Direct inhibition of the pacemaker (I-f) current in rabbit sinoatrial node cells by genistein

1 Genistein is a tyrosine kinase inhibitor which interferes with the activity of several ionic channels either by altering modulatory phosphorylating processes or by direct binding. In whole-cell conditions, genistein induces a partial inhibition of the pacemaker (If) current recorded in cardiac sinoatrial and ventricular myocytes. 2 We investigated the mechanism of action of genistein (50 mM) on the If current in whole-cell, cellattached, and inside-out configurations, and the measured fractional inhibitions were similar: 26.6, 27.2, and 33.6%, respectively. 3 When ATP was removed from the whole-cell pipette solution no differences were revealed in the effect of the drug when compared to metabolically active cells. Genistein fully maintained its blocking ability even when herbimycin, a tyrosine kinase inhibitor, was added to the whole-cell ATP-free pipette solution. 4 Genistein-induced block was independent of the gating state of the channel and did not display voltage or current dependence; this independence distinguishes genistein from all other f-channel blockers. 5 When inside-out experiments were performed to test for a direct interaction with the channel, genistein, superfused on the intracellular side of the membrane, decreased the maximal If conductance, and slightly shifted the current–activation curve to the left. Furthermore, the effect of genistein was independent of cAMP modulation. 6 We conclude that, in addition to its tyrosine kinase-inhibitory properties, genistein also blocks If by directly interacting with the channel, and thus cannot be considered a valuable pharmacological tool to investigate phosphorylation-dependent modulatory pathways of the If current and of cardiac rhythm.