Factors affecting membrane permeability and ionic homeostasis in the cold-submerged frog

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.