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 tes ted on the slow (L-type) Ca2+ current (I-Ca(L)) of vascular smooth mus cle cells from freshly isolated rat portal vein, using whole-cell volt age clamp. To isolate I-Ca(L), the pipette contained high Cs+ and the bath contained 140 mM tetraethylammonium (TEA) to block K+ currents. B ath application of genistein decreased I-Ca(L) in a concentration-depe ndent manner within 3-6 min. The concentration for half-maximal inhibi tion (IC50) was 54.9 mu M (at a holding potential of -40 mV). At a con centration 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 lef t by genistein (10.6 mV at 50 mu M), suggesting that genistein exerts a voltage-dependent block. Superfusion with daidzein, an inactive anal og 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+ c hannels in vascular smooth muscle cells are possibly modulated by endo genous tyrosine kinase activity. That is, tonic phosphorylation by tyr osine kinases maintains the Ca2+ channels in an available state for ac tivation by depolarization. Thus, the vascular tone may be controlled by tyrosine kinase activity.