Retention and degradation of 125I insulin by perfused livers from diabetic rats

Abstract

The retention and degradation of insulin by isolated perfused liver have been examined. Noncyclically perfused livers from streptozotocin diabetic rats retained 25% and degraded 10% of 125I insulin administered as a 1 min pulse. On gel filtration (Sephadex G50F), the degradation products released into the vascular effluent eluted in the salt peak. During the 45 min interval after the end of the 125I insulin infusion, 0.19% of the total dose was excreted in the bile. 60-90% of this material consisted of iodinated, low molecular weight degradation products. Inclusion of native insulin with the 125I insulin in the pulse depressed both the retention and degradation of iodinated material; however, this reflected increased retention and degradation of the total insulin dose (125I insulin plus native hormone). The log of the total amounts of insulin retained and degraded were linearly related to the log of the total amount of insulin infused at concentrations between 127 nM and 2.84 μM. Increasing the amount of native insulin in the infused pulse also depressed the total amount of iodinated material found in the bile and led to the appearance in the bile of intermediate sized degradation products that did not simultaneously appear in the vascular effluent. Addition of high concentrations of glucagon to the infused 125I insulin had no effect on the retention or degradation of the labeled hormone, or on the apparent size and amount of iodinated degradation products found in the vascular effluent or in the bile. Preinfusion of concanavalin A inhibited both 125I insulin retention and degradation. A greater depression by concanavalin A of degradation than binding was also observed with isolated hepatocytes. In contrast to 125I insulin, the retention and degradation of two iodinated insulin analogues of relative low biological potency, proinsulin and desalanyl desasparaginyl insulin, were small. The amount of radioactivity appearing in the bile after infusion of these analogues was almost negligible. However, degradation products of these analogues that appeared in the bile of the vascular effluent were qualitatively similar to those found after the infusion of 125I insulin. Our findings suggest that the rapid initial uptake of 125I insulin after its infusion into noncyclically perfused liver, as well as its subsequent degradation, behaves in a qualitatively similar fashion to the binding of 125I insulin and its degradation by isolated rat hepatocytes. This uptake and the subsequent phase of degradation may be attributable to binding of insulin at specific recognition sites, preliminary to its transfer to a degradative site(s) presumed to be located inside the cell.