Interaction of taurine with the fast Na-current in isolated rabbit myocytes

We studied in whole cell configuration with the patch clamp method the effect of taurine on the macroscopic Na current in adult ventricular rabbit myocytes. Because these cells have a large surface [13,750 +/- 704 microns2 (19), mean +/- S.E.M. (n)], we reduced [Na]o to 45 mM and worked at room temperature to obtain acceptable voltage control. When the cells were held at -80 mV, taurine (20 mM) had the following effects: 1) The current voltage relationships crossed over so that taurine increased INa at potentials negative to -45 mV, and at more positive potentials it depressed the current; 2) taurine reduced the maximal Na conductance from 536.3 +/- 72.2 to 253.6 +/- 33.6 microS.cm-2; 3) the crossing over of the I/V curves was mainly caused by a hyperpolarizing shift of V1/2 of the steady-state activation by 6.3 mV; 4) the crossing over was independent of a -4.6 mV shift of V1/2 of the steady-state inactivation and 5) taurine increased significantly the time constant of reactivation between -90 and -70 mV, but we did not find evidence that taurine changed the time constant of inactivation between -40 and +20 mV. We conclude that positive to -45 mV taurine causes a block of INa channels that resembles that of local anesthetic antiarrhythmic drugs. Negative to -45 mV taurine counteracts the local anesthetic effect causing increased excitability and improved conduction in the range of the threshold potential and -45 mV.