Subcellular distribution of ¹²⁵I-insulin by rat liver: Specificity and regulatory factors

Compared with pentobarbital-anesthetized, overnight-fasted rats injected with 200 μU ¹²⁵I-insulin/100 g rat (controls), animals injected simultaneously with 200 μU ¹²⁵I-insulin and 100,000 μU (4 μg) unlabeled insulin/100 g rat displayed a) a 56 and 75% reduction in peak accumulation of trichloroacetic acid (TCA)-precipitable and -soluble radioactivities, respectively, in crude liver homogenates; b) a fourfold reduction in rate of decline of TCA-precipitable radioactivity in the homogenates; and c) a decline of 64% in peak accumulation of TCA-precipitable radioactivity in the 105,000 g (microsomal) fraction, but only a 36% decline in the plasma membranes. A 2-min delay in the injection of 4 μg unlabeled insulin/100 g rat resulted in a 16 and 17% greater fall in TCA-precipitable radioactivity in the crude homogenate at 3 and 5 min after ¹²⁵I-insulin compared with controls but in a slower rate of decline of TCA-precipitable radioactivity thereafter. The simultaneous injection or 2 μg unlabeled glucagon and 200 μU ¹²⁵I insulin/100 g rat produced a slightly higher initial concentration and a slight delay in rate of decline of ¹²⁵I-insulin in plasma membranes compared with controls. The data are consistent with receptor-mediated internalization of insulin by the liver and suggest that insulin uptake might be controlled in an as yet unspecified way by the rate of intracellular processing and/or degradation of insulin rather than by the rate of transfer across the cell membrane. The results also suggest that the major portion of insulin removed by the liver is rapidly internalized and irreversibly bound in the microsomes.