Verapamil inhibits proliferation of LNCaP human prostate cancer cells influencing K+ channel gating

The mechanisms of verapamil and tetraethylammonium (TEA) inhibition of volt age-gated K+ channels in LNCaP human prostate cancer cells were studied in whole-cell and outside/inside-out patch-clamp configurations. Rapidly activ ating outward K+ currents (I-K) exhibited neither C-type, nor rapid (human ether a go-go-related gene-type) inactivation. With 2 mM [Mg2+](o), I-K act ivation kinetics was independent of holding potential, suggesting the absen ce of ether a go-go-type K+ channels. Extracellular applications of TEA and verapamil (IC50 = 11 muM) rapidly (12 s) inhibited I-K in LNCaP cells. Blo cking was also rapidly reversible. Intracellular TEA (1 mM), verapamil (1 m M), and membrane-impermeable N-methyl-verapamil (25 muM) did not influence whole-cell I-K, although both phenylalkylamines inhibited single-channel cu rrents in inside-out patches. Extracellular application of N-methyl-verapam ii (25 muM) had no in-fluence on I-K. Our results are compatible with the h ypothesis that, in LNCaP cells expressing C-type in activation-deficient vo ltage-activated K+ channels, phenylalkylamines interact with an intracellul ar binding site, and probably an additional hydrophobic binding site that d oes not bind charged phenylalkylamines. The inhibiting effects of verapamil and TEA on I-K were additive, suggesting independent K+-channel blocking m echanisms. Indeed, TEA (1 mM) reduced a single-channel conductance (from 7. 3 +/- 0.5 to 3.2 +/- 0.4 pA at a membrane potential of +50 mV, n = 6), wher eas verapamil (10 muM) reduced an open-channel probability (from 0.45 +/- 0 .1 in control to 0.1 +/- 0.09 in verapamil-treated cells, n = 9). The inhib iting effects of verapamil and TEA on LNCaP cell proliferation were not mul tiplicative, suggesting that both share a common antiproliferative mechanis m initiated through a K+ channel block.