Nickel block of three cloned T-type calcium channels: low concentrations selectively block alpha1H.
Nickel has been proposed to be a selective blocker of low-voltage-activated, T-type calcium channels. However, studies on cloned high-voltage-activated Ca(2+) channels indicated that some subtypes, such as alpha1E, are also blocked by low micromolar concentrations of NiCl(2). There are considerable differences in the sensitivity to Ni(2+) among native T-type currents, leading to the hypothesis that there may be more than one T-type channel. We confirmed part of this hypothesis by cloning three novel Ca(2+) channels, alpha1G, H, and I, whose currents are nearly identical to the biophysical properties of native T-type channels. In this study we examined the nickel block of these cloned T-type channels expressed in both Xenopus oocytes and HEK-293 cells (10 mM Ba(2+)). Only alpha1H currents were sensitive to low micromolar concentrations (IC(50) = 13 microM). Much higher concentrations were required to half-block alpha1I (216 microM) and alpha1G currents (250 microM). Nickel block varied with the test potential, with less block at potentials above -30 mV. Outward currents through the T channels were blocked even less. We show that depolarizations can unblock the channel and that this can occur in the absence of permeating ions. We conclude that Ni(2+) is only a selective blocker of alpha1H currents and that the concentrations required to block alpha1G and alpha1I will also affect high-voltage-activated calcium currents.
Lee
J H
JH
Department of Physiology, Loyola University Medical Center, Maywood, Illinois 60153 USA.
Gomora
J C
JC
Cribbs
L L
LL
Perez-Reyes
E
E
D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002121:Calcium Channel Blockers / Q000008:administration & dosage / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D003001:Cloning, Molecular; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009532:Nickel / Q000008:administration & dosage / Q000494:pharmacology*; D009865:Oocytes / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D014162:Transfection; D014982:Xenopus laevis
D002121:Calcium Channel Blockers; D020747:Calcium Channels, T-Type; D011994:Recombinant Proteins; D009532:Nickel