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The mechanism regulating lipoprotein lipase (LPL) expression in adipose tissue was examined in rats in the conditions of different calorie intakes with and without streptozotocin-induced (STZ-) diabetes. The LPL activity released from adipose tissue was greater with the higher calorie intake (20 g of normal chow diet per day) than with the lower calorie intake (13 g of normal chow diet per day), and was greater in normal rats than in STZ-diabetic rats. The LPL activity was proportional to the serum insulin level in all conditions. Dot-blot analysis showed that the amount of LPL mRNA in adipose tissue was increased by the higher calorie diet and that the increase was less in the diabetic state. Expression of mRNA was also nearly parallel with the serum insulin level. LPL activity released from the heart was not affected by either the calorie intake or the diabetic state. These results suggest that the mechanisms of LPL expression in adipose tissue and the heart are different, and that LPL expression in adipose tissue was closely dependent on the insulin level.

Fibroblast-like cells from subcutaneous adipose tissue and epididymal adipose tissue were prepared from rats and their proliferation and [14C]deoxy-glucose uptake and triglyceride synthesis from [14C]palmitate were investigated. The proliferation of fibroblast-like cells from subcutaneous adipose tissue was greater than that from epididymal adipose tissue. Insulin enhanced the proliferation of fibroblast-like cells derived from subcutaneous adipose tissue, but not from epididymal adipose tissue. The uptake of [14C]deoxy-glucose by fibroblast-like cells from the two sources was similar. Triglyceride synthesis from [14C]palmitate by epididymal fibroblast-like cells was higher than that by subcutaneous fibroblast-like cells. The [14C]deoxy-glucose uptake and triglyceride synthesis from [14C]palmitate by fibroblast-like cells from the two sources were not enhanced by the addition of insulin. These findings suggest the existence of different types of adipocytes in subcutaneous and epididymal adipose tissues in terms of proliferation, and show that the proliferation of fibroblast-like cells from subcutaneous adipose tissue is regulated by insulin. The triglyceride synthesis by immature adipocytes from both adipose tissues was not affected by insulin.

Plasma lipoprotein lipid, triglyceride clearance, and postheparin lipolytic activity were measured in young (2 months old) and aged (24 months old) male Wistar-King rats. Very low density lipoprotein and low density lipoprotein lipids (cholesterol, triglyceride, and phospholipid) and high density lipoprotein cholesterol were increased significantly in aged rats compared with the levels in young rats. Oral oil loading increased the plasma triglyceride levels to a greater extent in aged rats, and the clearance of plasma triglyceride following intravenously administered lipid occurred at a lower rate in the old rats. Lipoprotein lipase activities in post-heparin plasma and adipose tissue were higher in the young than that in the aged. These results suggest that hypertriglyceridemia in aged rats might be partly due to the delayed clearance of triglyceride by lipoprotein lipase.

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