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.
Shibata
S
S
Department of Pharmacology, Akita University School of Medicine, Akita 010-8543, Japan.
Ono
K
K
Iijima
T
T
D000818:Animals; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D019833:Genistein / Q000494:pharmacology*; D007473:Ion Channels / Q000187:drug effects* / Q000502:physiology; D007529:Isoflavones / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects*; D018408:Patch-Clamp Techniques; D017027:Protein Tyrosine Phosphatases / Q000037:antagonists & inhibitors; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors; D012849:Sinoatrial Node / Q000166:cytology / Q000187:drug effects* / Q000502:physiology; D013552:Swine; D020032:Tyrphostins / Q000494:pharmacology; D014638:Vanadates / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007473:Ion Channels; D007529:Isoflavones; D020032:Tyrphostins; D014638:Vanadates; C004742:daidzein; D019833:Genistein; D011505:Protein-Tyrosine Kinases; D017027:Protein Tyrosine Phosphatases