55.1 Saturday, Jan. 5 Glycerol Production and Cellular Uptake Mechanisms in Rainbow Smelt DRIEDZIC, W. R. ; Memorial University, NL, Canada email@example.com
Rainbow smelt is a small (≈10cm) fish that feeds under the sea ice during winter. Freeze resistance is achieved in part by the accumulation of high levels of glycerol (200 – 400 mM). Glycerol is produced primarily in the liver and builds up in all tissues via delivery through the circulatory system. Initial glycerol production is fuelled by liver glycogen but thereafter animals must continue to feed to survive as glycerol is continuously lost across the gills and skin at a rate of ≈10% per day. Dietary protein serves as a major source of glycerol. Regardless of the source of glycerol, the final metabolic steps involve the conversion of DHAP to G3P to glycerol. The glycerol cycle is controlled at the level of G3DPH, PEPCK, PDH, and enzymes of amino acid trafficking. Cellular uptake mechanisms remain to be addressed. Tissue glycerol equilibrates with glycerol in the plasma. At least in heart, glycerol appears to enter cells by passive diffusion down the concentration gradient with a linear relationship between extracellular glycerol and rate of uptake. Red blood cells (RBCs) present a different dynamic. In RBCs glycerol uptake shows two linear relationships with a transition point around 50 mM extracellular glycerol. The slope of the second phase is much steeper and is eliminated by phloretin, a blocker of facilitated transport. I propose RBCs have a low affinity aquaglyceroporin (AQGP) that facilitates glycerol entry at relatively high levels of extracellular glycerol. I further speculate that the presence of such an AQGP relates to the unique loading/unloading demand placed upon RBCs and no other tissues. Theoretically, RBCs show loose glycerol upon transiting the gill. Upon passage through the liver circulation the RBCs should reload at high extracellular glycerol levels.