Mg(2+) block unmasks Ca(2+)/Ba(2+) selectivity of alpha1G T-type calcium channels.
We have examined permeation by Ca(2+) and Ba(2+), and block by Mg(2+), using whole-cell recordings from alpha1G T-type calcium channels stably expressed in HEK 293 cells. Without Mg(o)(2+), inward currents were comparable with Ca(2+) and Ba(2+). Surprisingly, three other results indicate that alpha1G is actually selective for Ca(2+) over Ba(2+). 1) Mg(2+) block is approximately 7-fold more potent with Ba(2+) than with Ca(2+). With near-physiological (1 mM) Mg(o)(2+), inward currents were approximately 3-fold larger with 2 mM Ca(2+) than with 2 mM Ba(2+). The stronger competition between Ca(2+) and Mg(2+) implies that Ca(2+) binds more tightly than Ba(2+). 2) Outward currents (carried by Na(+)) are blocked more strongly by Ca(2+) than by Ba(2+). 3) The reversal potential is more positive with Ca(2+) than with Ba(2+), thus P(Ca) > P(Ba). We conclude that alpha1G can distinguish Ca(2+) from Ba(2+), despite the similar inward currents in the absence of Mg(o)(2+). Our results can be explained by a 2-site, 3-barrier model if Ca(2+) enters the pore 2-fold more easily than Ba(2+) but exits the pore at a 2-fold lower rate.
Serrano
J R
JR
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, USA.
Dashti
S R
SR
Perez-Reyes
E
E
Jones
S W
SW
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D020747:Calcium Channels, T-Type / Q000737:chemistry / Q000187:drug effects / Q000502:physiology*; D002413:Cations, Divalent / Q000494:pharmacology; D002460:Cell Line; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051381:Rats; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D014162:Transfection
D020747:Calcium Channels, T-Type; D002413:Cations, Divalent; D011994:Recombinant Proteins; D001464:Barium; D008274:Magnesium; D002118:Calcium