DIRECT BLOCK OF VOLTAGE-SENSITIVE SODIUM-CHANNELS BY GENISTEIN, A TYROSINE KINASE INHIBITOR

Genistein, an isoflavone inhibitor of tyrosine-specific protein kinase s, was shown to specifically block the Na-22(+) influx through voltage -sensitive Na+ channels in cultured rat brain neurons, whereas other t yrosine kinase antagonists such as lavendustin A, compound 5, tyrphost in A(47) and an erbstatin analog were inactive at concentrations known to block kinase activity in other neuronal systems. Dose-response cur ves for genistein indicated a half-maximum effect at 60 mu M. Daidzein , an inactive analog of genistein, had a similar inhibitory effect on the Na-22(+) influx with a half-maximum effect at 195 mu M. The time c ourse of genistein action was rapid, because maximum effect on Na-22() influx was obtained in less than 20 s at 100 mu M. Analysis of Na+ c urrents by the whole-cell recording technique showed that 20 mu M geni stein reduced the sodium current and shifted the voltage dependence of both activation and inactivation curves. No competition with [H-3]sax itoxin binding was observed, whereas the binding of [H-3]batrachotoxin in A 20-alpha-benzoate to rat brain synaptosomal membranes was partial ly inhibited, which suggested a direct or allosteric interaction with neurotoxin binding site 2. These data taken together clearly indicate that the inhibition of voltage-sensitive sodium channels by genistein is not mediated by tyrosine kinase inhibition.