Anomalous effect of permeant ion concentration on peak open probability of cardiac Na+ channels - PubMed (original) (raw)

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

C Townsend et al. J Gen Physiol. 1997 Jul.

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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Figures

Figure 10

Figure 10

Effect of external [Na+] on F1485Q tail currents. (A) Families of currents elicited by 9-ms depolarizations to voltages ranging from −80 to +70 mV. Holding potential, −140 mV. Currents shown were recorded from two cells bathed in either 150 mM Na+ (left) or 150 mM Cs+ at 8.0 and 7.1°C, respectively. (B) Tail currents were recorded from −80 to +70 mV after a brief depolarization (1.5 ms) to 0 mV from a holding potential of −140 mV. Currents recorded from the same two cells shown in A at the same temperatures.

Figure 5

Figure 5

Effect of extracellular Na+, Cs+, and Li+ on P open of F1485Q Na+ channels. P open (Po) was calculated from whole-cell peak currents (Fig. 2) and single-channel i-V relations (Figs. 3 and 4) as described in

methods

. NMG replaced Na+ in the 10 mM Na+ bath solution. Data are means ± SEM normalized to the maximum Po obtained in 150 mM Na+ o.

Figure 2

Figure 2

Effects of external cations on peak Na+ current-voltage relations. Currents were activated as described in the legend of Fig. 1. Normalized peak currents for WT- (A, n = 5) and F1485Q- (B, n = 3) transfected cells sequentially bathed in 10, 150, and 10 mM Na+. (C, D, and E) Current-voltage relations for F1485Q-transfected cells successively bathed in (in mM): 10 Na+ (140 Cs+), 150 Na+, and 10 Na+ (140 Cs+) (C, n = 3); 150 NMG, 150 Cs+, and 150 NMG (D, n = 5); 150 NMG, 150 Li+, and 150 NMG (E, n = 4); or 150+ Na, 150 Li+, and 150 Na+ (F, n = 3). Intracellular [Na+] was 150 mM in all cases.

Figure 3

Figure 3

Effects of [Na+]o on single-F1485Q channel currents. Selected single-channel current recordings from outside-out patches bathed in 10 (A) and 150 mM Na+ (B). Internal [Na+] was 150 mM. Currents were activated by 90-ms depolarizations (arrow) to voltages ranging from +20 to +80 mV (as indicated to the left of the traces) from a holding potential of −140 mV. The dotted lines represent the closed level. (C) Single-channel current-voltage relations in 10 (n = 2) and 150 mM (n = 4) Na+ o. Data points for 150 mM Na+ were fit by linear regression, yielding an estimate of the GHK permeability (solid line). The dotted line represents single-channel currents for 10 mM Na+ o as predicted by the GHK current equation.

Figure 4

Figure 4

Effects of external cations on single-channel outward currents. Currents were evoked as described in the legend of Fig. 3. (A) Selected single-channel recordings obtained in the presence of 150 mM NMG, 150 mM Cs+, and 150 mM Li+ in the bath solution (V = +60 mV). (B) Current-voltage relations for 150 mM NMGo (n = 2), 150 mM Cs+ o (n = 3), and 150 mM Li+ o (n = 3). Data points were fit to straight lines with slopes of 27, 31, and 35 pS for NMG, Cs+, and Li+, respectively.

Figure 1

Figure 1

WT and F1485Q hH1a Na+ channel currents in 10 and 150 mM [Na+]o. Currents were elicited by 9-ms depolarizations to voltages ranging from −80 to +70 mV in 10-mV increments from a holding potential of −140 mV and at a frequency of 0.5 Hz. Each panel shows families of Na+ currents obtained for one cell transfected with either WT (A) or F1485Q (B) Na+ channels and successively bathed in 10, 150, and 10 mM external Na+. Intracellular [Na+] was 150 mM.

Figure 6

Figure 6

Effects of impermeant cations on normalized P-V relationships. Peak P open (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.

Figure 7

Figure 7

Effect of hydrazinium on the I-V relationship and channel open probability. Currents were evoked as described in the legend of Fig. 1 except that 5-mV increments were used between each pulse. (A) I-V relationships obtained from F1485Q-transfected cells were successively bathed in Na+, NMG, Na+, hydrazinium, and Na+ (150 mM except [hydrazinium]o ∼138 mM, see

methods

). Data are means ± SEM from 3 cells. (B) Peak P open (Po) versus voltage relations for cells bathed in hydrazinium (138 mM, n = 3, see

methods

). The dotted line corresponds to the P-V curve obtained with 150 mM Na+ o. Data are means ± SEM with maximums normalized to unity.

Figure 8

Figure 8

Lack of effect of [Na+]o on activation kinetics. (A) Scaled Na+ currents from a cell bathed sequentially in Na+, Cs+, and Na+ (150 mM). V = +60 mV, −140 mV holding potential. (B) Time to peak versus voltage for cells successively bathed in Na+, Cs+, and Na+ (150 mM). Currents were evoked as described in the legend of Fig. 1 except that 5-mV increments were used. Data are means ± SEM from three cells.

Figure 9

Figure 9

Rapid kinetics of recovery after alteration of P open. Recovery at −140 mV after a 0.8-ms prepulse to +60 (A) or +20 mV (B, see insets). 16 0.8-ms recovery test pulses to +20 mV were given 0.1–18.1 ms after the prepulse (superimposed traces). Data are from one cell successively bathed in 150 mM Na+ and Cs+.

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References

    1. Armstrong CM, Bezanilla F. Charge movement associated with the opening and closing of the activation gates of the Na channels. J Gen Physiol. 1974;63:533–552. - PMC - PubMed
    1. Baukrowitz T, Yellen G. Modulation of K+ current by frequency and external [K+]: a tale of two inactivation mechanisms. Neuron. 1995;15:951–960. - PubMed
    1. Bezanilla, F., and A.M. Correa. 1995. Single-channel properties and gating of Na+ and K+ channels in the squid giant axon. In Cephalopod Neurobiology. N.J. Abbott, R. Williamson, and L. Maddock, editors. Oxford University Press, Oxford. 131–151.
    1. Brehm P, Eckert R. Calcium entry leads to inactivation of calcium channels in . Paramecium Science. 1978;202:1203–1206. - PubMed
    1. Chahine M, George AL, Jr, Zhou M, Ji S, Sun W, Barchi RL, Horn R. Sodium channel mutations in paramyotonia congenita uncouple inactivation from activation. Neuron. 1994;12:281–294. - PubMed

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