Inhibition of L-type Calcium Current in Rat Ventricular Cells by the Tyrosine Kinase Inhibitor, Genistein and its Inactive Analog, Daidzein

Effects of genistein, a specific inhibitor of tyrosine kinase, on the -type Ca2+channels were examined in freshly isolated young (days 10–18) rat ventricular cells using the whole-cell patch-clamp technique. Bath application of genistein decreased the -type Ca2+current [ICa(L)] in a concentration-dependent manner. The maximal inhibition of ICa(L)was about 40% (attained at concentrations above 100μ ); the concentration for half-inhibition (IC50) was 11μ . The effect of genistein (applied for about 5 min) was poorly reversible after washout for up to 5 min. The potential for half-inhibition (Vh) of the steady-state inactivation curve was shifted in the negative direction by 7 mV (at 100μ ) by genistein, and the slope factor was also slightly changed; the activation curve was not affected. Daidzein, which is structurally related to genistein, but has little inhibitory effect on tyrosine kinase activity (of the EGF receptor), unexpectedly had almost the same inhibitory effect on ICa(L). These observations suggest two possibilities for modulation of ICa(L)in rat ventricular cells by genistein: (a) phosphorylation of the slow Ca2+channels by tyrosine kinase; and (b) direct inhibition of the slow Ca2+channels (i.e. independent of inhibition of tyrosine kinase activity).