Thermodynamic and kinetic studies of the gating behavior of a K+-selective channel from the sarcoplasmic reticulum membrane.

Abstract

A voltage-dependent, K+-selective ionic channel from sarcoplasmic reticulum of rabbit skeletal muscle has been studied in a planar phospholipid bilayer membrane. The purpose of this work is to study the mechanism by which the channel undergoes transitions between its conducting and nonconducting states. Thermodynamic studies show that the "open" and "closed" states of the channel exist in a voltage-dependent equilibrium, and that the channel displays only a single open state; the channel conductance is 120 pmho in 0.1 M K +. The channel's gating process follows single exponential kinetics at all voltages tested, and the individual opening and closing rate constants are exponentially dependent on voltage. The individual rate constants may also be determined from a stochastic analysis of channel fluctuations among multiple conductance levels. Neither the thermodynamic nor the kinetic parameters of gating depend on the absolute concentration of channels in the bilayer. The results are taken as evidence that the channel gates by an unusually simple twostate conformational mechanism in which the equivalent of 1.1 net charges are moved across the membrane during the formation of the open channel.Of the events leading to the contraction of vertebrate skeletal muscle, those involved in the release of Ca ++ from the sarcoplasmic reticulum (SR) membrane are the least understood. It is clear that the permeability of the SR membrane to Ca ++ is greatly increased during the release process (Endo, 1977), but it is not known to what extent other ions participate in any voltage and conductance changes that may occur during Ca ++ release. One obvious question to arise from any consideration of Ca ++ movements is: what other ions move across the SR membrane to maintain electroneutrality and, hence, to permit the rapid, massive fluxes of Ca ++ into and out of the SR throughout the contraction-relaxation cycle? Indirect approaches have yielded estimates of the overall SR conductance (Vergara et al., 1978), but the ionic basis of this is entirely unknown. In particular, the role of K +, the overwhelmingly abundant ion on both sides of the SR membrane in vivo (Somlyo et al., 1977)) is obscure.

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study.

“…An explanation for this may be found in observations that show voltage-dependent gating of K channels in the isolated SR (58,70). Specifically, at pH 7 the K conductance of the SR channels incorporated into planar phospholipid bilayers is near zero at -50 and maximum at +100 mV applied potential.…”

“…Nonelectrogenic movements of Ca are also thought to occur in fragmented SR preparations (4) . A reduction in the K permeability of the SR during tetanus could be caused by the developed SR potential itself or by changes in pH or in cytoplasmic free Ca concentration (58,70). In addition to the movement ofprotons or organic ions, the potential due to Eca may also be partially balanced by an electrogenic Ca pump (111) or electrogenic Mg" flux into the SR.…”

BisG10, a K+ channel blocker, affects the calcium release channel from skeletal muscle sarcoplasmic reticulum

“…Since, a channel that displays a high conductance for K* in the SR membrane has been described and the electrical properties of th. ;s channel well defined [5][6][7]. The physiological role of this K ÷ permeability is still an important question.…”

A cation channel for K+ and Ca2+ from Tetrahymena cilia in planar lipid bilayers.

“…They are the patch-clamp method (11,20) and the planar lipid bilayer method (7,15,24,26). The latter method is particularly useful to investigate ion channels that are inaccessible to patch clamp pipettes because of the complex geometry of the membranes.…”