Cardiac Na + –Ca 2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca 2+ currents in guinea‐pig ventricular myocytes investigated under standard whole‐cell conditions (K + ‐free Tyrode's superfusate; EGTA‐buffered (pCa–10.5) Cs + dialysate). However, the inhibitors (100 μ M ) also induced membrane currents that reversed between −40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein‐induced current behaved like Cl − current, and was unaffected by either the addition of divalent cations (0.5 m M Cd 2+ ; 3 m M Ni 2+ ) that block the Na + –Ca 2+ exchanger (NCX), or the removal of external Na + and Ca 2+ . A23‐induced current was independent of Cl − driving force, and strongly suppressed by addition of Cd 2+ and Ni 2+ , and by removal of either external Na + or Ca 2+ . These and other results suggested that A23 activated an NCX current driven by submembrane Na + and Ca 2+ concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na + and Ca 2+ concentrations was obtained by suppressing cation influx (10 μ M verapamil) and raising dialysate Na + to 7 m M and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC 50 68±4 μ M and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK‐inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 m M ) antagonized the action of 100 μ M A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein‐insensitive TK.British Journal of Pharmacology (2004) 143 , 943–951. doi: 10.1038/sj.bjp.0706011