Decremental conduction in the posterior and anterior AV nodal inputs.
UNLABELLED: The role for fiber orientation as a determinant of conduction and block in the posterior (slow pathway, SP) and anterior (fast pathway, FP) AV nodal inputs was examined using multiple extracellular bipolar and intracellular microelectrode recordings in the superfused canine AV junction (N = 14). RESULTS: In both inputs, antegrade longitudinal conduction velocity decremented in association with decreased action potential amplitude and dV/dt(max). A similar decrement was also present in the SP transverse to fiber orientation. SP conduction block occurred preferentially near its insertion into the compact AV node with very slow conduction (0.05 +/- 0.01 M/sec) preceding conduction block. Distal antegrade FP conduction block occurred before conduction block occurred at more proximal FP sites. Conduction in the distal FP was maintained at a higher velocity (0.11 +/- 0.01 M/sec, p < 0.05 vs. SP) before 2:1 conduction block was observed. Conduction velocity, action potential amplitude, and dV/dt(max) were not different at any SP or FP site for paired activation transverse and longitudinal to fiber orientation. CONCLUSIONS: The data do not demonstrate a role for fiber orientation determining decremental conduction and block in transitional cell AV nodal inputs. Decremental conduction in both the SP and FP inputs is consistent with a proximal-to-distal gradient in resting membrane potential, action potential amplitude, dV/dt(max), and intracellular excitability in transitional cells during antegrade activation.