GENISTEIN INCREASES THE SENSITIVITY OF CARDIAC ION CHANNELS TO BETA-ADRENERGIC-RECEPTOR STIMULATION

The whole-cell patch-clamp technique was used to monitor the effects o f genistein, a tyrosine kinase inhibitor, on membrane currents recorde d from isolated guinea pig ventricular myocytes, Under control conditi ons, genistein (50 mu mol/L) did not activate the latent cAMP-regulate d Cl- current (I-Cl). However, in the presence of a subthreshold conce ntration (1 nmol/L) of the beta-adrenergic agonist isoproterenol (Iso) , genistein caused a near-maximal activation of this current. In the a bsence of genistein, Iso activated I-Cl with an EC50 of 5 nmol/L. In t he 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 daidze in (50 mu mol/L), an analogue of genistein that only weakly inhibits t yrosine kinase activity. Furthermore, peroxovanadate, a potent inhibit or of phosphotyrosine phosphatase activity, inhibited I-Cl activated b y Iso alone, and it blocked the stimulatory effect of genistein in the presence of Iso. To determine whether the stimulatory effect of genis tein was specific for I-Cl, we also studied its action on the cAMP-reg ulated delayed rectifier K+ current (I-K) and L-type Ca2+ current (ICa -L) present in these cells. Basal I-K and ICa-L were partially (approx imate to 30% to 40%) inhibited by genistein. However, this inhibitory effect was mimicked by daidzein, suggesting that inhibition of tyrosin e kinase activity is not involved. In addition to the nonspecific inhi bitory effect, genistein also caused a significant increase in the bet a-adrenergic sensitivity of the unblocked cationic currents. In the ab sence of genistein, 1 nmol/L Iso had no effect on either I-K or ICa-L. However, in the presence of genistein, 1 nmol/L Iso significantly inc reased the magnitude of both currents. These results suggest that tyro sine kinase activity may play an important role in regulating beta-adr energic responsiveness of the heart.