Effect of ZnCl2 and chelation of zinc ions by N,N-diethyldithiocarbamate (DEDTC) on the ERG b-wave amplitude from the isolated superfused vertebrate retina.

PURPOSE: NiCl(2) (15 microM) enhances the ERG b-wave amplitude of vertebrate retina, up to 1.5-fold by blocking E/R-type voltage-gated Ca(2+) channels, which is mediated by blocking the release of GABA onto ionotropic GABA-A and GABA-C receptors. In vivo, it is likely that zinc, rather than nickel ions, may be involved in the modulation of retinal signalling. Therefore, we tested the effect of both, ZnCl(2) (10 to 500 microM) and DEDTC (100 to 500 microM), which chelates zinc ions for the capacity to influence the ERG b-wave amplitude. METHODS: Transretinal potentials from the isolated bovine retina were recorded as electroretinograms and Ca(2+) inward currents by patch-clamp recordings of stably Ca(v)2.3 transfected HEK-293 cells, yielding an IC(50) value of 5.3 microM for ZnCl(2). RESULTS: ZnCl(2) (10-15 microM) increased the b-wave amplitude by 1.52-fold +/- 0.12 (n = 6 retinas), which was partially reversible upon washout. The same 1.5-fold stimulation of the b-wave amplitude was reported recently for 15 microM NiCl(2). The superfusion of isolated retinas by DEDTC (100 microM) caused a transient decrease of the ERG b-wave amplitude (0.75-fold +/- 0.06; n = 4), suggesting that the co-secretion of Zn(2+) ions may occur under scotopic conditions. CONCLUSION: The stimulatory effect of ZnCl(2) on the ERG b-wave amplitude resembles the stimulatory effect of NiCl(2) and may be mediated rather by the NiCl(2)-sensitive, Ca(v)2.3 E-/R-type voltage-gated Ca(2+) channels than by NiCl(2)-sensitive T-type channels.

Zonisamide block of cloned human T-type voltage-gated calcium channels.

SUMMARY: Zonisamide (ZNS) is a multi-target antiepileptic drug reported to be efficient in the treatment of both partial and generalized seizures, with T-type Ca(2+) channel blockade being one of its proposed mechanisms of action. In this study, we systematically investigated electrophysiological effects of ZNS on cloned human Ca(v)3.1-3.3 Ca(2+) channels in a heterologous HEK-293 expression system using whole cell patch-clamp technique. Concentration-response studies were performed in the range from 5 microM to 2mM for Ca(v)3.2 Ca(2+) channels exhibiting a 15.4-30.8% reduction of Ca(2+) influx within the maximum therapeutic plasma range (50-200 microM ZNS). The other T-type Ca(2+) channel entities, Ca(v)3.1 and Ca(v)3.3, were even less sensitive to ZNS. Both voltage- and concentration-dependence of inactivation kinetics remained unchanged for Ca(v)3.2 VGCC, whereas Ca(v)3.1 and Ca(v)3.3 exhibited minor, though significant reduction of inactivation-tau. Interestingly, ZNS block of Ca(v)3.2 VGCCs was not use-dependent and remained unaffected by changes in the holding potential. Steady-state inactivation studies did not display a significant shift in steady-state availability of Ca(v)3.2 channels at 100 microM ZNS (DeltaV(1/2)=3.1mV, p=0.071). Our studies indicate that ZNS is a moderate blocker of human Ca(v)3 T-type Ca(2+) channels with little or no effect on Ca(v)3.2 Ca(2+) channel inactivation kinetics, use- and state-dependence of blockade. These results suggest that T-type Ca(2+) channel inhibition only partially contributes to the anti-absence activity of ZNS antiepileptic drug.