Beta-adrenergic augmentation of flecainide-induced conduction slowing in canine Purkinje fibers.
BACKGROUND: This study was undertaken to test the hypothesis that beta-adrenergic stimulation in the setting of membrane depolarization will potentiate flecainide-induced conduction slowing. METHODS AND RESULTS: To elucidate the potential mechanism for the flecainide proarrhythmia observed in CAST, the voltage dependence of beta-adrenergic modulation of impulse propagation in eight flecainide-superfused canine Purkinje fibers was examined with a dual-microelectrode technique. At physiological membrane potentials (Vm) ([K+]o=5.4 micromol), 1 micromol flecainide decreased Vmax from 698+/-55 to 610+/-72 V/s (P=.003) and squared conduction velocity (theta2) from 2.11+/-1.1 to 1.72+/-0.9 (m/s)2 (P=.001). With K+ depolarization to Vm=-70 mV, flecainide further reduced Vmax from 306+/-101 to 245+/-65 V/s and theta2 from 1.12+/-0.4 to 0.99+/-0.6 (m/s)2, producing a 2.0-mV hyperpolarizing shift of apparent Na+ channel availability curves derived from theta2. The addition of 1 micromol isoproterenol to flecainide-superfused fibers at physiological Vm increased theta2 by 8% to 1.84+/-0.6 (m/s)2 (P<.01) without altering Vmax. At -70 mV, the addition of isoproterenol magnified the flecainide-induced reduction of Vmax an additional 24% to 185+/-52 V/s (P<.01) and theta2 by 17% to 0.82+/-0.5 (m/s)2 (P=.04), producing an additional 1.8-mV (P=.002) and 1.9-mV (P=.002) hyperpolarizing shift in the apparent Na+ channel inactivation curves generated from Vmax and theta2, respectively. At physiological Vm, the action potential duration (APD95) was reduced from 307+/-35 to 269+/-27 ms (P<.001) by flecainide and subsequently to 217+/-4 ms (P<.001) with isoproterenol addition. With 12 mmol/L K+, APD95 decreased from 198+/-23 to 182+/-17 ms (P=.005) with flecainide and to 164+/-10 ms (P=.004) with isoproterenol. CONCLUSIONS: At depolarized Vm, isoproterenol amplified the flecainide-induced reduction of Vmax and theta2, suggesting a further adrenergic-mediated reduction of Na+ current. Consequently, the synergy between catecholamines and flecainide at depolarized Vm and the shortened APD95 could facilitate arrhythmogenesis in the presence of underlying ischemia.[1]References
- Beta-adrenergic augmentation of flecainide-induced conduction slowing in canine Purkinje fibers. Cragun, K.T., Johnson, S.B., Packer, D.L. Circulation (1997) [Pubmed]
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