Ionic selectivity, saturation, and block in a K+-selective channel from sarcoplasmic reticulum.

Abstract

A B S T R A C TThe open-channel conductance properties of a voltage-gated channel from sarcoplasmic reticulum were studied in planar phospholipid membranes. The channel is ideally selective for K + over C1-and for K + over Ca ++. In symmetrical 1 M solutions, the single-channel conductance (in pmho) falls in the order: K + (214) > NH~-(157) > Rb § (125) > Na § (72) > Li + (8.1) > Cs + (<3). In neutral bilayers, the channel conductance saturates with ion activity according to a rectangular hyperbolic relation, with half-saturation activities of 54 mM for K + and 34 mM for Na +. Under symmetrical salt conditions, the K+:Na + channel conductance ratio increases with salt activity, but the permeability ratio, measured by single-channel bi-ionic potentials, is constant between 20 mM and 2.5 M salt; the permeability ratio is equal to the conductance ratio in the limit of Iow-sah concentration. The channel conductance varies <5% in the voltage range -100 to +70 mV. The maximum conductance ofK § and Na + is only weakly temperature dependent (AH:[: = 4.6 and 5.3 kcal/mol, respectively), but that of Li + varies strongly with temperature (~-/:~ = 13 kcal/mol). The channel's K + conductance is blocked asymmetrically by Cs +, and this block is competitive with K +. The results are consistent with an Eyring-type model of the conduction process in which the ion must traverse three kinetic barriers as it permeates the channel. The data conform to L~iuger's (1973. Biochim. Biophys. Acta. 311:423-441) predictions for a "pure" single-ion channel.A description of channel-mediated ion conductance requires the study of two separate processes: the formation of the conducting channel (the gating process) and the movement of current-carrying ions through the open channel (the conduction process). In the previous paper (Labarca et al., 1980) we described the gating properties of a K+-selective channel from sarcoplasmic reticulum (SR) of mammalian skeletal muscle. We now wish to describe this channel's conduction process. Earlier work (Miller, 1978) showed that SR vesicles can be made to donate ion-conducting channels to a planar phospholipid bilayer membrane. These channels display selectivity for monovalent cations, but this selectivity was not quantified previously, and no attempt was made to develop a picture of the conduction process. We now intend to do J. GEN. PnvsloL. 9 The Rockefeller University Press

Sheep cardiac sarcoplasmic reticulum calcium‐release channels: modification of conductance and gating by temperature.

“…This argument is supported by a comparison with the high conductance (Yrmax 230 pS) monovalent cation-selective channel of skeletal muscle SR. In this channel apparent activation enthalpies of 4-6 and 5.3 keal/mol have been determined for K+ and Na+ respectively which are virtually indistinguishable from the values for conductivity of such ions in aqueous solution (4 3-4 5 keal/mol; Coronado, Rosenberg & Miller, 1980). The relative permeability of the sheep cardiac SR calcium-release channel to calciuin and Tris is apparently unaffected by temperature variation, suggesting that the mechanisms governing the affinity of the sites of interaction for these ions are equally sensitive to changing temperature.…”

Immuno-proteomic approach to excitation–contraction coupling in skeletal and cardiac muscle: Molecular insights revealed by the mitsugumins

“…While monovalent cation-selective currents are known to exist in ER/SR membrane [10,14,[16][17][18], the identity of gene(s) encoding these channels was completely unknown. TRIC constitutes a candidate gene product for one such long-sought, counter ion channel in the ER/SR membrane.…”

“…Movement of ions across the SR membrane to counter this negative potential has been implicated in the efficient operation of VICR in skeletal muscle and CICR in cardiac muscle [10][11][12][13][14][15]. Although channels selective for monovalent cations have been reported in SR membrane [10,14,[16][17][18], until recently no gene has been identified that encodes an ER/SR localized cation-selective channel.…”

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.…”