Transport of Peptide Hormones Across Isolated Rat Mesentery: Effect of Human Serum-Bound Insulin

The exchange of ¹²⁵I-insulin, ¹²⁵I-glucagon, ¹²⁵I-proinsulin, ¹²⁵I-growth hormone, ¹³¹I-albumin, ¹⁴C-inulin, and ¹⁴C-dextran across isolated rat mesentery was studied in a diffusion cell. The passage of immunoprecipitable porcine ¹²⁵I-insulin (0.88 ng./ml.) was not affected by porcine proinsulin (145 ng./ml.), crystalline porcine insulin (17.4 ng./ml.), human growth hormone (87 ng./ml.), bovine serum albumin (4.5 mg./ml.), or normal guinea pig serum (840 μg. protein/ml.). However, the rate of insulin exchange was reduced by guinea pig anti-insulin antiserum and partially purified human serum-bound insulin (175 μg. protein/ml.). Bound insulin at the same concentration did not affect the exchange of ¹²⁵I-glucagon, ¹²⁵I-growth hormone, ¹⁴C-inulin, or ¹⁴C-dextran. Further purification of bound insulin by Sephadex G-100 chromatography yielded an approximately 45,000 molecular-weight fraction that at 5 μg. protein permilliliter allowed essentially no insulin transport. This same fraction of bound insulin significantly inhibited the disappearance of immunoprecipitable porcine ¹²⁵I-insulin from the incubation medium of isolated rat hemidiaphragms. Theses studies suggest that the transport of insulin across biologic membranes, mesothelium, and possible endothelium is specifically inhibited by bound insulin, a circulating macromolecule that possesses insulin-like activity.