Effects of insulin on glucose metabolism in isolated human fat cells

Isolated fat cells were used for the study of in vitro effects of insulin on glucose metabolism in human and rat adipose tissue. In human subcutaneous fat cells, effects of insulin could be detected at concentrations of glucose in the medium from 1 to 10 pmoles/ml. Cellular responsiveness was inversely propor- tional to the glucose level. At a constant concentration of 6 pmoles of glucose per ml, the effects of insulin at various con- centrations up to 500 pU/ml were investigated. At the highest concentration, which gave the maximal response, there was a 100% increase in the conversion of glucose-U-1% to glyceride- glycerol and a 40% increase in glucose oxidation. The dose- response curve was steepest between 2 and 20 pU/ml. Rat epididymal fat cells were much more responsive to in- sulin. Glucose lipogenesis and pentose cycle activity could also be demonstrated in rat cells, whereas these activities could not be shown in fat cells from human omental and subcutaneous tissue. The findings for human cells are attributed to changes in cellular activity during preparation. in vitro response to changes in insulin concentration over the physiological range has yet to be demonstrated in human adipose tissue. The present investigation was undertaken to further explore the effects of insulin on glucose metabolism in adipose tissue from adult man. Isolated fat cells were em- ployed in preference to intact tissues because epididymal adipose cells have proved to be more sensitive and repro- ducible in their responses to hormones than the fat pad (12). The results obtained here indicate that the uptake of glucose and its conversion to glyceride-glycerol by human fat cells are stimulated by concentrations of insulin similar to those assayed in plasma after glucose ingestion. However, glucose lipogenesis was notably ab- sent in this preparation.