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Membrane potential stabilization in amphibian skeletal muscle fibres in hypertonic solutions
Author(s) -
Ferenczi Emily A.,
Fraser James A.,
Chawla Sangeeta,
Skepper Jeremy N.,
Schwiening Christof J.,
Huang Christopher L.H.
Publication year - 2004
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.2003.058545
Subject(s) - extracellular , cotransporter , bumetanide , tonicity , biophysics , chemistry , ouabain , intracellular , membrane potential , osmotic concentration , extracellular fluid , membrane , sodium , biochemistry , biology , organic chemistry
This study investigated membrane transport mechanisms influencing relative changes in cell volume ( V ) and resting membrane potential ( E m ) following osmotic challenge in amphibian skeletal muscle fibres. It demonstrated a stabilization of E m despite cell shrinkage, which was attributable to elevation of intracellular [Cl − ] above electrochemical equilibrium through Na + –Cl − and Na + −K + −2Cl − cotransporter action following exposures to extracellular hypertonicity. Fibre volumes ( V ) determined by confocal microscope xz‐ scanning of cutaneous pectoris muscle fibres varied linearly with [1/extracellular osmolarity], showing insignificant volume corrections, in fibres studied in Cl − ‐free, normal and Na + ‐free Ringer solutions and in the presence of bumetanide, chlorothiazide and ouabain. The observed volume changes following increases in extracellular tonicity were compared with microelectrode measurements of steady‐state resting potentials ( E m ). Fibres in isotonic Cl − ‐free, normal and Na + ‐free Ringer solutions showed similar E m values consistent with previously reported permeability ratios P Na / P K ( 0.03–0.05) and P Cl / P K (∼2.0) and intracellular [Na + ], [K + ] and [Cl − ]. Increased extracellular osmolarities produced hyperpolarizing shifts in E m in fibres studied in Cl − ‐free Ringer solution consistent with the Goldman‐Hodgkin‐Katz (GHK) equation. In contrast, fibres exposed to hypertonic Ringer solutions of normal ionic composition showed no such E m shifts, suggesting a Cl − ‐dependent stabilization of membrane potential. This stabilization of E m was abolished by withdrawing extracellular Na + or by the combined presence of the Na + –Cl − cotransporter (NCC) inhibitor chlorothiazide (10 μ m ) and the Na + −K + −2Cl − cotransporter (NKCC) inhibitor bumetanide (10 μ m ), or the Na + −K + ‐ATPase inhibitor ouabain (1 or 10 μ m ) during alterations in extracellular osmolarity. Application of such agents after such increases in tonicity only produced a hyperpolarization after a time delay, as expected for passive Cl − equilibration. These findings suggest a model that implicates the NCC and/or NKCC in fluxes that maintain [Cl − ] i above its electrochemical equilibrium. Such splinting of [Cl − ] i in combination with the high P Cl / P K of skeletal muscle stabilizes E m despite volume changes produced by extracellular hypertonicity, but at the expense of a cellular capacity for regulatory volume increases (RVIs). In situations where P Cl / P K is low, the same cotransporters would instead permit RVIs but at the expense of a capacity to stabilize E m .