Genistein Increases the Sensitivity of Cardiac Ion Channels to ß-Adrenergic Receptor Stimulation

From the Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio.

Correspondence to Robert D. Harvey, PhD, Department of Physiology and Biophysics, Case Western Reserve University, 2109 Adelbert Rd, Cleveland, OH, 44106-4970. E-mail rdh3{at}po.cwru.edu

Abstract—The whole-cell patch-clamp technique was used to monitor the effects of genistein, a tyrosine kinase inhibitor, on membrane currents recorded from isolated guinea pig ventricular myocytes. Under control conditions, genistein (50 µmol/L) did not activate the latent cAMP-regulated Cl^- current (I_Cl). However, in the presence of a subthreshold concentration (1 nmol/L) of the ß-adrenergic agonist isoproterenol (Iso), genistein caused a near-maximal activation of this current. In the absence of genistein, Iso activated I_Cl with an EC₅₀ of 5 nmol/L. In the presence of genistein, Iso activated I_Cl with an EC₅₀ of 0.3 nmol/L. This facilitatory effect was not observed in the presence of daidzein (50 µmol/L), an analogue of genistein that only weakly inhibits tyrosine kinase activity. Furthermore, peroxovanadate, a potent inhibitor of phosphotyrosine phosphatase activity, inhibited I_Cl activated by Iso alone, and it blocked the stimulatory effect of genistein in the presence of Iso. To determine whether the stimulatory effect of genistein was specific for I_Cl, we also studied its action on the cAMP-regulated delayed rectifier K⁺ current (I_K) and L-type Ca²⁺ current (I_Ca-L) present in these cells. Basal I_K and I_Ca-L were partially (30% to 40%) inhibited by genistein. However, this inhibitory effect was mimicked by daidzein, suggesting that inhibition of tyrosine kinase activity is not involved. In addition to the nonspecific inhibitory effect, genistein also caused a significant increase in the ß-adrenergic sensitivity of the unblocked cationic currents. In the absence of genistein, 1 nmol/L Iso had no effect on either I_K or I_Ca-L. However, in the presence of genistein, 1 nmol/L Iso significantly increased the magnitude of both currents. These results suggest that tyrosine kinase activity may play an important role in regulating ß-adrenergic responsiveness of the heart.

Key Words: cardiac cystic fibrosis transmembrane conductance regulator Cl^- current • delayed rectifier K⁺ current • L-type Ca²⁺ current • tyrosine kinase • phosphotyrosine phosphatase

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