Taurine depresses INa and depolarises the membrane but does not affect membrane surface charges in perfused rabbit hearts.

Study Objective: The aim was to determine whether taurine influences the membrane surface charges in cardiac muscle.

Design: Screening of the negative charges at the outside surface of the membrane results in a shift of the steady state inactivation of the sodium system towards less negative potentials. This feature was used to study eventual effects of taurine on surface charges and the data were compared to the known influence of varying extracellular calcium.

Experimental Material: New Zealand rabbits (6-7 weeks, 1.25-1.75 kg) were anesthetised and the hearts were rapidly excised and perfused with the Langendorff technique.

Measurements And Main Results: Standard microelectrodes were used to determine the effects of 20 mM taurine and varying Ca concentrations (from 0.3 to 5.0 mM) on action potential parameters. The resting potential was varied by changing extracellular K between 2.5 and 10 mM. Taurine significantly depolarised the membrane by about 3 mV between 5 and 10 mM Ko but not at 2.5 mM; the maximum rate of depolarisation (dV/dTmax) decreased significantly at all Ko except at 10 mM where taurine caused arrhythmias or cardiac arrest. The dV/dTmax upsilon resting potential relationship (a measure for the steady state sodium current inactivation) was not changed by taurine, but the current was depressed as a function of membrane potential, the depression being more pronounced at more positive membrane potentials. An increase in Cao from 0.3 to 5.0 mM displaced the half maximal value of the dV/dtmax upsilon resting potential relationship from -79 to -67 mV, showing that the screening effect of Ca on the negative charges at the outside surface of the membrane could be detected with this experimental approach.

Conclusions: The decrease of the fast Na current by taurine can explain the arrhythmias observed at 10 mM external potassium, whereas the surface charges of the glycocalix were not affected.