title: Factors affecting membrane permeability and ionic homeostasis in the cold-submerged frog.
abstract: Frogs (Rana temporaria) were submerged at 3 degrees C in either normoxic (P(O2)=155 mmHg, P(O2)=20 kPa) or hypoxic (P(O2)=60 mmHg; P(O2)=8 kPa) water for up to 16 weeks, and denied air access, to mimic the conditions of an ice-covered pond during the winter. The activity of the skeletal muscle Na(+)/K(+) pump over the first 2 months of hibernation, measured by ouabain-inhibitable (22)Na(+) efflux, was reduced by 30 % during normoxia and by up to 50 % during hypoxia. The reduction in Na(+)/K(+) pump activity was accompanied by reductions in passive (22)Na(+) influx and (86)Rb(+) efflux (effectively K(+) efflux) across the sarcolemma. This may be due to a decreased Na(+) permeability of the sarcolemma and a 75 % reduction in K(+) leak mediated by ATP-sensitive K(+) channels ('K(ATP)' channels). The lowered rates of (22)Na(+) and (86)Rb(+) flux are coincident with lowered transmembrane ion gradients for [Na(+)] and [K(+)], which may also lower Na(+)/K(+) pump activity. The dilution of extracellular [Na(+)] and intracellular [K(+)] may be partially explained by increased water retention by the whole animal, although measurements of skeletal muscle fluid compartments using (3)H-labelled inulin suggested that the reduced ion gradients represented a new steady state for skeletal muscle. Conversely, intracellular ion homeostasis within ventricular muscle was maintained at pre-submergence levels, despite a significant increase in tissue water content, with the exception of the hypoxic frogs following 4 months of submergence. Both ventricular muscles and skeletal muscles maintained resting membrane potential at pre-submergence levels throughout the entire period of hibernation. The ability of the skeletal muscle to maintain its resting membrane potential, coincident with decreased Na(+)/K(+) pump activity and lowered membrane permeability, provided evidence of functional channel arrest as an energy-sparing strategy during hibernation in the cold-submerged frog.
authors.affiliation: Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. paul.donohoe@nimr.mrc.ac.uk
authors.forename: P H
authors.initials: PH
authors.lastname: Donohoe
authors.forename: T G
authors.initials: TG
authors.lastname: West
authors.forename: R G
authors.initials: RG
authors.lastname: Boutilier
mesh_terms: D000818:Animals; D001692:Biological Transport; D002463:Cell Membrane Permeability / Q000502:physiology*; D003080:Cold Temperature; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005905:Glyburide / Q000494:pharmacology; D006605:Hibernation / Q000502:physiology; D006706:Homeostasis / Q000502:physiology*; D007101:Immersion; D066298:In Vitro Techniques; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D010100:Oxygen / Q000502:physiology; D011188:Potassium / Q000097:blood / Q000378:metabolism*; D011898:Ranidae / Q000378:metabolism*; D012508:Sarcolemma / Q000378:metabolism; D012964:Sodium / Q000097:blood / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
chemical_list: D004791:Enzyme Inhibitors; D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium; D010100:Oxygen; D005905:Glyburide