Time course of thermal adaptation in plasma membranes of trout kidney. I. Headgroup composition.

The phospholipid composition of plasma membranes from the kidney of rainbow trout, Salmo gairdneri, was determined over a period of 21 days as fish were acclimating between temperatures of 5 and 20 degrees C. Proportions of phosphatidylethanolamine (PE) were significantly higher (29.03 vs. 23.26%) in membranes of 5 degrees C- than 20 degrees C-acclimated trout, whereas levels of phosphatidylcholine (PC) were similar. During acclimation, changes in headgroup composition were restricted primarily to PE and PC, with proportions of the former increasing and the latter decreasing on cold exposure. Headgroup composition changed rapidly as indicated by variations in PC/PE, which dropped from 1.71 +/- 0.13 to 0.78 +/- 0.11 within 8 h of cold acclimation and rose from 1.31 +/- 0.15 to 2.0 +/- 0.13 by the 2nd day of warm acclimation. Differences between warm- and cold-acclimating fish in the proportion of both PC and PE attained during the acclimation time course exceeded the magnitude of the differences between fully acclimated fish. Because of differences in molecular geometry and physical properties between PC and PE, the observed changes in headgroup composition may result in thermal compensation of membrane function, and the more rapid response to cold as opposed to warm stress suggests a primary role in adaptation to cold temperatures. However, headgroup modulation is most likely to contribute to homeoviscous adaptation only during the initial stages of the acclimatory response.