Anomalous Effect of Permeant Ion Concentration on Peak Open Probability of Cardiac Na 1 Channels

Human heart Na 1 channels were expressed transiently in both mammalian cells and Xenopus oocytes, and Na 1 currents measured using 150 mM intracellular Na 1 . Decreasing extracellular permeant ion concentration decreases outward Na 1 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 1 , Li 1 , or hydrazinium) is substituted for a relatively impermeant cation (K 1 , Rb 1 , Cs 1 , N -methylglucamine, Tris, choline, or tetramethylammonium). With high concentrations of extracellular permeant cations, the peak open probability of Na 1 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 1 60 mV recovers within 5 ms upon repolarization to 2 140 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. key words: sodium channels • gating • single channel recording • kinetics i n t r o d u c t i o n The selective pores of ion channels are said to be opened and closed by “gates,” which may be directly regulated by a variety of factors including the concentrations of agonists and antagonists (Hille, 1992). The gating of voltage-dependent ion channels (e.g., the Na 1 , Ca 2 1 , and K 1 channels of nerve and striated muscle) is primarily controlled by transmembrane potential. The conformational transitions underlying voltagedependent gating are also influenced by the concentration and identity of the permeant ion, sometimes in profound ways. For example, some Ca 2 1 channels inactivate strongly when Ca 2 1 ions carry the inward current but not when Ba 2 1 is the permeant ion (Brehm and Eckert, 1978; Tillotson, 1979). The voltage-dependent gating of K 1 channels is also affected by K 1 concentration. In general an increase in extracellular K 1 concentration increases open probability at any membrane potential, inhibits inactivation, and increases the rate of recovery from inactivation (Swenson and Armstrong, 1981; Clay, 1986; Matteson and Swenson, 1986; Demo and Yellen, 1991; Pardo et al., 1992; Lopez-Barneo et al., 1993; Gomez-Lagunas and Armstrong, 1994; Baukrowitz and Yellen, 1995; Levy and Deutsch, 1996 a , b ). These effects are usually interpreted as a consequence of the binding of K 1 ions in or near the ion-conducting pore. For example, K 1 ions may bind near the extracellular mouth of the pore and interfere with the closing of either an activation or an inactivation gate. Alternatively the binding of K 1 ions inside the pore may enhance the opening of an inactivation gate. The open probability of a class of Cl 2 channels is also enhanced by increasing extracellular Cl 2 concentration (Pusch et al., 1995; Chen and Miller, 1996). Although intracellular Na 1 concentration affects the rates and steady-state levels of inactivation of Na 1 channels (e.g., Chandler and Meves, 1965; 1970; Oxford and Yeh, 1985), changes of extracellular Na 1 concentration between 15 mM and 4 M have relatively small effects on either the kinetics or voltage dependence of Na 1 currents of squid axon (Armstrong and Bezanilla, 1974; Oxford and Yeh, 1985; Correa and Bezanilla, 1994; Bezanilla and Correa, 1995; A.M. Correa and F. Bezanilla, personal communication). Our own examination of currents from cardiac Na 1 channels also shows little effect of external Na 1 concentration on fast inactivation kinetics (O’Leary et al., 1994; Tang et al., 1996). We report here an anomalous effect of changing extracellular permeant ion concentration, namely a decrease in Na 1 current in response to an increase of Address correspondence to Dr. Richard Horn, Department of Physiology, Institute of Hyperexcitability, Jefferson Medical College, 1020 Locust Street, Philadelphia, PA 19107. Fax: 215-503-2073; E-mail: [email protected] on Jne 0, 2017 D ow nladed fom Published July 1, 1997