Inhibition by genistein of the hyperpolarization‐activated cation current in porcine sino‐atrial node cells

1 The hyperpolarization-activated cation current (If) was recorded in single pacemaker cells of the porcine sino-atrial node, and the effects of genistein, an isoflavone inhibitor of tyrosine-specific protein kinases was investigated by the whole-cell patch clamp technique. 2 Genistein (20-500 microM) decreased If in a dose-dependent manner with an IC50 value of 62.3 microM and a maximum inhibition of 45.3%. 3 The effect on If appeared without altering the half-activation potential (control, -88.3+/-2.8 mV; genistein, -87.0+/-1.8 mV) and the slope factor (control, 8.0+/-0.3 mV; genistein, 8.6+/-0.7 mV) of the steady-state activation curve. No significant voltage-dependency was detected in the fully-activated current-voltage relation measured by the double-pulse protocols. 4 The inactive form of genistein analogue, daidzein (500 microM) or genistin (200 microM), were without effect. If was not affected by another tyrosine kinase inhibitor, tryphostin-47 (100 microM), but tyrphostin-25 (100-200 microM) suppressed If in an irreversible manner. 5 Neither bath nor intracellular application of the tyrosine phosphatase inhibitor, orthovanadate, affected If, and subsequent application of genistein inhibited If significantly. 6 These data indicate that the inhibition of If by genistein is not mediated through tyrosine kinase inhibition but through nonselective block of the If channels.