells; all P=NS), but not the inactive KN93 analogue, KN92. Results for 0 versus 1 Hz cells were not significantly different for any variable.
Conclusions: In vitro atrial-cardiomyocyte tachystimulation for 24 hrs reproduces the characteristic remodeling effects of in-vivo atrial tachycardia (decreased APD, 2+-dependent, CAMK-II, calcineurin, and protein-phosphatase related pathways. ICa-L, Ito) via Ca This system provides a potentially valuable tool with which to obtain new insights into molecular mechanisms underlying AF-related electrical remodeling.