Scintigraphic distribution of 123 I labelled proinsulin, split conversion intermediates and insulin in rats

Pharmacokinetic profile of orexin A and effects on plasma insulin and glucagon in the rat

Orexin A (OXA) is found in the central nervous system (CNS) and in the gut. Peripheral administration of OXA to rats results in an inhibition of fasting motility. Plasma OXA increases during fasting and central administration of OXA increases food intake. The aim of the present study was to assess the pharmacokinetic profile of OXA and the effect of intravenously (i.v.) administered OXA on plasma concentrations of insulin and glucagon concentrations. Rats were given OXA i.v. (100 pmol kg(-1) min(-1)) for time periods of 0, 10, 20, 30 min and for 10, 20, 30 min after ceasing a 30-min infusion. After each time period, rats were then sacrificed and blood obtained. OXA was also administered at increasing doses (0, 100, 300 and 500 pmol kg(-1) min(-1)) for 30 min and blood was obtained. Plasma OXA, insulin and glucagon levels were measured using commercially available radioimmunoassay (RIA) kits. The plasma half-life of OXA was 27.1+/-9.5 min. Stepwise increasing infusion rates of OXA confirmed a linear concentration-time curve and thus first-order kinetics. Its volume of distribution indicated no binding to peripheral tissues. Plasma glucagon decreased during infusion of OXA, while insulin was unaffected. Plasma OXA was raised fourfold after food intake. Thus, OXA has a longer plasma half-life than many other peptides found in the gut. This needs to be taken into account when assessing effects of OXA on biological parameters after peripheral administration.

Cell itinerary and metabolic fate of proinsulin in rat liver: in vivo and in vitro studies

Proinsulin, the insulin precursor in pancreatic beta-cells, displays a slower hepatic clearance than insulin and exerts a more prolonged metabolic effect on liver in vivo. To elucidate the mechanisms underlying these differences, the cellular itinerary and processing of proinsulin and insulin in rat liver have been comparatively studied using cell fractionation. As [125I]-insulin, [125I]-proinsulin taken up into liver in vivo was internalized and accumulated in endosomes, in which it underwent dissociation from the insulin receptor and degradation in a pH- and ATP-dependent manner. However, relative to [125I]-insulin, [125I]-proinsulin showed a delayed and prolonged in vivo association with endosomes, a slower in vivo and cell-free endosomal processing, and a higher cell-free endosome-lysosome transfer. Endosomal extracts degraded to a lesser extent proinsulin than insulin at acidic pH; so did, and even proportionally less, at neutral pH, plasma membrane and cytosolic fractions. Proinsulin degradation products generated by soluble endosomal extracts were isolated by HPLC and characterized by mass spectrometry. Under conditions resulting in multiple cleavages in insulin, proinsulin was cleaved at eight bonds in the C peptide but only at the Phe24-Phe25 bond in the insulin moiety. As native insulin, native proinsulin induced a dose- and time-dependent endocytosis and tyrosine phosphorylation of the insulin receptor; but at an inframaximal dose, proinsulin effects on these processes were of longer duration. We conclude that a reduced proteolysis of proinsulin in endosomes, and probably also at the plasma membrane, accounts for its slower hepatic clearance and prolonged effects on insulin receptor endocytosis and tyrosine phosphorylation.

Importance of validation of immunoassays for intact proinsulin

The aim of this study was to compare results obtained from two commercially available immunoassay kits for intact proinsulin. Fasting and post-prandial samples were obtained from both healthy subjects and patients with type 2 diabetes mellitus and assays were carried out according to the manufacturers' instructions. Coefficient of variation of the duplicates in both assays was acceptable with the MLT Intact Proinsulin assay giving slightly better overall precision. Regression analysis indicated a good correlation between the assays (r=0.97), however, a procedure better designed to compare analytical methods demonstrated a considerable lack of agreement for some samples. Dilution of samples in the Dako assay greatly affected the results when compared to samples assayed undiluted, whereas in the MLT assay, dilution of samples produced the expected results. Repeat comparison, assaying samples neat in the MLT assay and diluted 1:5 in the Dako assay, resulted in a considerable improvement in the agreement between the Dako and MLT assays. This study underlines the importance of the use of validation procedures which demonstrate quantitative analytical recoveries from a variety of specimens over the working range of the assay method in question.

Insulin-like growth factor binding protein-1 in NIDDM: relationship with the insulin resistance syndrome

OBJECTIVE

In order to examine the role of insulin-like growth factors in the pathogenesis of accelerated macrovascular disease in noninsulin-dependent diabetes mellitus (NIDDM), we investigated the relationship between the insulin resistance syndrome and the IGF axis.

DESIGN

Cross-sectional analysis of the relationship between insulin resistance syndrome variables and concentrations of IGF-1, IGF-2, IGFBP-1 and IGFBP-3 in 80 subjects with NIDDM.

RESULTS

After correcting for age, sex and body mass index, concentrations of IGFBP-1, correlated with those of HDL-cholesterol (r = 0.40; P < 0.001), triglycerides (r = -0.24; P = 0.04), insulin (r = -0.39; P < 0.001), intact proinsulin (r = -0.32; P = 0.006), des 31,32 proinsulin (r = -0.40; P = 0.001), and with insulin sensitivity (r = 0.38; P = 0.001) and PAI-1 activity (r = -0.24; P = 0.05); IGF-1 levels only correlated with those of HDL-cholesterol (r = -0.33; P = 0.005), and this was not explained by IGFBP-1 or insulin sensitivity. With additional correction for insulin, concentrations of IGFBP-1 still correlated with HDL-cholesterol (r = 0.40; P < 0.001), but not those of triglycerides or PAI-1 activity. There were no significant relationships between levels of IGF-2 and any of the variables investigated, and IGFBP-3 levels only correlated with those of total cholesterol (r = 0.24, P = 0.04).

CONCLUSIONS

In NIDDM, concentrations of IGFBP-1 are related to those of insulin, insulin sensitivity, serum lipoproteins and PAI-1 activity. The relationship between concentrations of IGFBP-1 and HDL-cholesterol is not explained by insulin. Concentrations of IGF-1 are linked to HDL-cholesterol, and this is not explained by levels of IGFBP-1. IGFBP-1 concentrations were related to PAI-1 activity, and this may be explained by insulin, which regulates the production of IGFBP-1 and PAI-1.

First direct assay for intact human proinsulin

We describe a sensitive two-site sandwich enzyme-linked immunosorbent assay for the measurement of intact human proinsulin in 100 μL of serum or plasma. The assay is based on the use of two monoclonal antibodies specific for epitopes at the C-peptide/insulin A chain junction and at the insulin B chain/C-peptide junction, respectively. Cross-reactivities with insulin, C-peptide, and the four proinsulin conversion intermediates were negligible. The detection limit in buffer was 0.2 pmol/L (3 standard deviations from zero). The working range was 0.2–100 pmol/L. The mean intra- and interassay coefficients of variation were 2.4% and 8.9%, respectively. The mean recovery of added proinsulin was 103%. Dilution curves of 40 serum samples are parallel to the proinsulin calibration curve. Proinsulin concentrations in 20 fasting healthy subjects were all above the limit of detection: median (range), 2.7 pmol/L (1.1–6.9 pmol/L). Six fasting non-insulin-dependent diabetes mellitus and five insulinoma patients had proinsulin concentrations significantly higher than healthy subjects: median (range), 7.7 pmol/L (3.2–18 pmol/L) and 153 pmol/L (98–320 pmol/L), respectively.

Insulin propeptides in conditions associated with insulin resistance in humans and their relevance to insulin measurements

Routine insulin assays measure not only biologically active insulin but also the relatively inactive propeptides, proinsulin and desdipeptide proinsulin. Such measurements may be misleading if insulin propeptide levels are increased, as has been reported in patients with non-insulin-dependent diabetes mellitus (NIDDM). Inferences regarding insulin resistance, based on hyperinsulinemia, could thus be invalidated where routine insulin assays have been used. We have measured plasma insulin levels using a routine assay, together with measurements of the major circulating insulin propeptides, intact proinsulin and des 31,32proinsulin, in various clinical situations associated with apparently increased insulin levels and insulin resistance. Major increases of insulin propeptide levels relative to insulin levels were not seen in obese subjects or in patients taking oral contraceptives or danazol, or in obese subjects compared with non-obese controls. Although the insulinemic responses observed with routine radioimmunoassay in these situations associated with insulin resistance are not confounded by major changes in the proportion of circulating insulin propeptides, further studies will be necessary to validate investigations in other insulin-resistant states.