title: Adaptations of plasma membrane glucose transport facilitate cryoprotectant distribution in freeze-tolerant frogs.
abstract: Natural freeze tolerance in several anuran species involves the accumulation of high concentrations of glucose as a cryoprotectant in body fluids and tissues. The present study identifies an important new molecular mechanism supporting freeze tolerance, an adaptive increase in the capacity for facilitated transport of cryoprotectant across plasma membranes by increasing the numbers and/or activity of plasma membrane glucose transporters. Glucose transport by membranes isolated from liver and skeletal muscle was analyzed in two species, the freeze-tolerant wood frog Rana sylvatica and the freeze-intolerant leopard frog Rana pipiens. Membranes from both liver and muscle of R. sylvatica displayed much higher rates of carrier-mediated glucose transport, measured by a rapid filtration technique, compared with corresponding rates for R. pipiens membranes. For the liver Vmax values for glucose transport by membrane vesicles were 69 +/- 18 and 8.4 +/- 2.3 nmol.mg protein-1.s-1 at 10 degrees C for R. sylvatica and R. pipiens, respectively. This difference was due primarily to a greater number of glucose transporters in R. sylvatica liver membranes; the total number of transporter sites, determined by cytochalasin B binding, was 4.7-fold higher in the freeze-tolerant species. For muscle membranes, the Vmax for glucose transport was 4.9 +/- 1 and 0.6 +/- 0.16 nmol.mg-1 x s-1 at 22 degrees C for R. sylvatica and R. pipiens, respectively. However, in muscle there were no differences in the number of membrane transporters between species.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Medicine, University of Vermont, Burlington 05405.
authors.forename: P A
authors.initials: PA
authors.lastname: King
authors.forename: M N
authors.initials: MN
authors.lastname: Rosholt
authors.forename: K B
authors.initials: KB
authors.lastname: Storey
mesh_terms: D009597:4-Nitrophenylphosphatase / Q000032:analysis; D015720:5'-Nucleotidase / Q000032:analysis; D000064:Acclimatization; D000249:Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D015415:Biomarkers / Q000032:analysis; D002250:Carbon Radioisotopes; D002458:Cell Fractionation; D005615:Freezing; D005947:Glucose / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism*; D011865:Radioisotope Dilution Technique; D011894:Rana pipiens / Q000502:physiology*; D011898:Ranidae / Q000502:physiology*; D014316:Tritium
chemical_list: D015415:Biomarkers; D002250:Carbon Radioisotopes; D009004:Monosaccharide Transport Proteins; D014316:Tritium; D000249:Adenosine Monophosphate; D009597:4-Nitrophenylphosphatase; D015720:5'-Nucleotidase; D005947:Glucose