A quantitative and comparative study of the effects of a synthetic ciguatoxin CTX3C on the kinetic properties of voltage-dependent sodium channels

1 Ciguatoxins (CTXs) are known to bind to receptor site 5 of the voltage-dependent Na channel, but the toxin's physiological effects are poorly understood. In this study, we investigated the effects of a ciguatoxin congener (CTX3C) on three different Na-channel isoforms, rNa(v)1.2, rNa(v)1.4, and rNa(v)1.5, which were transiently expressed in HEK293 cells. 2 The toxin ( 1.0 mumol l(-1)) shifted the activation potential ( V-1/2 of activation curve) in the negative direction by 4 - 9 mV and increased the slope factor (k) from 8 mV to between 9 and 12 mV ( indicative of decreased steepness of the activation curve), thereby resulting in a hyperpolarizing shift of the threshold potential by 30 mV for all Na channel isoforms. 3 The toxin ( 1.0 mumol l(-1)) significantly accelerated the time-to-peak current from 0.62 to 0.52 ms in isoform rNa(v)1.2. Higher doses of the toxin ( 3 - 10 mumol l(-1)) additionally decreased time-to-peak current in rNav1.4 and rNav1.5. 4 A toxin effect on decay of I-Na at - 20 mV was either absent or marginal even at relatively high doses of CTX3C. 5 The toxin ( 1 mumol l(-1)) shifted the inactivation potential (V-1/2 of inactivation curve) in the negative direction by 15 - 18 mV in all isoforms. 6 I-Na maxima of the I - V curve (at -20 mV) were suppressed by application of 1.0 mumol l(-1) CTX3C to a similar extent ( 80 - 85% of the control) in all the three isoforms. Higher doses of CTX3C up to 10 mumol l(-1) further suppressed I-Na to 61 - 72% of the control. 7 Recovery from slow inactivation induced by a depolarizing prepulse of intermediate duration ( 500 ms) was dramatically delayed in the presence of 1.0 mumol l(-1) CTX3C, as time constants describing the monoexponential recovery were increased from 38 +/- 8 to 588 +/- 151 ms ( n = 5), 53 +/- 6 to 338 +/- 85 ms ( n = 4), and 23 +/- 3 to 232 +/- 117 ms ( n = 3) in rNa(v)1.2, rNa(v)1.4, and rNa(v)1.5, respectively. 8 CTX3C exerted multimodal effects on sodium channels, with simultaneous stimulatory and inhibitory aspects, probably due to the large molecular size ( 3 nm in length) and lipophilicity of this membrane-spanning toxin.