Kinetic relationships between insulin receptor binding and effects on glucose transport in isolated rat adipocytes

The role of insulin receptor occupancy in the stimulation of glucose transport has been studied in isolated rat adipocytes. At 37 degrees C, under steady-state conditions, the time needed to fill the fraction of receptors (less than 1%) required for an initial measurable effect varied with insulin concentration from less than 10 s at 100 ng/mL to 90 s at 0.5 ng/mL. However, at all insulin concentrations there was an initial lag period before any activation was seen. The length of the initial lag was inversely related to the insulin concentration, lasting 2 min at 0.5 ng/mL and only 30-40 s at 5-500 ng/mL (maximal levels). A similar discrepancy was noted between dissociation of prebound insulin and the loss of insulin's effects on transport. At an insulin concentration of 0.3 ng/mL, half of the insulin effect was lost within 12 min; the t1/2 of dissociation was 8 min. When the insulin concentration was increased to 10 ng/mL, the t1/2 of dissociation increased only to 10 min while the t1/2 of deactivation was now 60 min. In conclusion, (1) kinetic studies reveal a time-requiring step between insulin binding and early effects on glucose transport, (2) a low level of insulin binding (less than 1% occupancy) is all that is necessary to initiate the insulin stimulus-response sequence, and (3) the rate of deactivation is closely related to the steady-state level of insulin binding, and with increasing insulin concentrations this rate slows and diverges from the rate of dissociation of insulin from receptors.