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 L-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 L-type Ca2+ current [I-Ca( L)] in a concentration-dependent manner. The maximal inhibition of I-C a(L) was about 40% (attained at concentrations above 100 mu M); the co ncentration for half-inhibition (IC50) was 11 mu M. The effect of geni stein (applied for about 5 min) was poorly reversible after washout fo r up to 5 min, The potential for half-inhibition (V-h) of the steady-s tate inactivation curve was shifted in the negative direction by 7 mV (at 100 mu M) by genistein, and the slope factor was also slightly cha nged; the activation curve was not affected. Daidzein, which structura lly related to genistein, but has little inhibitory effect on tyrosine kinase activity (of the EGF receptor), unexpectedly had almost the sa me inhibitory effect on I-Ca(L). These observations suggest two possib ilities for modulation of I-Ca(L) in rat ventricular cells by genistei n: (a) phosphorylation of the slow Ca2+ channels by tyrosine kinase; a nd (b) direct inhibition of the slow Ca2+ channels (i.e. independent o f inhibition of tyrosine kinase activity). (C) 1996 Academic Press Lim ited