ADAPTATIONS OF PLASMA-MEMBRANE GLUCOSE-TRANSPORT FACILITATE CRYOPROTECTANT DISTRIBUTION IN FREEZE-TOLERANT FROGS

Natural freeze tolerance in several anuran species involves the accumm ulation of high concentrations of glucose as a cryoprotectant in body fluids and tissues. The present study identifies an important new mole cular mechanism supporting freeze tolerance, an adaptive increase in t he 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 li ver and skeletal muscle was analyzed in two species, the freeze-tolera nt wood frog Rana sylvatica and the freeze-intolerant leopard frog Ran a pipiens. Membranes from both liver and muscle of R. sylvatica displa yed 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 V(max) values for glucose transpo rt by membrane vesicles were 69 +/- 18 and 8.4 +/- 2.3 nmol . mg prote in-1 . s-1 at 10-degrees-C for R. sylvatica and R. pipiens, respective ly. This difference was due primarily to a greater number of glucose t ransporters in R. sylvatica liver membranes; the total number of trans porter sites, determined by cytochalasin B binding, was 4.7-fold highe r in the freeze-tolerant species. For muscle membranes, the V(max) for glucose transport was 4.9 +/- 1 and 0.6 +/- 0.16 nmol . mg-1 . s- 1 a t 22-degrees-C for R. sylvatica and R. pipiens, respectively. However, in muscle there were no differences in the number of membrane transpo rters between species. We conclude that the greater capacity for plasm a membrane glucose transport in R. sylvatica is a key adaptation that facilitates the rapid export of glucose cryoprotectant from liver and its uptake into other organs during the early hours of freezing exposu re.