Na+/Ca2+ exchange current: lack of effect of taurine

The intracellular taurine content is very high in some types of cells, including myocardial cells, and taurine becomes depleted in failing hearts. The effect of taurine on inward (forward) and outward (reversal) Na+/Ca2+ exchange current (INa/Ca) was examined in freshly isolated rat (11-20-day-old) ventricular cells using whole-cell voltage clamp. INa/Ca was evoked by a 300 ms ramp hyperpolarizing pulse at a rate of 0.67 V/s (from +60 to -140 mV) from a holding potential of -40 mV. Under the condition of 500 nM Ca2+ in the pipette, when the bath solution was changed from 150 mM Li+ to 150 mM Na+, an inward INa/Ca was initiated. On the other hand, under the condition of 50 mM Na+ and 56 nM Ca2+ in the pipette, when the bath solution was changed from Na(+)-free, Ca(2+)-free solution to Na(+)-free, 2 mM Ca2+, an outward INa/Ca was initiated. Both inward and outward INa/Ca were completely blocked by 5 mM Ni2+, helping to identify the current as INa/Ca. Forward and reversal INa/Ca was measured in the absence (control) and presence of 30 mM taurine added to the bath. The current densities (pA/pF) of inward INa/Ca at -50 mV were 0.75 +/- 0.12 in control and 0.73 +/- 0.11 in presence of 30 mM taurine. Outward INa/Ca at +50 mV were 0.73 +/- 0.08 in control and 0.65 +/- 0.02 in 30 mM taurine. There were no significant differences between control and taurine for both inward and outward INa/Ca. Therefore, the various effects of taurine reported on the heart are not caused by a direct effect of taurine on Na/Ca exchange.