Interaction of somatomedin-C with an antibody directed against the synthetic C-peptide region of insulin-like growth factor-I

Insulin-like growth factor-I is a human plasma peptide with strong structural homology to human prosinsulin. This peptide has been classified as a somatomedin on the basis of its biological actions and growth hormone dependence. We have generated an antibody to the synthetic 12 amino acid C-peptide region of insulin-like growth factor-I and used it to compare three somatomedin preparations to insulin-like growth factor-I. We also compared these somatomedin preparations to insulin-like growth factor-I using the standard SM-C RIA. We conclude that insulin-like growth factor-I and somatomedin-C behave identically in these two RIA's and are very similar if not identical molecules.

Comparison of [125I]somatomedin A and [125I]somatomedin C radioreceptor assays for somatomedin peptide content in whole and acid-chromatographed plasma

The placental membrane radioreceptor assay was used to measure the levels of somatomedin (SM) peptides in plasma. Displacement of both [125I]somatomedin A ([125I]SM-A) and [125I]somatomedin C ([125I]SM-C) by normal whole plasma, the peptide fraction of acid-chromatographed plasma, and a partially purified, insulin-free SM preparation were compared. The peptide fraction of plasma was isolated by acid chromatography over Sephadex G-50 in 0.25 M formic acid with a yield of greater than or equal to 90%, as determined by bioassay and [125I]SM. In the case of [125I]SM-A, the dose-response curves for whole plasma, acid-chromatographed plasma, and the standard SM preparation were parallel (P less than 0.2). In contrast, for [125I]SM-C, the dose-response curves for acid-chromatographed plasma and the purified SM preparation were parallel (P less than 0.2), but both differed significantly from that of whole plasma (P less than 0.001). In addition, there was less variability in the assay of acid-chromatographed plasma compared to whole plasma. The results indicate that radioreceptor assay of unextracted normal plasma using [125I]SM-A is a valid measure of SM peptide concentration, while radioreceptor assay of unextracted normal plasma using [125I]SM-C, in our hands, is not. Acid chromatography of plasma before its assay is an uncomplicated procedure which allows valid and precise measurement of SM peptide content using either [125I]SM-A or [125I]SM-C.

Synthesis of heteroaromatic potential beta-adrenergic antagonists by the glycidol route

The synthesis of several 3-alkylamino-2-hydroxypropyl heteroaryl ethers (13-15, 17, and 18) is described. These compounds were prepared by the alkylamination of the corresponding glycidyl ethers (6-8, 10, and 11), which in turn were obtained from the requisite heteroaryl halides and the sodium salt of glycidol. The above basic ethers exhibited beta-blocking activity, but the potency of the tested compounds was considerably less than that of propanolol. Only 3-tert-butylamino-2-hydroxyl-1-(1,2,4-thiadiazol-5-yl) propyl ether (13) showed some selective myocardial beta-blocking activity.

Demonstration of specific plasma protein binding sites for somatomedin

Native somatomedin (SM) has an apparent mol wt of at least 60,000 daltons, while all the purified SM peptides have mol wt 5-8,000 daltsons. We have studied the behavior of somatomedin in plasma during gel filtration at pH 8.1 and pH 2.8, and by the recombination of fractions. Using porcine bioassay for SM, displacement of 125I-insulin by SM, and binding of 125I-SM to plasma proteins, we can demonstrate that the disparity in apparent mol wt of SM is due to reversible binding to plasma proteins which are themselves biologically inactive. This binding site is specific for the SM peptides, saturable, and has high affinity and relatively low capacity.

Comparison of 125-I-insulin binding and degradation to isolated rat hepatocytes and liver membranes

We have compared the ability of rat liver plasma membranes and isolated hepatocytes to bind and degrade insulin. Isolated cells were prepared in two different ways: by mechanical separation of cells and by collagenase digestion of extracellular matric. In all studies the binding and degradative characteristics of both types of hepatocyte preparations were identical. Furthermore, with one exception, the binding characteristics of membranes and cells were also quite similar. The only exception concerned the amount of insulin bound by hepatocytes as compared to liver membranes. Thus, at concentrations of cells (1.2 times 10(6) cells per milliliter) and membranes (150 mug. protein per milliliter) that gave equal binding at insulin concentrations less than 100 ng./ml., the amount of insulin specifically bound at insulin concentrations greater than 100 ng./ml. was greater with use of hepatocytes. Additional studies indicated that, in contrast to membranes, at the higher insulin concentration only 75 per cent of the previously bound insulin could be recovered from hepatocytes. Thus, a nondissociable component exists; which probably represents intracellular radioactivity and appears to account for the higher specific insulin binding by cells at higher insulin concentrations. When insulin degradation was studied at the above hepatocyte and plasma membrane concentrations, cells degraded 30 per cent more insulin than did membranes. Kinetic analysis of these data revealed that the Km for insulin degradation (5 times 10(-7) M at 37 degrees) was the same for both systems wereas the Vmax was greater with use of hepatocytes.

IN CONCLUSION

(1) Preparation of hepatocytes by collagenase digestion does not appear to alter insulin binding or degradation; (2) studies of liver membranes and isolated hepatocytes obtained from normal rats should yield similar information about insulin-receptor interaction as long as insulin concentrations less than 100 ng./ml. are used; (3) at very high insulin concentrations, some of the radioactivity appears to enter the cells; (4) the kinetics of insulin degradation by hepatocytes and liver membranes are similar; and (5) insulin degradation appears to be primarily a membrane phenomenon.

The effects of acute and chronic dexamethasone administration on insulin binding to isolated rat hepatocytes and adipocytes

In an effort to determine the possible relationship between changes in insulin-receptor binding and the glucocorticoid-induced insulin-resistant state, we studied insulin binding to specific receptors located on isolated adipocytes and hepatocytes obtained from dexamethasone (D)-treated rats. Three groups of D-treated rats were studied: (1) acute high-dose treatment (1.5 mg/kg/6 days), (2) acute low-dose treatment (0.125 mg/kg/6 days), and (3) chronic low-dose treatment (0.125 mg/kg/21 days). When insulin binding to isolated hepatocytes was studied, we found that binding to isolated hepatocytes was studied, we found that binding was only 30%-50% of control values when cells from the D-treated animals were used. This decrease in binding was greatest for cells from the acute high-dose group, indicating a dose-response effect, and least for cells from the chronic group, suggesting a tendency toward return of insulin-receptor binding during chronic treatment. When insulin binding to isolated adipocytes was studied, binding was 50%-60% of control values when cells from both acute D-treated groups were used. While the magnitude of the decrease in insulin binding was not as great as that seen with hepatocytes, the decrease was still greatest using cells from the acute high-dose group as compared to the acute low-dose group. Thus, a dose-response effect was suggested in both tissues. On the other hand, the effects of chronic D treatment on insulin binding were strikingly different in the two cell systems. After chronic D treatment, insulin binding to adipocytes returned to near-normal levels, while a 55% decrease in binding to hepatocytes persisted. Thus, the tendency toward return of insulin binding after chronic D treatment seen with hepatocytes was almost fully expressed by adipocytes. This might be related to the amelioration of the corticosteroid-induced insulin-resistant state which has been reported after chronic corticosteroid administration to humans. In conclusion, (1) a decrease in insulin binding is associated with corticosteroid excess, and it is possible that this decreased binding is related to the insulin resistance which results from corticosteroid administration; (2) the return of insulin binding toward normal after chronic D treatment could well be related to the improvement in insulin resistance seen during chronic corticosteroid administration to humans; and (3) the difference in effects of chronic D treatment on insulin binding to hepatocytes versus adipocytes indicates that changes in insulin binding can be tissue specific.

Long term results of gastrectomy with respect to blood lipids, blood pressure, weight and living habits

A sample of ambulant Japanese-American men (ages 45-69 years), was divided into those having a previous partial gastrectomy and a control non-gastrectomy population. Three-hundred-and-forty-seven men with a history of partial gastrectomy weighed less and had lower values for serum cholesterol, triglyceride, and blood pressure than did the control population of 7,598 men. The depressed lipid and blood pressure values could not be entirely explained by the reduced weight. Likewise, none of these differences appeared related to diet or living habits. Those operated on for gastric ulcer had, on the average, lower systolic pressures than duodenal ulcer patients and those with gastrojejunal anastamoses had lower cholesterol levels than patients with a gastroduodenostomy.