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Anomalous effect of permeant ion concentration on peak open probability of cardiac Na+ channels.

Townsend C, Hartmann HA, Horn R - J. Gen. Physiol. (1997)

Bottom Line: This anomalous effect of permeant ion concentration, especially obvious in a mutant (F1485Q) in which fast inactivation is partially abolished, is due to an alteration of open probability.The effect is only observed when a highly permeant cation (Na+, Li+, or hydrazinium) is substituted for a relatively impermeant cation (K+, Rb+, Cs+, N-methylglucamine, Tris, choline, or tetramethylammonium).There is little effect of permeant ion concentration on activation kinetics at depolarized voltages.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Jefferson Medical College, Philadelphia, Pennsylvania 19107, USA.

ABSTRACT
Human heart Na+ channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na+ currents measured using 150 mM intracellular Na+. Decreasing extracellular permeant ion concentration decreases outward Na+ current at positive voltages while increasing the driving force for the current. This anomalous effect of permeant ion concentration, especially obvious in a mutant (F1485Q) in which fast inactivation is partially abolished, is due to an alteration of open probability. The effect is only observed when a highly permeant cation (Na+, Li+, or hydrazinium) is substituted for a relatively impermeant cation (K+, Rb+, Cs+, N-methylglucamine, Tris, choline, or tetramethylammonium). With high concentrations of extracellular permeant cations, the peak open probability of Na+ channels increases with depolarization and then saturates at positive voltages. By contrast, with low concentrations of permeant ions, the open probability reaches a maximum at approximately 0 mV and then decreases with further depolarization. There is little effect of permeant ion concentration on activation kinetics at depolarized voltages. Furthermore, the lowered open probability caused by a brief depolarization to +60 mV recovers within 5 ms upon repolarization to -140 mV, indicative of a gating process with rapid kinetics. Tail currents at reduced temperatures reveal the rapid onset of this gating process during a large depolarization. A large depolarization may drive a permeant cation out of a site within the extracellular mouth of the pore, reducing the efficiency with which the channel opens.

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Effects of impermeant cations on normalized P-V relationships. Peak Popen (Po) was determined from whole-cell and single-channel current-voltage relations as described in methods.  Data from F1485Q-transfected cells bathed in 150 mM of the indicated cations. In each panel, the dotted line corresponds to the  P-V curve obtained with 150 mM Na+o. Data are means ± SEM  with maximums normalized to unity.
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Figure 6: Effects of impermeant cations on normalized P-V relationships. Peak Popen (Po) was determined from whole-cell and single-channel current-voltage relations as described in methods. Data from F1485Q-transfected cells bathed in 150 mM of the indicated cations. In each panel, the dotted line corresponds to the P-V curve obtained with 150 mM Na+o. Data are means ± SEM with maximums normalized to unity.

Mentions: We obtained single channel records at positive voltages. To estimate a normalized Popen-voltage (P-V) relationship over the entire range of activation, we assumed that the single-channel current-voltage relationship had the form of the Goldman-Hodgkin-Katz (GHK) current equation (Hille, 1992, 341–345). Although the GHK model is insufficient to describe all details of permeation in Na+ channels (Hille, 1975), this model fit our data well over a range of positive voltages when the external cation was predominantly Na+ or NMG (see Fig. 3 C). In the case of symmetrical concentrations of Na+, the GHK current-voltage relationship is linear, so that the P-V relationship is equivalent to a conductance-voltage relationship. We estimated the relative permeability of other cations to Na+ from biionic reversal potentials and calculated the current-voltage relationship for a single channel (i-V) obeying the GHK current equation. The normalized P-V relationship was then obtained by dividing the peak macroscopic I-V relationship by the GHK i-V relationship and scaling the maximum to unity (see Fig. 6).


Anomalous effect of permeant ion concentration on peak open probability of cardiac Na+ channels.

Townsend C, Hartmann HA, Horn R - J. Gen. Physiol. (1997)

Effects of impermeant cations on normalized P-V relationships. Peak Popen (Po) was determined from whole-cell and single-channel current-voltage relations as described in methods.  Data from F1485Q-transfected cells bathed in 150 mM of the indicated cations. In each panel, the dotted line corresponds to the  P-V curve obtained with 150 mM Na+o. Data are means ± SEM  with maximums normalized to unity.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2229355&req=5

Figure 6: Effects of impermeant cations on normalized P-V relationships. Peak Popen (Po) was determined from whole-cell and single-channel current-voltage relations as described in methods. Data from F1485Q-transfected cells bathed in 150 mM of the indicated cations. In each panel, the dotted line corresponds to the P-V curve obtained with 150 mM Na+o. Data are means ± SEM with maximums normalized to unity.
Mentions: We obtained single channel records at positive voltages. To estimate a normalized Popen-voltage (P-V) relationship over the entire range of activation, we assumed that the single-channel current-voltage relationship had the form of the Goldman-Hodgkin-Katz (GHK) current equation (Hille, 1992, 341–345). Although the GHK model is insufficient to describe all details of permeation in Na+ channels (Hille, 1975), this model fit our data well over a range of positive voltages when the external cation was predominantly Na+ or NMG (see Fig. 3 C). In the case of symmetrical concentrations of Na+, the GHK current-voltage relationship is linear, so that the P-V relationship is equivalent to a conductance-voltage relationship. We estimated the relative permeability of other cations to Na+ from biionic reversal potentials and calculated the current-voltage relationship for a single channel (i-V) obeying the GHK current equation. The normalized P-V relationship was then obtained by dividing the peak macroscopic I-V relationship by the GHK i-V relationship and scaling the maximum to unity (see Fig. 6).

Bottom Line: This anomalous effect of permeant ion concentration, especially obvious in a mutant (F1485Q) in which fast inactivation is partially abolished, is due to an alteration of open probability.The effect is only observed when a highly permeant cation (Na+, Li+, or hydrazinium) is substituted for a relatively impermeant cation (K+, Rb+, Cs+, N-methylglucamine, Tris, choline, or tetramethylammonium).There is little effect of permeant ion concentration on activation kinetics at depolarized voltages.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, Jefferson Medical College, Philadelphia, Pennsylvania 19107, USA.

ABSTRACT
Human heart Na+ channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na+ currents measured using 150 mM intracellular Na+. Decreasing extracellular permeant ion concentration decreases outward Na+ current at positive voltages while increasing the driving force for the current. This anomalous effect of permeant ion concentration, especially obvious in a mutant (F1485Q) in which fast inactivation is partially abolished, is due to an alteration of open probability. The effect is only observed when a highly permeant cation (Na+, Li+, or hydrazinium) is substituted for a relatively impermeant cation (K+, Rb+, Cs+, N-methylglucamine, Tris, choline, or tetramethylammonium). With high concentrations of extracellular permeant cations, the peak open probability of Na+ channels increases with depolarization and then saturates at positive voltages. By contrast, with low concentrations of permeant ions, the open probability reaches a maximum at approximately 0 mV and then decreases with further depolarization. There is little effect of permeant ion concentration on activation kinetics at depolarized voltages. Furthermore, the lowered open probability caused by a brief depolarization to +60 mV recovers within 5 ms upon repolarization to -140 mV, indicative of a gating process with rapid kinetics. Tail currents at reduced temperatures reveal the rapid onset of this gating process during a large depolarization. A large depolarization may drive a permeant cation out of a site within the extracellular mouth of the pore, reducing the efficiency with which the channel opens.

Show MeSH
Related in: MedlinePlus