Membrane fluidity and hemilayer temperature sensitivity in trout hepatocytes during brief in vitro cold exposure

Fluorescent membrane probes were used to assess the fluidity of hepatocyte plasma membranes (PM) from 20 degrees C-acclimated trout after exposure to 20 and 5 degrees C. PM isolated from cells after 6 h at 5 degrees C were significantly more fluid [fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene (DPH)] than control membranes at both temperatures. The increased fluidity was sufficient to offset 45-50% of the cold-induced membrane ordering. In contrast, the fluidity of PM in intact cells from 20 degrees C-acclimated fish remained constant when exposed to 5 degrees C for a similar period. In addition, the fluidity of the inner hemilayer [1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene, p-toluenesulfonate (TMA-DPH)] was significantly less sensitive to temperature change than was the fluidity of the outer hemilayer [3-(p-(6-phenyl)-1,3,5-hexatrienyl)phenylpropionic acid (PA-DPH)]. Because the isolated membrane preparation was most likely enriched with canalicular membranes (based on 5'-nucleotidase recovery), these results suggest that the canalicular domain of the plasma membrane is preferentially modified during short-term cold exposure and that the fluidity of the inner hemilayer of the plasma membrane of intact cells is relatively temperature insensitive, thus requiring fewer modifications than the outer hemilayer during temperature acclimation.