Limits...
Ni2+ block of CaV3.1 (alpha1G) T-type calcium channels.

Obejero-Paz CA, Gray IP, Jones SW - J. Gen. Physiol. (2008)

Bottom Line: Na(+)).We conclude that both fast and slow block of Ca(V)3.1 by Ni(2+) are most consistent with occlusion of the pore.The exit rate of Ni(2+) for slow block is reduced at high Ni(2+) concentrations, suggesting that the site responsible for fast block can "lock in" slow block by Ni(2+), at a site located deeper within the pore.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106, USA.

ABSTRACT
Ni(2+) inhibits current through calcium channels, in part by blocking the pore, but Ni(2+) may also allosterically affect channel activity via sites outside the permeation pathway. As a test for pore blockade, we examined whether the effect of Ni(2+) on Ca(V)3.1 is affected by permeant ions. We find two components to block by Ni(2+), a rapid block with little voltage dependence, and a slow block most visible as accelerated tail currents. Rapid block is weaker for outward vs. inward currents (apparent K(d) = 3 vs. 1 mM Ni(2+), with 2 mM Ca(2+) or Ba(2+)) and is reduced at high permeant ion concentration (110 vs. 2 mM Ca(2+) or Ba(2+)). Slow block depends both on the concentration and on the identity of the permeant ion (Ca(2+) vs. Ba(2+) vs. Na(+)). Slow block is 2-3x faster in Ba(2+) than in Ca(2+) (2 or 110 mM), and is approximately 10x faster with 2 vs. 110 mM Ca(2+) or Ba(2+). Slow block is orders of magnitude slower than the diffusion limit, except in the nominal absence of divalent cations ( approximately 3 muM Ca(2+)). We conclude that both fast and slow block of Ca(V)3.1 by Ni(2+) are most consistent with occlusion of the pore. The exit rate of Ni(2+) for slow block is reduced at high Ni(2+) concentrations, suggesting that the site responsible for fast block can "lock in" slow block by Ni(2+), at a site located deeper within the pore. In contrast to the complex pore block observed for Ca(V)3.1, inhibition of Ca(V)3.2 by Ni(2+) was essentially independent of voltage, and was similar in 2 mM Ca(2+) vs. Ba(2+), consistent with inhibition by a different mechanism, at a site outside the pore.

Show MeSH

Related in: MedlinePlus

Effects of 3 mM Ni2+, with 110 mM Ca2+ or Ba2+. (A and B) Inhibition of IIV currents by Ni2+ in Ca2+ (A) and Ba2+ (B). (C) Inhibition of chord conductances by Ni2+, calculated from the data of A and B. (D and E) Effects of Ni2+ on tail current time constants in Ca2+ (D) and Ba2+ (E). (F) Pseudo first-order rate constants for Ni2+ block, calculated from biexponential fits. (G and H) Effects of 3 mM Ni2+ on I-V relationships in 110 mM Ca2+ (G) and 110 mM Ba2+ (H). (I) Effect of Ni2+ on chord conductances, from the data of G and I. Smooth curves are Woodhull (1973) fits to data in Ca2+ (solid curve) and Ba2+ (dotted curve), constrained to have the same voltage dependence (see text for parameters). n = 5 (Ca2+), n = 4 (Ba2+).
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
getmorefigures.php?uid=PMC2483332&req=5

fig8: Effects of 3 mM Ni2+, with 110 mM Ca2+ or Ba2+. (A and B) Inhibition of IIV currents by Ni2+ in Ca2+ (A) and Ba2+ (B). (C) Inhibition of chord conductances by Ni2+, calculated from the data of A and B. (D and E) Effects of Ni2+ on tail current time constants in Ca2+ (D) and Ba2+ (E). (F) Pseudo first-order rate constants for Ni2+ block, calculated from biexponential fits. (G and H) Effects of 3 mM Ni2+ on I-V relationships in 110 mM Ca2+ (G) and 110 mM Ba2+ (H). (I) Effect of Ni2+ on chord conductances, from the data of G and I. Smooth curves are Woodhull (1973) fits to data in Ca2+ (solid curve) and Ba2+ (dotted curve), constrained to have the same voltage dependence (see text for parameters). n = 5 (Ca2+), n = 4 (Ba2+).

Mentions: “Instantaneous” inhibition of currents by Ni2+. Initial current amplitudes were measured (see Materials and methods) from the protocol of Fig. 1, in 2 mM Ca2+ (A) or 2 mM Ba2+ (D). (B and E) Chord conductances, calculated from the data of A and D, respectively. (C and F) The conductance in Ni2+ as a fraction of the control conductance in 2 mM Ca2+ (C) or 2 mM Ba2+ (F). A ratio of 1.0 (dashed lines) represents zero inhibition. Different Ni2+ concentrations are indicated by the symbol shapes and color coding defined in B and E in all panels, and also in Figs. 3, 5, and 8 below. In Ca2+, 4 cells were tested in 0.3 mM Ni2+, 3 cells in 1 mM, and 5 cells in 3 mM. In Ba2+, 3 cells were tested in 0.1 mM Ni2+, 4 cells in 0.3 mM, and 4 cells in 1 mM. Note that 3 mM Ni2+ was used only in Ca2+, and 0.1 mM Ni2+ only in Ba2+.


Ni2+ block of CaV3.1 (alpha1G) T-type calcium channels.

Obejero-Paz CA, Gray IP, Jones SW - J. Gen. Physiol. (2008)

Effects of 3 mM Ni2+, with 110 mM Ca2+ or Ba2+. (A and B) Inhibition of IIV currents by Ni2+ in Ca2+ (A) and Ba2+ (B). (C) Inhibition of chord conductances by Ni2+, calculated from the data of A and B. (D and E) Effects of Ni2+ on tail current time constants in Ca2+ (D) and Ba2+ (E). (F) Pseudo first-order rate constants for Ni2+ block, calculated from biexponential fits. (G and H) Effects of 3 mM Ni2+ on I-V relationships in 110 mM Ca2+ (G) and 110 mM Ba2+ (H). (I) Effect of Ni2+ on chord conductances, from the data of G and I. Smooth curves are Woodhull (1973) fits to data in Ca2+ (solid curve) and Ba2+ (dotted curve), constrained to have the same voltage dependence (see text for parameters). n = 5 (Ca2+), n = 4 (Ba2+).
© Copyright Policy
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2483332&req=5

fig8: Effects of 3 mM Ni2+, with 110 mM Ca2+ or Ba2+. (A and B) Inhibition of IIV currents by Ni2+ in Ca2+ (A) and Ba2+ (B). (C) Inhibition of chord conductances by Ni2+, calculated from the data of A and B. (D and E) Effects of Ni2+ on tail current time constants in Ca2+ (D) and Ba2+ (E). (F) Pseudo first-order rate constants for Ni2+ block, calculated from biexponential fits. (G and H) Effects of 3 mM Ni2+ on I-V relationships in 110 mM Ca2+ (G) and 110 mM Ba2+ (H). (I) Effect of Ni2+ on chord conductances, from the data of G and I. Smooth curves are Woodhull (1973) fits to data in Ca2+ (solid curve) and Ba2+ (dotted curve), constrained to have the same voltage dependence (see text for parameters). n = 5 (Ca2+), n = 4 (Ba2+).
Mentions: “Instantaneous” inhibition of currents by Ni2+. Initial current amplitudes were measured (see Materials and methods) from the protocol of Fig. 1, in 2 mM Ca2+ (A) or 2 mM Ba2+ (D). (B and E) Chord conductances, calculated from the data of A and D, respectively. (C and F) The conductance in Ni2+ as a fraction of the control conductance in 2 mM Ca2+ (C) or 2 mM Ba2+ (F). A ratio of 1.0 (dashed lines) represents zero inhibition. Different Ni2+ concentrations are indicated by the symbol shapes and color coding defined in B and E in all panels, and also in Figs. 3, 5, and 8 below. In Ca2+, 4 cells were tested in 0.3 mM Ni2+, 3 cells in 1 mM, and 5 cells in 3 mM. In Ba2+, 3 cells were tested in 0.1 mM Ni2+, 4 cells in 0.3 mM, and 4 cells in 1 mM. Note that 3 mM Ni2+ was used only in Ca2+, and 0.1 mM Ni2+ only in Ba2+.

Bottom Line: Na(+)).We conclude that both fast and slow block of Ca(V)3.1 by Ni(2+) are most consistent with occlusion of the pore.The exit rate of Ni(2+) for slow block is reduced at high Ni(2+) concentrations, suggesting that the site responsible for fast block can "lock in" slow block by Ni(2+), at a site located deeper within the pore.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106, USA.

ABSTRACT
Ni(2+) inhibits current through calcium channels, in part by blocking the pore, but Ni(2+) may also allosterically affect channel activity via sites outside the permeation pathway. As a test for pore blockade, we examined whether the effect of Ni(2+) on Ca(V)3.1 is affected by permeant ions. We find two components to block by Ni(2+), a rapid block with little voltage dependence, and a slow block most visible as accelerated tail currents. Rapid block is weaker for outward vs. inward currents (apparent K(d) = 3 vs. 1 mM Ni(2+), with 2 mM Ca(2+) or Ba(2+)) and is reduced at high permeant ion concentration (110 vs. 2 mM Ca(2+) or Ba(2+)). Slow block depends both on the concentration and on the identity of the permeant ion (Ca(2+) vs. Ba(2+) vs. Na(+)). Slow block is 2-3x faster in Ba(2+) than in Ca(2+) (2 or 110 mM), and is approximately 10x faster with 2 vs. 110 mM Ca(2+) or Ba(2+). Slow block is orders of magnitude slower than the diffusion limit, except in the nominal absence of divalent cations ( approximately 3 muM Ca(2+)). We conclude that both fast and slow block of Ca(V)3.1 by Ni(2+) are most consistent with occlusion of the pore. The exit rate of Ni(2+) for slow block is reduced at high Ni(2+) concentrations, suggesting that the site responsible for fast block can "lock in" slow block by Ni(2+), at a site located deeper within the pore. In contrast to the complex pore block observed for Ca(V)3.1, inhibition of Ca(V)3.2 by Ni(2+) was essentially independent of voltage, and was similar in 2 mM Ca(2+) vs. Ba(2+), consistent with inhibition by a different mechanism, at a site outside the pore.

Show MeSH
Related in: MedlinePlus