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

The hyperpolarization‐activated cation current (I f ) 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. Genistein (20–500 μ M ) decreased I f in a dose‐dependent manner with an IC 50 value of 62.3 μ M and a maximum inhibition of 45.3%. The effect on I f 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. The inactive form of genistein analogue, daidzein (500 μ M ) or genistin (200 μ M ), were without effect. I f was not affected by another tyrosine kinase inhibitor, tryphostin‐47 (100 μ M ), but tyrphostin‐25 (100–200 μ M ) suppressed I f in an irreversible manner. Neither bath nor intracellular application of the tyrosine phosphatase inhibitor, orthovanadate, affected I f , and subsequent application of genistein inhibited I f significantly. These data indicate that the inhibition of I f by genistein is not mediated through tyrosine kinase inhibition but through nonselective block of the I f channels.British Journal of Pharmacology (1999) 128 , 1284–1290; doi: 10.1038/sj.bjp.0702903