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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.

1. The effect of the antiarrhythmic drug dronedarone on the Acetylcholine-activated K(+) current (I(K(ACh))) was investigated in single cells isolated from sinoatrial node (SAN) tissue of rabbit hearts. 2. Externally perfused dronedarone (0.001 - 1 microM) caused a potent, voltage independent block of I(K(ACh)). Fitting of the dose response curve of I(K(ACh)) block yielded an IC(50) value of 63 nM, a value over one order of magnitude lower than those reported for dronedarone block of other cardiac currents. 3. I(K(ACh)) block was not due to an inhibitory action of dronedarone on the muscarinic M2 receptor activation, since the drug was effective on I(K(ACh)) constitutively activated by intracellular perfusion with GTP-gammaS. 4. External cell perfusion with dronedarone inhibited the activity of I(K(ACh)) channels recorded from cell-attached patches by reducing the channel open probability (from 0.56 to 0.11) without modification of the single-channel conductance. 5. These data suggest that dronedarone blocks I(K(ACh)) channels either by disrupting the G-protein-mediated activation or by a direct inhibitory interaction with the channel protein.

A series of racemic 3-phenyl-4-(1-adamantyl)-5-X-phenyl- delta 2-1,2,4-oxadiazo lines (PAdOx) were directly resolved by HPLC using a Pirkle-type stationary phase containing N,N'-(3,5-dinitrobenzoyl)-1(R),2(R)-diaminocyclohexane as chiral selector. The more retained enantiomers have S configuration, as demonstrated by X-ray crystallography and circular dichroism measurements. The influence of aromatic ring substituents on enantioselective retention was quantitatively assessed by traditional linear free energy-related (LFER) equations and comparative molecular field analysis (CoMFA). In good agreement with previous findings, the results from this study indicate that the increase in retention (k') is favoured mainly by the phi-basicity and the hydrophilicity of solute, whereas enantioselectivity (alpha) can be satisfactorily modeled by electronic and bulk parameters or CoMFA descriptors. The LFER equations and CoMFA models gave helpful insights into chiral recognition mechanisms.