Insulin binding in epithelial cells isolated from human gallbladder.

The binding of 125I-insulin was studied in epithelial cells isolated from human gallbladder. Kinetic studies at 22 degrees C showed that binding of 125I-insulin to gallbladder cells was rapid (maximum 1 h.) and reversible. In the presence of added excess of unlabelled insulin, 50% of initially bound insulin is dissociated in 10-15 min. At apparent equilibrium (1 h at 22 degrees C) unlabelled insulin (10(-10) to 10(-7) M) inhibited competitively tracer (5 X 10(-11) M) binding, with 50% inhibition at about 2 X 10(-9) M. Scatchard analysis gave curvilinear plots, that may be attributed either to negative cooperativity, or to two orders of binding sites: In the first case, the extreme average affinities are Ke = 1.8 X 10(8) M-1, and Kf = 3.2 X 10(7) M-1 for 49,000 sites/cell. In the latter case, gallbladder cells present 8,000 sites/cell of high affinity (Kd = 1.16 X 10(-9) M) and 42,000 sites/cell of low affinity (Kd = 3.3 X 10(-8) M). In addition, in the same cellular preparation at 37 degrees C, insulin at a concentration of 10(-8) M significantly stimulated (p less than 0.01) 3H-leucine incorporation into acid-insoluble fraction by 1.4 +/- 0.1 fold at 30 min. A maximal effect is obtained at 10(-7) M insulin (2.11 +/- 0.06 fold). Our results suggest a stimulatory effect of insulin or related "insulin-like" peptides on growth in human gut.