Undiluted cytosolic extracts were prepared from fast glycolytic muscle tissue of white perch (Morone americanus). Diffusion coefficients (D) through the cytosol preparations were estimated in vitro for a series of selected low molecular weight compounds using an experimental diffusion chamber. Determinations were made at 5 degrees and 25 degrees C to assess thermal sensitivity of the process. Non-metabolizable analogues of naturally occurring compounds were employed to avoid chemical alteration of solutes by the catalytically competent preparations during diffusion experiments. Kinematic viscosity of cytosolic extracts, which is a major determinant of diffusive resistance, increases from 2.94 +/− 0.06 to 5.35 +/− 0.02 X 10(−2) cm2 s-1 between temperatures of 25 degrees and 5 degrees C (Q10 = 1.35 +/− 0.01). The diffusion coefficients (D) of D-lactic acid are 2.26 +/− 0.84 and 0.79 +/− 0.15 X 10(−6) cm2s-1 at 25 degrees and 5 degrees C, respectively (Q10 = 1.84 +/− 0.36). The D values of 2-deoxyglucose are 2.87 +/− 1.01 and 1.22 +/− 0.36 X 10(−6) cm2s-1 at 25 degrees and 5 degrees C (Q10 = 1.75 +/− 0.54). The D values of Ca2+ are 2.47 +/− 0.28 and 1.09 +/− 0.36 X 10(−6) cm2s-1 at 25 degrees and 5 degrees C (Q10 = 2.04 +/− 0.36). The D values for the ATP analogue, AMP-PNP, are 0.87 +/− 0.33 and 0.81 +/− 0.15 X 10(−6) cm2s-1 at 25 degrees and 5 degrees C (Q10 = 0.98 +/− 0.12). AMP-PNP is the only compound tested which did not show significant thermal sensitivity of diffusion. Recently reported changes in muscle cell ultrastructure induced by temperature acclimation of fishes may serve to counteract the effect of temperature change on diffusion of key small molecules through the aqueous cytoplasm, thus maintaining flux rates between cellular compartments. These mechanisms may be of considerable import in achieving relative temperature independence of cellular function that is characteristic of many eurythermal aquatic animals.

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