BLOCK OF HUMAN VOLTAGE-SENSITIVE NA-SY5Y CELLS BY LIFARIZINE( CURRENTS IN DIFFERENTIATED SH)

1 The ability of lifarizine (RS-87476) to block human voltage Na+ chan nel currents was studied by use of whole cell patch clamp recording fr om differentiated neuroblastoma cells (SH-SY5Y). 2. The Na+ conductanc e in differentiated SH-SY5Y cells (24.0 +/- 2.4 nS, n = 11) was half-m aximally activated by 10 ms depolarizations to - 37 +/- 2 mV and was h alf-maximally inactivated by predepolarizing pulses of 200 ms duration to - 86 +/- 3 mV (n = 11). 3 At low stimulus frequencies (0.1 to 0.33 Hz) voltage-dependent sodium currents were completely blocked, in a c oncentration-dependent manner, by extracellular application of either tetrodotoxin (EC(50) = 4 +/- 1 nM, n = 12) or by lifarizine (EC(50) = 783 +/- 67 nM, n = 9). The onset of block by lifarizine (tau = 91 +/- 14 s at 10 mu M) was considerably slower than that of tetrodotoxin (ta u = 16 +/- 3 s at 100 nM). 4 Lifarizine (1 mu M) reduced the peak sodi um conductance in each cell (from 26.4 +/- 2.0 nS to 15.1 +/- 2.7 nS, n = 4) without changing the macroscopic kinetics of sodium current act ivation or inactivation (V1/2 = - 35 +/- 1 mV and - 87 +/- 4 mV respec tively, n = 4). Similarly, lifarizine (1 mu M) did not affect the reve rsal potential of the macroscopic sodium current (+ 14 +/- 5 mV in con trol and + 16 +/- 2 mV in 1 mu M lifarizine; n = 4) or reactivation ti me-constant (tau = 14.0 +/- 4.4 ms). 5 Block of the sodium channel ope n state by tetrodotoxin (30 nM) did not prevent the inhibition caused by a subsequent application of lifarizine (3 mu M). In contrast the de pression caused by lifarizine was readily reversible after pretreatmen t of cells with the local anaesthetic, lignocaine (10 mM). 6 These dat a demonstrate that lifarizine is a use- and voltage-dependent antagoni st of human voltage-sensitive sodium currents. The slow kinetics and p harmacology of the block by lifarizine indicate that access of this dr ug to the channel is more restricted than that of tetrodotoxin and may involve an allosteric site or state of the channel that is also regul ated by local anaesthetics.