1. Isoprenaline (1 μmol/l) and glucagon (1 μmol/l) raised the levels of cyclic AMP in isolated adipose cells of the rat to a maximum after incubation for approximately 15 min.
2. The effect of glucagon was impaired by pretreatment of adipose cells with trypsin (0.2 mg/ml and 2.0 mg/ml) and N-ethylmaleimide (5 mmol/l). The β-adrenergic receptor was insensitive to such forms of treatment.
3. Insulin (430 μunits/ml) lowered the intracellular levels of cyclic AMP in adipose cells stimulated with isoprenaline (1 μmol/l) and glucagon (1 μmol/l). This effect was observed after incubation for 5 min.
4. Pretreatment of cells with trypsin (0.2 mg/ml and 2.0 mg/ml) and N-ethylmaleimide (0.5 and 5 mmol/l) abolished the effect of insulin in decreasing the intracellular levels of cyclic AMP. At the higher concentration of trypsin a rise in intracellular levels of cyclic AMP was observed in the presence of insulin.
5. Similar concentrations of trypsin and N-ethylmaleimide decreased the disappearance of unlabelled insulin from the incubation medium and also decreased the binding of 125I-labelled insulin to isolated fat cells.
6. The effect of insulin on decreasing the intracellular levels of cyclic AMP in modified adipose cells significantly correlated with the disappearance of unlabelled insulin from the medium and with the percentage of total 125I-labelled insulin bound to cells.
7. The possibility is discussed of using the disappearance of insulin or binding of 125I-labelled insulin to adipose cells as a measure of insulin-receptor availability on the plasma membrane of fat-cells.
Insulin-Receptor Interaction in Isolated Fat Cells
