The possible existence of regional differences in the antilipolytic action of insulin in human adipose tissue was investigated in vitro. Insulin-induced inhibition of glycerol release and insulin binding, measured in terms of receptor number, receptor affinity, and dissociation rates, were determined in omental and subcutaneous adipose tissue segments and isolated fat cells of 16 nonobese subjects who were undergoing elective abdominal surgery but were otherwise healthy. The sensitivity of the antilipolytic effect of insulin was higher in subcutaneous than in omental adipose tissue; the half-maximal effect was obtained with 1 and 3 microU/ml of insulin, respectively (P less than 0.01). Responsiveness of the antilipolytic effect of insulin was threefold higher in the subcutaneous than in the omental region (P less than 0.005). Insulin receptor affinity was significantly higher in subcutaneous than in omental fat cells, but there was no difference in receptor number (about 300,000 sites/cell). 125I-insulin dissociated more rapidly from omental than from subcutaneous adipocytes in both the absence and the presence of excess native insulin. The data suggest that significant regional differences exist in the antilipolytic action of insulin in man; omental fat being less responsive than subcutaneous fat. The difference involves the insulin receptor affinity, which is caused at least partly by variations in the insulin dissociation rate but is also due to differences in insulin action at the postreceptor level.
The glucuronyltransferase activity towards morphine was measured in microsomes isolated from liver specimens obtained from human fetuses and cancer patients. All the fetal livers investigated had measurable UDP-glucuronyltransferase activity towards morphine. There was no correlation between the gestational age (15 to 27 weeks) and the glucuronidation rate. The mean value of the enzymatic activities was higher in fetal livers obtained by hysterotomy (0.20 nmoles x min-1 x mg-1) than in livers obtained after induced abortion (0.11 nmoles x min-1 x mg-1). The average rate of glucuronidation in microsomes from adult liver (mean 1.15 nmoles x min-1 x mg-1) was 6 to 10 times higher than in the fetal liver microsomes. Together with previous investigations on human adult and fetal liver glucuronidation, the present results support the theory of heterogeneity of human UDP-glucuronyltransferase.
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