Fast pacing facilitates discontinuous action potential propagation betweenrabbit atrial cells

We examined the critical coupling conductance (GC) for propagation at diffe rent pacing cycle lengths (CLs) (1,000 and 400 ms). As G(C) was progressive ly reduced, propagation failed at a CL of 1,000 ms, whereas propagation suc ceeded at a CL of 400 ms over a range of G(C) values before failing at a CL of 400 ms at a lower G(C), showing facilitation of propagation at the shor ter CL. Critical G(C) was (means +/- SE) 0.8 +/- 0.1 nS for a CL of 400 ms and 1.3 +/- 0.1 nS for a CL of 1,000 ms (a 63% increase, P < 0.002, n = 9 c ell pairs). In 14 uncoupled cells, action potential duration at 30% repolar ization (APD(30)) increased from 19.9 <plus/minus> 2.5 to 41.8 +/- 2.6 ms ( P < 0.001) as CL decreased from 1,000 to 400 ms. In five cell pairs, critic al G(C) with 4-aminopyridine (4-AP) was reduced to 0.4 <plus/minus> 0.1 nS at a CL of 1,000 ms (P < 0.05 compared with control solution), and critical G(C) in 4-AP was unchanged by decreasing CL to 400 ms. It is possible that the "remodeling" of atrial cells due to atrial fibrillation or tachycardia , which has been shown to produce a decrease in the transient outward curre nt, may result in an enhanced ability to propagate, possibly facilitating f urther development of fibrillation under conditions of decreased cellular c oupling.