[Effect of ethmozin on the electrophysiologic parameters of Purkinje cells in the sheep heart. The frequency-dependent blockade of sodium channels].

: The effect of ethmozin (E) on action potential (AP) duration (ADP90, APD50), AP upstroke velocity (Vmax) and cable properties (Ri, Rm, Rin, Cm, lambda m, tau m) was studied. Vmax was significantly reduced by E (0.2 to 5 mkM). Duration of AP plateau (APD50) was more sensitive to E than APD90. Thus, application of 0.5 mkM E resulted in 30% decrease of APD90 and 60% of APD50. This suggested that E not only inhibits INa, what is reflected by Vmax reduction, but also can affect other currents involved in plateau phase. Cable properties remained unchanged in the wide concentration range of E. Blockade of Vmax by E was use-dependent. Uptake rates of ethmozin by sodium channel for different stimulation frequencies were estimated and kinetic binding and unbinding constants (k = 40322 M-1 ms-1; 1 = 7.17 X 10(-5) ms-1) with unguarded receptor were calculated using novel drug-channel interaction model (Starmer and Grant, 1985).