Is fluid-phase endocytosis conserved in hepatocytes of species acclimated and adapted to different temperatures?

Our primary objective was to determine if rates of fluid-phase endocytosis (FPE) were conserved in hepatocytes from organisms acclimated and adapted t o differ ent temperatures. To this aim, the fluorescent dye Lucifer yellow was employed to measure FPE at different assay temperatures (AT) in hepatoc ytes from 5 degrees C- and 20 degrees C-acclimated trout, Oncorhynchus myki ss (at 5 and 20 degrees C AT), 22 degrees C- and 35 degrees C-acclimated ti lapia, Oreochromis nilotica (at 22 and 35 degrees C AT), and the Sprague-Da wley rat (at 10, 20, and 37 degrees C AT). FPE was also studied in rats fed a long-chain polyunsaturated fatty acid (PUFA)-enriched diet (at 10 degree s C AT). Despite being temperature dependent, endocytic rates (values in pl .cell(-1).h(-1)) in both species of fish were compensated after a period of acclimation. For example, in 20 degrees C-acclimated trout, the rate of en docytosis declined from 1.84 to 1.07 when the AT was reduced from 20 to 5 d egrees C; however, after a period of acclimation at 5 degrees C, the rate ( at 5 degrees C AT) was largely restored (1.80) and almost perfectly compens ated (95%). In tilapia, endocytic rates were also temperature compensated, although only partially (36%). Relatively similar rates obtained at 5 degre es C in 5 degrees C-acclimated trout (1.8), at 20 degrees C in 20 degrees C -acclimated trout (1.84), and at 22 degrees C in 22 degrees C-acclimated ti lapia (2.2) suggest that endocytic rates are somewhat conserved in these tw o species of fish. In contrast, the rate in rat measured at 37 degrees C (1 6.83) was severalfold greater than in fish at their respective body tempera tures. A role for lipids in determining rates of endocytosis was supported by data: obtained at 10 degrees C in hepatocytes isolated from rats fed a l ong-chain PUFA-enriched diet: endocytic rates were higher (5.35 pl.cell(-1) h(-1)) than those of rats fed a standard chow diet (2.33 pl.cell(-1).h(-1)) . The conservation of endocytic rates in fish may be related to their abili ty to conserve other membrane characteristics (i.e., order or phase behavio r) by restructuring their membrane lipid composition or by modulating the a ctivities of proteins that regulate endocytosis and membrane traffic, where as the lack of conservation between fish and rat may be due to differences in metabolic rate.