Key words: K+ conductance — Voltage-dependent channel gating — K+-dependent/stochastic gating model
Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract. The effect of extracellular cation concentration and membrane voltage on the current carried by outward-rectifying K+ channels was examined in stomatal guard cells of Vicia faba L. Intact guard cells were impaled with double-barrelled microelectrodes and the K+ current was monitored under voltage clamp in 0.1–30 mm K+ and in equivalent concentrations of Rb+, Cs+ and Na+. From a conditioning voltage of −200 mV, clamp steps to voltages between −150 and +50 mV in 0.1 mm K+ activated current through outward-rectifying K+ channels (I K, out) at the plasma membrane in a voltage-dependent fashion. Increasing [K+] o shifted the voltage-sensitivity of I K, out in parallel with the equilibrium potential for K+ across the membrane. A similar effect of [K+] o was evident in the kinetics of I K, out activation and deactivation, as well as the steady-state conductance- (g K −) voltage relations. Linear conductances, determined as a function of the conditioning voltage from instantaneous I-V curves, yielded voltages for half-maximal conductance near −130 mV in 0.1 mm K+, −80 mV in 1.0 mm K+, and −20 mV in 10 mm K+. Similar data were obtained with Rb+ and Cs+, but not with Na+, consistent with the relative efficacy of cation binding under equilibrium conditions (K+≥ Rb+ 〉 Cs+ 〉 〉 Na+). Changing Ca2+ or Mg2+ concentrations outside between 0.1 and 10 mm was without effect on the voltage-dependence of g K or on I K, out activation kinetics, although 10 mm [Ca2+] o accelerated current deactivation at voltages negative of −75 mV. At any one voltage, increasing [K+] o suppressed g K completely, an action that showed significant cooperativity with a Hill coefficient of 2. The apparent affinity for K+ was sensitive to voltage, varying from 0.5 to 20 mm with clamp voltages near −100 to 0 mV, respectively. These, and additional data indicate that extracellular K+ acts as a ligand and alters the voltage-dependence of I K, out gating; the results implicate K+-binding sites accessible from the external surface of the membrane, deep within the electrical field, but distinct from the channel pore; and they are consistent with a serial 4-state reaction-kinetic model for channel gating in which binding of two K+ ions outside affects the distribution between closed states of the channel.
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