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 beta-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-3Ca2+ 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 microM of the blocker of the taurine-Na+ cotransporter, beta-alanine, prevented the amino acid-induced increase of total [Ca]i. On the other hand, application of beta-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 beta-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.