Effect of tetrodotoxin on the transmembrane potential of the pacemaker cell in the sinus venosus of toad

Abstract

Transmembrane potentials of single pacemaker cells in the sinus venosus of toad have been recorded with floating microelectrode, and it was found that the action potential amplitudes were not identical in different pacemaker cells. In the recorded potential range of maximum diastolic potential (MDP) 40 – 75 mv, there was a linear correlation between MDP and maximum upstroke velocity (Vmax) (r = 0.80, n = 16), and cells with different MDP reacted differently to tetrodotoxin (TTX). When MDP was equal to or less than 56 mv, the transmembrane potential did not show any significant change after TTX; when the MDP was higher than 56 mv, the transmembrane potential exhibited significant changes, mainly in reduction of APA and Vmax after TTX. The experimental results indicate that, when MDP is below 56 mv, the fast sodium channels are totally inactivated, whereas with MDP higher than 56 mv some of the fast sodium channels are available and contribute to the rising phase of the action potential.