Genistein, an isoflavone inhibitor of tyrosine-specific protein kinases, was shown to specifically block the 22Na+ influx through voltage-sensitive Na+ channels in cultured rat brain neurons, whereas other tyrosine kinase antagonists such as lavendustin A, compound 5, tyrphostin A47 and an erbstatin analog were inactive at concentrations known to block kinase activity in other neuronal systems. Dose-response curves for genistein indicated a half-maximum effect at 60 microM. Daidzein, an inactive analog of genistein, had a similar inhibitory effect on the 22Na+ influx with a half-maximum effect at 195 microM. The time course of genistein action was rapid, because maximum effect on 22Na+ influx was obtained in less than 20 s at 100 microM. Analysis of Na+ currents by the whole-cell recording technique showed that 20 microM genistein reduced the sodium current and shifted the voltage dependence of both activation and inactivation curves. No competition with [3H]saxitoxin binding was observed, whereas the binding of [3H]batrachotoxinin A 20-alpha-benzoate to rat brain synaptosomal membranes was partially 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.
