Postfreeze viability of erythrocytes from Dryophytes chrysoscelis

Dryophytes chrysoscelis (formerly Hyla chrysoscelis, Cope's gray treefrog) is a freeze-tolerant anuran that accumulates glycerol and urea during cold acclimation and freezing. It is hypothesized that glycerol and urea function as cryoprotectants by minimizing osmotically induced cell damage during freezing and thawing, thereby improving the postfreeze viability of red blood cells (RBCs) when frozen in medium containing those solutes. To test this, erythrocytes were obtained from warm (22°C) and cold-acclimated (4°C) frogs and suspended in 280 mOsM phosphate-buffered saline (PBS). RBCs were frozen in 280 mOsM, isosmotic/isotonic, PBS, or in PBS made hyperosmotic by addition of 150 mM solutes. Postfreeze viability was determined with a hemolysis assay. Postfreeze viability of cells from warm-acclimated frogs improved from 18.9 ± 1.3% in PBS to 47.4 ± 5.2% in PBS with urea ( p < 0.01). The addition of other solutes (glycerol, glucose, NaCl, or sorbitol) had no effect. RBCs from cold-acclimated frogs had 45.8 ± 3.4% viability when frozen in 280 mOsM PBS, and this improved to 71.6 ± 8.9% or 71.9 ± 1.6%, respectively, when frozen with glycerol ( p < 0.01) or urea ( p < 0.001). The viability of RBCs from cold-acclimated frogs was not different between unfrozen cells 86.7-88.4%) and those frozen with glycerol (71.6 ± 8.9%, p > 0.05) or with urea (71.9 ± 1.6%, p > 0.05). These data suggest that (a) cold acclimation induces cellular changes in RBCs that result in improved postfreeze viability, and (b) glycerol and urea are part of a complex cryoprotectant system in D. chrysoscelis.