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Effect of alkali metal cations on slow inactivation of cardiac Na+ channels.

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

Bottom Line: In whole cell recordings, raising [Na+]zero from 10 to 150 mM increases the rate of recovery from slow inactivation at -140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction.Single channel recordings of F1485Q show a decrease in the number of blank (i.e., ) records when [Na+]0 is increased.Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]0, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate.

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+. The kinetics of decaying outward Na+ current in response to 1-s depolarizations in the F1485Q mutant depends on the predominant cation in the extracellular solution, suggesting an effect on slow inactivation. The decay rate is lower for the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ than for the organic cations Tris, tetramethylammonium, N-methylglucamine, and choline. In whole cell recordings, raising [Na+]zero from 10 to 150 mM increases the rate of recovery from slow inactivation at -140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction. Single channel recordings of F1485Q show a decrease in the number of blank (i.e., ) records when [Na+]0 is increased. Significant clustering of blank records when depolarizing at a frequency of 0.5 Hz suggests that periods of inactivity represent the sojourn of a channel in a slow-inactivated state. Examination of the single channel kinetics at +60 mV during 90-ms depolarizations shows that neither open time, closed time, nor first latency is significantly affected by [Na+]0. However raising [Na+]0 decreases the duration of the last closed interval terminated by the end of the depolarization, leading to an increased number of openings at the depolarized voltage. Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]0, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate.

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Maximum likelihood estimates of the rate constants for  the kinetic model. Data are means ± SEM from 13 patches. P values are derived by ANOVA from the natural logarithm of the rate  constants.
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Figure 6: Maximum likelihood estimates of the rate constants for the kinetic model. Data are means ± SEM from 13 patches. P values are derived by ANOVA from the natural logarithm of the rate constants.

Mentions: This model includes an open state, O0, and 3 inactivated states, I1, I2 and I3. Because the data were collected at +60 mV, the channels were maximally activated and transitions to or from a closed state in the activation pathway are extremely unlikely. Transitions between open and inactivated states are included because, for F1485Q channels, the repeated transitions between open and closed levels observed during a depolarization to large positive voltages are thought to arise from recurrent entry and recovery from fast-inactivated states destabilized by the mutation (Hartmann et al., 1994; Lawrence et al., 1996). To account for the long inactivated state from which channels cannot reopen (blank records), an absorbing inactivated state (I3) is included. I3 is identified with the slow-inactivated state in this model. Although this model is sequential, in that fast inactivation must precede slow inactivation, a nonsequential model produced equivalent results (see discussion). We estimated the rate constants for the above sequential model from the gating transitions of idealized single-channel data (Horn and Lange, 1983). Fig. 6 shows these estimates for 13 single-channel patches in which [Na+]o was changed. The model describes the data quite well, as shown by the open probability calculated from the estimated rate constants (Fig. 7 A). Fig. 7 represents analysis of the same patch shown in Fig. 5 during sequential exposure to 150, 10, and 150 mM Na+.


Effect of alkali metal cations on slow inactivation of cardiac Na+ channels.

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

Maximum likelihood estimates of the rate constants for  the kinetic model. Data are means ± SEM from 13 patches. P values are derived by ANOVA from the natural logarithm of the rate  constants.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2229358&req=5

Figure 6: Maximum likelihood estimates of the rate constants for the kinetic model. Data are means ± SEM from 13 patches. P values are derived by ANOVA from the natural logarithm of the rate constants.
Mentions: This model includes an open state, O0, and 3 inactivated states, I1, I2 and I3. Because the data were collected at +60 mV, the channels were maximally activated and transitions to or from a closed state in the activation pathway are extremely unlikely. Transitions between open and inactivated states are included because, for F1485Q channels, the repeated transitions between open and closed levels observed during a depolarization to large positive voltages are thought to arise from recurrent entry and recovery from fast-inactivated states destabilized by the mutation (Hartmann et al., 1994; Lawrence et al., 1996). To account for the long inactivated state from which channels cannot reopen (blank records), an absorbing inactivated state (I3) is included. I3 is identified with the slow-inactivated state in this model. Although this model is sequential, in that fast inactivation must precede slow inactivation, a nonsequential model produced equivalent results (see discussion). We estimated the rate constants for the above sequential model from the gating transitions of idealized single-channel data (Horn and Lange, 1983). Fig. 6 shows these estimates for 13 single-channel patches in which [Na+]o was changed. The model describes the data quite well, as shown by the open probability calculated from the estimated rate constants (Fig. 7 A). Fig. 7 represents analysis of the same patch shown in Fig. 5 during sequential exposure to 150, 10, and 150 mM Na+.

Bottom Line: In whole cell recordings, raising [Na+]zero from 10 to 150 mM increases the rate of recovery from slow inactivation at -140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction.Single channel recordings of F1485Q show a decrease in the number of blank (i.e., ) records when [Na+]0 is increased.Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]0, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate.

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+. The kinetics of decaying outward Na+ current in response to 1-s depolarizations in the F1485Q mutant depends on the predominant cation in the extracellular solution, suggesting an effect on slow inactivation. The decay rate is lower for the alkali metal cations Li+, Na+, K+, Rb+, and Cs+ than for the organic cations Tris, tetramethylammonium, N-methylglucamine, and choline. In whole cell recordings, raising [Na+]zero from 10 to 150 mM increases the rate of recovery from slow inactivation at -140 mV, decreases the rate of slow inactivation at relatively depolarized voltages, and shifts steady-state slow inactivation in a depolarized direction. Single channel recordings of F1485Q show a decrease in the number of blank (i.e., ) records when [Na+]0 is increased. Significant clustering of blank records when depolarizing at a frequency of 0.5 Hz suggests that periods of inactivity represent the sojourn of a channel in a slow-inactivated state. Examination of the single channel kinetics at +60 mV during 90-ms depolarizations shows that neither open time, closed time, nor first latency is significantly affected by [Na+]0. However raising [Na+]0 decreases the duration of the last closed interval terminated by the end of the depolarization, leading to an increased number of openings at the depolarized voltage. Analysis of single channel data indicates that at a depolarized voltage a single rate constant for entry into a slow-inactivated state is reduced in high [Na+]0, suggesting that the binding of an alkali metal cation, perhaps in the ion-conducting pore, inhibits the closing of the slow inactivation gate.

Show MeSH
Related in: MedlinePlus