Lipoprotein Lipase Activity in Skeletal Muscle Is Related to Insulin ...

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1202 Arteriosclerosis and Thrombosis Vol 11, No 5 September/October 1991 was expressed per gram. However, it may be questioned that LPL activity should be expressed per 10 6 cells because we analyzed the heparin-releasable (extracellular) part. Thus, by analogy, muscle LPL activity should have been expressed per 10 6 muscle fibers because muscle fiber size is also related to obesity. However, we regard the significant correlations between TG concentration in serum and muscle LPL activity expressed per gram as an indication of the importance of muscle LPL and as an indication that this way of expressing the LPL activity is physiologically relevant. Hepatic lipase activity is known to be regulated by sex steroids. For example, during treatment with estrogens, activity of this enzyme is decreased, but when estrogen is combined with a gestagen with androgenic properties, hepatic lipase activity is upregulated, being reflected primarily in increases and decreases, respectively, in HDL cholesterol concentrations. 41 " 43 However, an inverse relation between the degree of physical fitness (Vo 2max ) and hepatic lipase activity has been reported. 44 More physically fit men have lower insulin concentrations than do less fit ones. 4546 These observations are compatible with the present finding of a positive correlation between the area under the curve of insulin concentrations and hepatic lipase activity. It is thus possible that insulin (or insulinrelated cellular functions) may also be involved in the regulation of hepatic lipase activity. It may be suggested that the increases in hepatic lipase activity may have been partly due to overexposure of the liver to FFAs. This could induce hepatic insulin resistance, further diminish hepatic insulin extraction, 3 - 31 - 32 and reduce the hepatic catabolism of sex steroids. 47 In conclusion, this study has demonstrated that muscle LPL activity is correlated with the serum TG concentration in the fasting state and intravenous fat tolerance, indicating a role for impaired hydrolysis of circulating TGs in the hypertriglyceridemia of obesity and diabetes. Furthermore, muscle LPL activity was related to glucose infusion rate, suggesting that the hyperinsulinemia associated with insulin resistance may have a downregulating effect on muscle LPL activity. The present results demonstrate that several risk factors for coronary heart disease, such as serum TG and HDL cholesterol concentrations and blood pressure, are correlated with insulin resistance, supporting the concept that insulin resistance is an important underlying pathogenic condition. References 1. 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