Cardiac Na⁺–Ca²⁺ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Abstract

1 Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca 2 þ currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K þ -free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs þ dialysate). However, the inhibitors (100 mM) also induced membrane currents that reversed between À40 and 0 mV, and the objective of the present study was to characterize these currents.2 Genistein-induced current behaved like Cl À current, and was unaffected by either the addition of divalent cations (0.5 mM Cd 2 þ ; 3 mM Ni 2 þ ) that block the Na þ -Ca 2 þ exchanger (NCX), or the removal of external Na þ and Ca 2 þ . 3 A23-induced current was independent of Cl À driving force, and strongly suppressed by addition of Cd 2 þ and Ni 2 þ , and by removal of either external Na þ or Ca 2 þ . These and other results suggested that A23 activated an NCX current driven by submembrane Na þ and Ca 2 þ concentrations higher than those in the bulk cytoplasm. 4 Improved control of intracellular Na þ and Ca 2 þ concentrations was obtained by suppressing cation influx (10 mM verapamil) and raising dialysate Na þ to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC 50 6874 mM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 mM A23. 5 The results suggest that activation of cardiac NCX by A23 is due to inhibition of genisteininsensitive TK.

Regulation of the Na+/Ca2+ Exchanger by Pyridine Nucleotide Redox Potential in Ventricular Myocytes

“…The failure of exogenous H 2 O 2 to mimic NCX inhibition by NADH implies that NADH-induced H 2 O 2 generation by the DPI-sensitive enzyme, and the subsequent NCX inhibition, are highly site-specific and finely regulated processes. H 2 O 2 is known to be a signaling molecule that regulates a large variety of protein kinases and phosphatases (46), including PKA and PKC, which have been shown to activate NCX (47,48), and tyrosine kinases, which have been reported to inhibit NCX current (49). Bulliard et al (50) reported that, in brain synaptosomes, millimolar concentrations of either the reduced or oxidized forms of NAD(P)H inhibited NCX and observed alterations in protein phosphorylation.…”

Phosphorylation and Other Conundrums of Na/Ca Exchanger, NCX1

“…The guinea pig ventricular myocytes showed indirect evidence of the tyrosine kinase activity on the NCX1. The tyrosine kinase inhibitor, tyrphostin A23, induced a current in guinea pig ventricular myocytes, which is sensitive to the application of Ni 2+ and removal of Na + or Ca 2+ in the external solution 39 . Recently, NCX1 appears to regulate the Ca 2+ store in microglia and the NCX1 activity is influenced by tyrosine kinase 40 .…”

“…The tyrosine kinase inhibitor, tyrphostin A23, induced a current in guinea pig ventricular myocytes, which is sensitive to the application of Ni 2+ and removal of Na + or Ca 2+ in the external solution. 39 Recently, NCX1 appears to regulate the Ca 2+ store in microglia and the NCX1 activity is influenced by tyrosine kinase. 40 In conclusion, PKA, PKC, CamKII, and tyrosine kinases are shown to regulate NCX1 but more experiments are needed to explain the conditions under which these kinases regulate the NCX1.…”

Regulation of cardiac Na–Ca exchange activity by selective tyrosine kinase inhibition

“…At first glance, this observation would seem to be at odds with a new study in the current issue of the British Journal of Pharmacology. Missan & McDonald (2004) report that the tyrosine kinase inhibitors tyrphostin A23 and tyrphostin A25 stimulate membrane current generated by NCX in cardiac myocytes, suggesting that basal tyrosine phosphorylation actually inhibits the exchanger. Missan and McDonald also find that the effect of these tyrphostins does not appear to be mimicked by genistein.…”