In vivo imaging and quantitative analysis of insulin-receptor interaction in lean and obese Zucker rats

Abstract

Summary, Imaging and quantitative analysis of insulin-receptor interaction was studied in vivo in lean and obese Zucker rats, using a recently developed technique in which purified Tyr M4 123I-monoiodoinsulin is intravenously injected and the tracer followed by scintillation scanning. The obese rats were 72% overweight, had near normal blood glucose concentrations and an ll-fold increase in plasma insulin concentration. In both groups of rats, the tracer was rapidly taken up by the liver (by a receptor mediated mechanism) and the kidneys (by a non-receptor mediated process). Past this maximum, radioactivity decreased in both organs as 123I-insulin was degraded and free ~z3I-iodide was released into the plasma compartment. Heart radioactivity (i. e. blood pool) mirrored that of the liver and kidneys. The rapid initial decrease of blood radioactivity was concomitant with liver and kidney uptake of 123I-insulin. Release of free iodide from these organs induced a slow secondary rise of blood radioactivity followed by a final decline corresponding to clearance of plasma iodide, mainly by urinary excretion. Liver radioactivity profiles of lean and obese rats were parallel. When expressed per g weight, liver radioactivity was significantly decreased in obese rats. However, due to hepatomegaly in obese rats, total liver radioactivity was significantly higher in homozygous fa/fa rats than in lean littermates. Furthermore, if the marked hyperinsulinaemia of the obese rats is taken into account, total bound insulin was enhanced in the liver of fa/fa rats whatever reference is used, either g weight or total liver. The kidney profile of radioactivity of both rats was not significantly different. In conclusion: (1) obese rats are insulin resistant as near normal glycaemia is achieved at the price of a marked hyperinsulinaemia; (2) liver uptake of insulin is enhanced in obese rats, and (3) the insulin resistance syndrome of fa/fa rats is not due to a decrease in liver insulin receptor number and/or affinity but rather to as yet unknown event(s) subsequent to receptor binding.

Differing effects of antiinsulin serum and antiinsulin receptor serum on 123I-insulin metabolism in rats.

“…Computer analysis of kinetic data was performed as previously described (2). Briefly, the actual injected radioactivity (microcuries) was measured by counting the activity of the '23I-insulin-containing syringe before and after injection.…”

Scintigraphic distribution of complexes of antiinsulin antibodies and 123I-insulin. In vivo studies in rats.

“…cretion and active transport by the gastric mucosa, salivary and thyroid glands, so that at a later time after '231-ins injection, e.g. 30 min, the biodistribution of radioactivity is essentially that of iodide and characterized by dense activity in the bladder, stomach, and neck (8,19,21).…”

“…3 A). The radioactivity in the stomach has been demonstrated to be due to 123I-iodide from the degradation of '231-ins (8,19). The quantitative determination of the percentage of apparent total radioactivity in the liver and spleen of the AIIgG2-1231-ins injected rat was 96% (Table III).…”

“…Dynamic images were analyzed as previously described ( 19). Briefly, regions of interest were defined over the right upper part of the liver and the lower pole of each kidney, and the heart and time-activity curves were generated.…”

Implications of the Dominant Role of Transporters in Drug Uptake by Cells (Supplementary Material)

“…However, in the general drug discovery environment a drug-specific uptake assay will usually not be available, and so general systems that can be applied to track virtually any molecule are preferable. Radiolabelled drugs are widely used in transporter activity assays, uptake being determined by scintillation counting on extracted cell contents [225][226][227][228][229][230][231][232]. However, it is often difficult to source radiolabelled compounds.…”