Taurine indirectly increases [Ca]i by inducing Ca2+ influx through the Na+-Ca2+ exchanger

Recent studies in heart cells have shown taurine to induce a sustained increase of both intracellular Ca²⁺ and Na⁺. These results led us to believe that the increase in Na⁺ by taurine could be due to Na⁺ entry through the taurine-Na⁺ cotransporter which in turn favours transarcolemmal Ca²⁺ influx through Na⁺-Ca²⁺ exchange. Therefore, we investigated the effect of -alanine, a blocker of the taurine-Na⁺ cotransporter and low concentrations of CBDMB (a pyrazine derivative, 5-(N-4chlorobenzyl)-2,4-dimethylbenzamil), a Na⁺-Ca²⁺ exchanger blocker on taurine-induced Ca]_i increase in embryonic chick heart cells. Using Fura-2 Ca²⁺ imaging and Fluo-3 Ca²⁺ confocal microscopy techniques, taurine (20 mM) as expected, induced a sustained increase in Ca]_i at both the cytosolic and the nuclear levels. Preexposure to 500 M of the blocker of the taurine-Na⁺ cotransporter, -alanine, prevented the amino acid-induced increase of total Ca]_i. On the other hand, application of -alanine did not reverse the action of taurine on total Ca]_i. However, low concentrations of the Na⁺-Ca²⁺ exchanger blocker, CBDMB, reversed the taurine-induced sustained increase of cytosolic and nuclear free calcium (in presence or absence of -alanine). Thus, the effect of taurine on Ca]_i in heart cells appears to be due to Na⁺ entry through the taurine-Na⁺ cotransporter which in turn favours transarcolemmal Ca²⁺ influx through the Na⁺-Ca²⁺ exchanger.