Binding and degradation of 125I-insulin by renal glomeruli and tubules isolated from rats.

Abstract

Isolated rat renal glomeruli and tubules were shown to exhibit specific binding of 125I-insulin and enzymatic degradation of the hormone. Binding to both renal fractions reached a plateau by 1h at 22 °C and increased linearly with increasing protein concentrations. Binding was inhibited in both preparations by insulin and its analogues in the order of relative potency: insulin > despentapeptide insulin > proinsulin, but insulin was ten times more potent in inhibiting 125I-insulin binding to glomeruli than that to tubules, indicating a different affinity of receptors for the hormone in the two renal fractions (about 17 versus 210 μg unlabelled insulin/l inhibiting 50% of the 125I-insulin binding to glomeruli and tubules, respectively). Bound 125I-insulin dissociated at a faster rate from tubules than from glomeruli; this release was accelerated by unlabelled insulin in both renal fractions, but to a greater extent in glomeruli than in tubules. Two-thirds of the total bound material released from glomeruli was found to be intact insulin as measured by trichloroacetic acid precipitation, whereas only one-third of the material released from tubules was intact. No direct relationship between binding and degradation of 125I-insulin in these renal fractions could be demonstrated, however, because of the release of proteolytic enzymes into the incubation medium resulting in almost all degradation being extracellular. Although differing in their affinity for 125I-insulin the high affinity glomerular insulin receptor and the lower affinity tubular insulin receptor have characteristics similar to those of insulin receptors in insulin responsive tissues.