Physical-chemical and biochemical differences in liver plasma membranes in aging F-344 rats

Age-associated differences in the microviscosity and the activities of enzymes (Mg++-ATPase, Na+,K+-ATPase and 5'-nucleotidase) in the liver plasma membrane were investigated in male and female rats of various ages ranging from 2 to 30 months. The membrane microviscosity, as determined by fluorescence polarization using 1,6-diphenyl-1,3,5,-hexatriene (DPH) as a probe, increased progressively with age after 2 months in male rats, whereas in female rats the microviscosity began to increase only after 24 months. On the other hand, age-associated differences in the activities of membrane-bound enzymes were generally minimal or not significant with the exception of the 5'-nucleotidase activity determined at the pH 9.1, which progressively decreased with age in male rats. The Na+, K+-ATPase activity tended to decrease in both sexes. These decrements in activity did not appear to be large enough, however, to be of definitive physiological significance. These results suggest that age may change the physical-chemical and biochemical qualities of the rat's hepatocyte plasma membrane, but the relationship between the membrane microviscosity and the activities of membrane bound enzymes is not a simple parallelism.