Direct inhibition of the pacemaker (If) current in rabbit sinoatrial node cells by genistein.
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 (I(f)) current recorded in cardiac sinoatrial and ventricular myocytes. We investigated the mechanism of action of genistein (50 microM) on the I(f) current in whole-cell, cell-attached, and inside-out configurations, and the measured fractional inhibitions were similar: 26.6, 27.2, and 33.6%, respectively. 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. 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. 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 I(f) conductance, and slightly shifted the current-activation curve to the left. Furthermore, the effect of genistein was independent of cAMP modulation. We conclude that, in addition to its tyrosine kinase-inhibitory properties, genistein also blocks I(f) by directly interacting with the channel, and thus cannot be considered a valuable pharmacological tool to investigate phosphorylation-dependent modulatory pathways of the I(f) current and of cardiac rhythm.
Altomare
Claudia
C
Department of Biomolecular Sciences and Biotechnology, Laboratory of Molecular Physiology and Neurobiology, University of Milan, via Celoria 26, 20133 Milano, Italy.
Tognati
Agnese
A
Bescond
Jocelyn
J
Ferroni
Arnaldo
A
Baruscotti
Mirko
M
D000818:Animals; D002478:Cells, Cultured; D019833:Genistein / Q000494:pharmacology*; D032383:Myocytes, Cardiac / Q000187:drug effects*; D010138:Pacemaker, Artificial; D011505:Protein-Tyrosine Kinases / Q000037:antagonists & inhibitors*; D011817:Rabbits; D012849:Sinoatrial Node / Q000166:cytology / Q000187:drug effects*
D019833:Genistein; D011505:Protein-Tyrosine Kinases