Tyrosine kinase inhibitor, genistein, inhibits macroscopic L-type calcium current in rat portal vein smooth muscle cells

The effect of genistein, a specific tyrosine kinase inhibitor, was tested on the slow (L-type) Ca2+ current (I-Ca(L)) of vascular smooth muscle cells from freshly isolated rat portal vein, using whole-cell voltage clamp. To isolate I-Ca(L), the pipette contained high Cs+ and the bath contained 140 mM tetraethylammonium (TEA) to block K+ currents. Bath application of genistein decreased I-Ca(L) in a concentration-dependent manner within 3-6 min. The concentration for half-maximal inhibition (IC50) was 54.9 mu M (at a holding potential of -40 mV). At a concentration of 300 mu M, genistein produced nearly complete inhibition of I-Ca(L). The inhibitory effect of genistein was not reversed after washout for up to 5 min. The potential for half-inhibition (V-1/2) of the steady-state inactivation curve for I-Ca(L) was shifted to the left by genistein (10.6 mV at 50 mu M), suggesting that genistein exerts a voltage-dependent block. Superfusion with daidzein, an inactive analog of genistein, had no inhibitory effect on I-Ca(L) at concentrations as high as 300 mu M. These results may suggest that the L-type Ca2+ channels in vascular smooth muscle cells are possibly modulated by endogenous tyrosine kinase activity. That is, tonic phosphorylation by tyrosine kinases maintains the Ca2+ channels in an available state for activation by depolarization. Thus, the vascular tone may be controlled by tyrosine kinase activity.