title: Membrane potentials in Rana temporaria muscle fibres in strongly hypertonic solutions.
abstract: Conventional microelectrode methods were used to measure variations in resting membrane potentials, E(m), of intact amphibian skeletal muscle fibres over a wide range of increased extracellular tonicities produced by inclusion of varying extracellular concentrations of sucrose. Moderate increases in extracellular tonicity to up to 2.6x normal (2.6tau) under Cl(-) free conditions produced negative shifts in E(m) that followed expectations for the K(+) Nernst equation (E(K)) applied to a perfect osmometer containing a conserved intracellular K(+) content despite any accompanying cell volume change. In contrast, E(m) remained stable in fibres studied in otherwise similar Cl(-) containing solutions, consistent with E(m) stabilization despite negative shifts in E(K) through inward cation-Cl(-) co-transport activity. Short exposures to higher tonicities (>3tau) similarly produced negative shifts in E(m) in Cl(-) free but not Cl(-) containing solutions. However, prolonged exposures to solutions of >3tau caused gradual net positive changes in E (m) in both Cl(-) containing and Cl(-) free solutions suggesting that these changes were independent of cation-Cl(-) transport. Indeed, there was no evidence of cation-Cl(-) co-transport activity in strongly hypertonic solutions despite its predicted energetic favourability, suggesting its possible regulation by E (m) in muscle. Additional findings implicated a failure to maintain greatly increased transmembrane [K(+)] gradients in these E(m) changes. Thus: (1) halving or doubling [K(+)](e) produced negative or positive shifts in E(m), respectively in isotonic or moderately hypertonic (<2.7tau), but not strongly hypertonic (>3tau) solutions; (2) subsequent restoration of isotonic extracellular conditions produced further positive changes in E(m) consistent with a dilution of the depleted [K(+)](i) by fibres regaining their original resting volumes; (3) quantitative modelling similarly predicted a gradual net efflux of K(+) as the balance between active and passive [K(+)] fluxes altered due to increased transmembrane [K(+)] gradients in hypertonic and low [K(+)](e) solutions. However, the observed positive changes in E(m) in the most strongly hypertonic solutions eventually exceeded these predictions suggesting additional limitations on Na(+)/K(+)-ATPase activity in strongly hypertonic solutions.
authors.affiliation: Physiological Laboratory, University of Cambridge, Cambridge CB2 3EG, UK. jaf21@cam.ac.uk
authors.forename: James A
authors.initials: JA
authors.lastname: Fraser
authors.forename: Kai Yuen
authors.initials: KY
authors.lastname: Wong
authors.forename: Juliet A
authors.initials: JA
authors.lastname: Usher-Smith
authors.forename: Christopher L-H
authors.initials: CL
authors.lastname: Huang
mesh_terms: D000818:Animals; D006863:Hydrogen-Ion Concentration; D006982:Hypertonic Solutions / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology*; D009994:Osmolar Concentration; D011896:Rana temporaria; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors
chemical_list: D006982:Hypertonic Solutions; D049994:Sodium Potassium Chloride Symporter Inhibitors; D028021:Sodium-Potassium-Chloride Symporters; D000254:Sodium-Potassium-Exchanging ATPase