Circulation, Vol 88, 1634-1646, Copyright © 1993 by American Heart Association
Y Liu, W Zeng, M Delmar and J Jalife
BACKGROUND. The term "concealed conduction" is used in electrocardiography
when a proximal (atrial or ventricular) impulse penetrates the
atrioventricular (AV) node but fails to traverse it completely. Its
penetration into the node is inferred by its after effects on the
propagation of succeeding impulses. Concealed AV nodal conduction is a
well-established phenomenon, but its precise cellular and subcellular
mechanisms are unknown. It has been suggested that concealed conduction
results from a transient impairment of excitability caused by the
subthreshold depolarization (ie, electrotonic inhibition) that is elicited
downstream of the site of block. METHODS AND RESULTS. We studied the ionic
mechanism of electrotonic inhibition and concealed conduction in single
myocytes isolated from the rabbit AV node. Cells were paced using
just-threshold current pulses delivered at a constant (basic) cycle length
of 1 second. Appropriately timed interpolation of a conditioning pulse of
depolarizing but subthreshold current led to failure of the subsequent,
previously successful activation. The ability of the subthreshold response
to inhibit subsequent excitation was increased when the interval between
the conditioning and succeeding pulses was shortened, when the amplitude of
the conditioning pulse was increased, or when the inward sodium current was
blocked by superfusion with tetrodotoxin (30 microM). Voltage clamp
analysis demonstrated that electrotonic inhibition results from partial
inactivation of the transient calcium current (ICa.T). Similar results were
obtained using a mathematical model (Hodgkin-Huxley type) of the AV nodal
myocyte. Additional simulations in a linear array of AV nodal cells showed
that when a premature impulse fails to traverse the AV node, the
subthreshold depolarization elicited downstream of the site of block may
lead to a transient reduction of excitability with consequent delay or
block of the succeeding impulse. CONCLUSIONS. The overall data strongly
suggest that some of the electrocardiographic manifestations of concealed
AV conduction are the result of electrotonic inhibition of excitability
secondary to a transient decrease in ICa.T.
ARTICLES
Ionic mechanisms of electronic inhibition and concealed conduction in rabbit atrioventricular nodal myocytes
Department of Pharmacology, SUNY Health Science Center at Syracuse 13210.
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