Insulin-induced insulin resistance of lipolysis in human adipocytes in organ culture.
Adipose tissue derived from open biopsies was used to develop a system for studying insulin resistance in human tissue in vitro. Subcutaneous adipose tissue obtained from obese donors was incubated in Parker's medium 199 in the absence or presence of insulin for 24 h under sterile conditions. Adipocytes were then isolated by collagenase digestion, washed thoroughly, and incubated for 2 h with multiple insulin concentrations in Krebs-Ringer phosphate buffer with 4% bovine serum albumin. Lipolysis was estimated by measuring glycerol. Basal lipolysis in adipocytes cultured with insulin did not differ significantly from that of adipocytes cultured without insulin (2.49 +/- 0.18 vs. 2.67+/- 0.58 mumol glycerol/mmol triglyceride). The maximum acute response in adipocytes prepared from tissue exposed to insulin during culture was 55% inhibition of basal lipolysis, whereas the maximum response in cells prepared from tissue not exposed to insulin chronically was 80%. Statistical analysis by paired t test showed a significant difference (P < 0.01) in the reaction of the two groups of cells to acute exposure to insulin. The insulin dose required to produce the half-maximal effect was increased from 3 to 24 microU/ml. Thus, after chronic exposure to insulin, adipocytes were not as responsive to the acute antilipolytic action of the hormone. We conclude that chronic exposure to insulin induces insulin resistance in human adipocytes.