SEASONAL-CHANGES IN PLASMA-MEMBRANE GLUCOSE TRANSPORTERS ENHANCE CRYOPROTECTANT DISTRIBUTION IN THE FREEZE-TOLERANT WOOD FROG

One of the critical adaptations for freeze tolerance by the wood frog, Rana sylvatica, is the production of large quantities of glucose as a n organ cryoprotectant during freezing exposures. Glucose export from the liver, where it is synthesized, and its uptake by other organs is dependent upon carrier-mediated transport across plasma membranes by g lucose-transporter proteins. Seasonal changes in the capacity to trans port glucose across plasma membranes were assessed in liver and skelet al muscle of wood frogs; summer-collected (June) frogs were compared w ith autumn-collected (September) cold-acclimated (5 degrees C for 3-4 weeks) frogs. Plasma membrane vesicles prepared from liver of autumn-c ollected frogs showed 6-fold higher rates of carrier-mediated glucose transport than vesicles from summer-collected frogs, maximal velocity (V-max) values for transport being 72 +/- 14 and 12.0 +/- 2.9 nmol . m g protein(-1). s(-1), respectively (at 10 degrees C). However, substra te affinity constants for carrier-mediated glucose transport (K-1/2) d id not change seasonally. The difference in transport rates was due to greater numbers of glucose transporters in liver plasma membranes fro m autumn-collected frogs. The total number of transporter sites, as de termined by cytochalasin B binding, was 8.5-fold higher in autumn than in summer. Glucose transporters in wood frog liver membranes cross-re acted with antibodies to the rat GluT-2 glucose transporter (the mamma lian liver isoform), and Western blots further confirmed a large incre ase in transporter numbers in liver membranes from autumn- versus summ er-collected frogs. By contrast with the liver, however, there were no seasonal changes in glucose-transporter activity or numbers in plasma membranes isolated from skeletal muscle. We conclude that an enhanced capacity for glucose transport across liver, but not muscle, plasma m embranes during autumn cold-hardening is an important adaptation that anticipates the need for rapid export of cryoprotectant from liver dur ing natural freezing episodes.