THYROID-HORMONE STIMULATES NA-CA2+ EXCHANGER EXPRESSION IN RAT CARDIAC MYOCYTES()

We investigated whether thyroid hormone directly affects Na+-Ca2+ exch anger expression in cardiac myocytes. Cultured neonatal rat cardiocyte s were prepared from 1-day-old Sprague-Dawley rats. Intracellular Naconcentration ([Na+](i)) in cardiocytes was measured by using the Na+- sensitive dye sodium-binding benzofran isophthalate (SBFI). Na+-Ca2+ e xchanger messenger RNA (mRNA) and protein expression were assayed by N orthern and Western blotting, respectively Triiodothyronine (T-3; 10(- 8) M) showed no effect on [Na+](i) in cardiocytes, whereas ouabain (10 0 mu M) caused a significant increase in [Na+](i) from 11.3+/-5.0 to 2 1.8+/-5.0 mM. Exposure of cardiocytes to ouabain caused a rapid increa se in Na+-Ca2+ exchanger mRNA accumulation, with a maximal twofold ele vation at 12 h. The ouabain-induced Na+-Ca2+ exchanger mRNA accumulati on was still observed in the Ca2+-free culture medium. On the other ha nd, exposure of cardiocytes to T-3 induced a gradual increase in Na+ e xchanger mRNA accumulation, with a maximal threefold increase at 24 h. Even in Na+-free medium, Tg still induced a twofold increase in Na+-C a2+ exchanger mRNA accumulation in cardiocytes. Exposure of cardiocyte s to T-3 for 24-48 h also caused a marked increase in Na+-Ca2+ exchang er protein accumulation. In conclusion, thyroid hormone directly incre ases cardiac Na+-Ca2+ exchanger expression, independent of alterations in Na+ mobilization. These findings suggest also that thyroid hormone and Na+ regulate Na+-Ca2+ exchanger gene expression through distinct molecular regulatory pathways.