Frozen storage of ovine and rat tissues adversely affects lipoprotein lipase activity

Abstract

The purpose of this study was to determine effects of frozen storage (frozen in liquid N, stored for 6 weeks at −80°C) of ovine adipose, and skeletal and cardiac muscle tissues on dietary induced changes in total (intra- and extracellular) lipoprotein lipase (LPL) activity previously determined in fresh tissue. Ewes were grown on either a low- (LE) or high-energy (HE) diet for 120 days (six per diet). In fresh tissue of HE ewes, LPL activity was higher ( P < 0.05) for adipose tissue, and lower ( P < 0.05) for skeletal and cardiac muscles than fresh tissues of LE ewes. Frozen storage altered these dietary effects and decreased ( P < 0.01) LPL activity in adipose and cardiac tissues; treatment differences were maintained in skeletal muscle, and LPL activity was similar for fresh and frozen skeletal muscle of LE ewes. In subsequent experiments, ovine adipose and cardiac tissues were subjected to frozen storage, and LPL activity assayed with either a protease inhibitor (aprotinin, 2 μg/mL), or with 1 mM/L EGTA and without Ca 2+ in the medium. Either treatment inhibited loss of LPL activity in frozen tissues. Frozen storage of hearts from five male, Sprague-Dawley rats decreased ( P < 0.01) LPL activity; no decrease in activity was observed when protease inhibitor was used. However, it is not certain whether inclusion of protease inhibitor and (or) elimination of Ca 2+ in frozen-thawed tissue prevents loss of effects on LPL activity that would be observed in fresh tissue in reponse to dietary changes or other physiological perturbations. In conclusion, frozen storage causes loss of LPL activity in ovine and rat tissues. Thus, frozen storage of tissue may be inappropriate when subsequent analysis of LPL activity is of interest.