Free and total insulin integrated concentrations in insulin dependent diabetes

Tissue barriers and novel approaches to achieve hepatoselectivity of subcutaneously-injected insulin therapeutics

Current subcutaneously (s.c.)-injected insulin (INS) products result in a hyperinsulin exposure to peripheral tissues (skeletal muscle and adipose) while INS hardly accesses to liver after injection. This unphysiological distribution raises risks of hypoglycemia episode and causes weight gain after long term treatment. An ideal INS replacement therapy requires the distribution or action of exogenous INS to more closely mimic physiological INS in terms of its preferential hepatic action. However, there are 2 factors that limit the ability of s.c. injected INS to restore the liver: peripheral gradient in INS deficient diabetes patients: (1) the transport of INS in capillary endothelium and peripheral tissues from the injection site; and (2) peripheral INS receptor (IR) mediated INS degradation. In this review, the tissue barriers against efficient liver targeting of s.c. injected INS are discussed and current advances in developing hepatoselective insulin therapeutics are introduced.

Pseudomonas aeruginosa biofilms perturb wound resolution and antibiotic tolerance in diabetic mice

Diabetic patients are more susceptible to the development of chronic wounds than non-diabetics. The impaired healing properties of these wounds, which often develop debilitating bacterial infections, significantly increase the rate of lower extremity amputation in diabetic patients. We hypothesize that bacterial biofilms, or sessile communities of bacteria that reside in a complex matrix of exopolymeric material, contribute to the severity of diabetic wounds. To test this hypothesis, we developed an in vivo chronic wound, diabetic mouse model to determine the ability of the opportunistic pathogen, Pseudomonas aeruginosa, to cause biofilm-associated infections. Utilizing this model, we observed that diabetic mice with P. aeruginosa-infected chronic wounds displayed impaired bacterial clearing and wound closure in comparison with their non-diabetic littermates. While treating diabetic mice with insulin improved their overall health, it did not restore their ability to resolve P. aeruginosa wound infections or speed healing. In fact, the prevalence of biofilms and the tolerance of P. aeruginosa to gentamicin treatment increased when diabetic mice were treated with insulin. Insulin treatment was observed to directly affect the ability of P. aeruginosa to form biofilms in vitro. These data demonstrate that the chronically wounded diabetic mouse appears to be a useful model to study wound healing and biofilm infection dynamics, and suggest that the diabetic wound environment may promote the formation of biofilms. Further, this model provides for the elucidation of mechanistic factors, such as the ability of insulin to influence antimicrobial effectiveness, which may be relevant to the formation of biofilms in diabetic wounds.

Immunological responses to exogenous insulin

Regardless of purity and origin, therapeutic insulins continue to be immunogenic in humans. However, severe immunological complications occur rarely, and less severe events affect a small minority of patients. Insulin autoantibodies (IAAs) may be detectable in insulin-naive individuals who have a high likelihood of developing type 1 diabetes or in patients who have had viral disorders, have been treated with various drugs, or have autoimmune disorders or paraneoplastic syndromes. This suggests that under certain circumstances, immune tolerance to insulin can be overcome. Factors that can lead to more or less susceptibility to humoral responses to exogenous insulin include the recipient's immune response genes, age, the presence of sufficient circulating autologous insulin, and the site of insulin delivery. Little proof exists, however, that the development of insulin antibodies (IAs) to exogenous insulin therapy affects integrated glucose control, insulin dose requirements, and incidence of hypoglycemia, or contributes to beta-cell failure or to long-term complications of diabetes. Studies in which pregnant women with diabetes were monitored for glycemic control argue against a connection between IAs and fetal risk. Although studies have shown increased levels of immune complexes in patients with diabetic microangiopathic complications, these immune complexes often do not contain insulin or IAs, and insulin administration does not contribute to their formation. The majority of studies have shown no relationship between IAs and diabetic angiopathic complications, including nephropathy, retinopathy, and neuropathy. With the advent of novel insulin formulations and delivery systems, such as insulin pumps and inhaled insulin, examination of these issues is increasingly relevant.

High cumulative insulin exposure: a risk factor of atherosclerosis in type 1 diabetes?

BACKGROUND

Since insulin therapy might have an atherogenic effect, we studied the relationship between cumulative insulin dose and atherosclerosis in type 1 diabetes. We have focused on patients with type 1 diabetes instead of type 2 diabetes to minimise the effect of insulin resistance as a potential confounder.

METHODS

An observational study was performed in 215 subjects with type 1 diabetes treated with multiple insulin injection therapy. Atherosclerosis was assessed by measurement of carotid intima-media thickness (CIMT).

RESULTS

The cumulative dose of regular insulin showed a positive and significant relation with CIMT: increase of 21 microm in CIMT per S.D. of insulin use (95% CI: 8-35 adjusted for gender and age), which remained unchanged after adjustment for duration of diabetes, HbA1c, BMI, pulse pressure, physical activity and carotid lumen diameter. A similar relation was found for intermediate-acting insulin: 15.5 microm per S.D. (2-29), which was no longer present after further adjustment.

CONCLUSIONS

These findings provide evidence that a high cumulative dose of regular insulin is a risk factor for atherosclerosis.

Insulin treatment and cardiovascular disease; friend or foe? A point of view

BACKGROUND

Several observational studies have shown that higher insulin levels are associated with an increased risk of cardiovascular disease. If higher endogenous insulin levels are causally related to cardiovascular disease, one might expect an increased risk of cardiovascular disease in patients treated with insulin, as this results in high circulating insulin levels. Such risk elevation might counteract the benefits of tight glucose control. Our objective was to explore the relationship between insulin therapy and cardiovascular disease in Type 1 and Type 2 diabetes mellitus using information from available literature.

SUMMARY OF COMMENT

Several experimental studies in animals and humans support the presence of a harmful effect of insulin on the vascular endothelium. In prospective follow-up studies increased insulin dosage was associated with increased risks of cardiovascular disease, although confounding by indication could not be excluded. Randomized controlled trials in diabetic patients, comparing conventional with intensive glucose-lowering treatment, although showing a reduction in microvascular disease, showed no significant difference in the incidence of cardiovascular disease. The results with respect to exposure to insulin are, however, difficult to interpret due to insufficient information on exposure to insulin levels as well as confounding by glycaemic control and body mass index. In addition, these studies were not designed to address the question whether higher insulin use relates to increased cardiovascular risk.

CONCLUSION

Published research provides conflicting evidence as to whether exposure to high levels of exogenous insulin in diabetes mellitus affects the risk of cardiovascular disease. The currently available studies have a number of serious methodological restraints that limit accurate interpretation and conclusions in this area.

Patterns of hyperinsulinaemia in type 1 diabetic patients with and without nephropathy

The prevalence and patterns of insulinaemia in five groups of patients with Type 1 diabetes have been reinvestigated using a free insulin assay which minimizes in vitro redistribution of the free and antibody-bound insulin components. In 18 diabetic patients managed by conventional insulin injection treatment and 19 patients treated by continuous subcutaneous insulin infusion (CSII), the mean 24-h serum free insulin level exceeded a non-diabetic reference range in 78% (injections) and 68% (CSII). Twenty-four-hour profiles showed that hyperinsulinaemia occurred in the basal state before meals and at night, but not immediately post-prandially. The serum free insulin concentration at 0800 h, 1200 h, and 1600 h was significantly (p less than 0.001) correlated with mean 24-h free insulin in injection-treated and CSII patients, and samples at one of these time-points may thus provide a simple, single measure of integrated insulinaemia for population studies. There was no significant difference in 24-h mean free insulin levels in 7 patients randomly crossed-over between injection treatment and CSII, but the profiles had a more physiological pattern during CSII, with a mean post-prandial serum free insulin level which was significantly higher on CSII than injections (mean +/- SE = 37.2 +/- 3.1 vs 26.9 +/- 2.5 mU l-1, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The course and determinants of insulin action in type 1 (insulin-dependent) diabetes mellitus

The course and determinants of insulin action were investigated in 8 newly diagnosed Type 1 (insulin-dependent) diabetic patients, who were studied every 3 months for one year, and in three groups of 8 patients each with 5, 10 and 20 years diabetes, studied once. Fifteen healthy subjects matched for age, sex and body weight served as control subjects. Dose-response curves were constructed using sequential euglycaemic (5.0 mmol/l) clamps (insulin infusion rates: 0.5, 1.0, 2.0 and 5.0 mU.kg-1.min-1 in periods of 2h). After 1/2 month of insulin treatment, insulin responsiveness was normal, but sensitivity was decreased (ED50 70 +/- 7 mU/l (SEM) vs 54 +/- 4 mU/l in control subjects, p less than 0.05). After 6 months, insulin sensitivity was improved (ED50 57 +/- 4 mU/l, p less than 0.01 vs 1/2 month and not significant (NS) vs control subjects); but after 9 and 12 months, it was reduced again, similarly to 0.5 month. Insulin responsiveness remained normal at all time-points. In the three groups of patients with longstanding diabetes, impaired insulin sensitivity with normal responsiveness was noted also (ED50 73 +/- 9 mU/l, p less than 0.02 vs control subjects). At 6, 9 and 12 months, glycaemic control (HbA1) and insulin dose were inverse correlates for insulin action; in patients with longstanding disease, this was noted for HbA1 and body weight, in control subjects for body weight. In conclusion, decreased insulin sensitivity re-develops in Type 1 diabetes within the first year following an initial improvement.(ABSTRACT TRUNCATED AT 250 WORDS)

The variability in the action of unmodified insulin is more dependent on changes in tissue insulin sensitivity than on insulin absorption

Eight normal subjects were studied twice for 360 min after the subcutaneous injection of unmodified insulin (0.15 U kg-1) during euglycaemic clamps. Insulin absorption was assessed by both the area under the insulin-time curve above baseline (AUC) and time course of absorption (time to 25% and 50% of total AUC). Insulin action was measured as the amount of glucose infused. The maximal serum insulin concentration was 0.27 +/- 0.02 (+/- SE) nmol l-1 at 112 +/- 10 min. Fifty percent of total glucose infused occurred at 218 +/- 7 min. The maximal glucose infusion rate was 5.11 +/- 0.70 mg kg-1 min-1 and occurred at 256 +/- 12 min. Intrasubject coefficients of variation (CV) for total insulin AUC (11.2%), time to 25% of maximum AUC (12.1%) and time to 50% of maximum AUC (10.2%) were considerably lower than that for total insulin action (22.6%). Total insulin AUC did not correlate with total glucose utilization (r = 0.06, NS). We conclude that when glucose concentrations are maintained by euglycaemic clamps the peak of unmodified insulin action is later and the duration longer than traditionally recognized, insulin AUC does not predict insulin action, and the higher variability of insulin action compared with the indices of absorption suggests that day-to-day changes in tissue insulin sensitivity contribute more to the variability in insulin action than changes in absorption.

Free insulin concentrations in immediately extracted plasma samples and their relationships to clinical and metabolic parameters in insulin-treated diabetic patients

The relationships between free insulin and various clinical and metabolic parameters in insulin-treated diabetic patients are still not clear, possibly because of the technical difficulties in measuring free insulin. Recently, it has been demonstrated that in the presence of insulin antibodies only immediate centrifugation of blood and extraction of insulin antibodies provide an accurate evaluation of in vivo free insulin concentrations. In this study we evaluated the relationships between free and bound insulin levels, insulin antibodies, metabolic control and insulin requirement in 38 insulin-treated diabetic patients, in whom plasma free insulin was assayed in immediately processed samples. The main findings of our study are as follows. Free insulin concentrations ranged from 2.5 to 54 microU/ml; no difference was found between males and females; the unbound hormone level was inversely correlated to fasting plasma glucose (p less than 0.01) and HbA1c (p less than 0.02); a positive correlation was shown between free insulin and daily insulin dose; finally, free insulin concentrations were not correlated with insulin antibody binding.

Macrovascular disease and hyperinsulinaemia

The evidence that hyperinsulinaemia represents an independent risk factor for cardiovascular disease is tantalizing but the hypothesis cannot be said to be proven. The inconsistencies arising from the major prospective studies require that further work be done. Hyperinsulinaemia may not carry the same implications in all subjects and its interactions with other risk factors and with blood glucose are not well described. Possible further research has been discussed and outlined at a recent meeting (Colwell, 1985). The suggestions include delineating the action of growth factors and insulin in defined serum-free tissue culture, and the use of more sophisticated culture models, such as smooth muscle covered by vascular endothelium. The choice of human or primate tissue is desirable because of the species specificity of the atherosclerotic lesions. Prospective trials of modifying peripheral insulin levels in treated diabetic patients are probably still impracticable. The case for attempting to achieve normoglycaemia in diabetes to avoid microvascular complications is strong, and current insulin treatment regimens accept peripheral hyperinsulinaemia as a consequence of achieving portal insulin concentrations sufficient to suppress hepatic glucose output. It is hard to envisage a trial to examine reduced peripheral insulin concentrations which would not give unacceptably poor blood glucose control. Current studies of different methods and degrees of control of blood glucose might be used to provide some indication of whether such a trial could ever be justified. The Diabetes Control and Complication Trial (DCCT) is a prospective multicentre study of intensive versus conventional insulin treatment in insulin-dependent diabetic patients in the USA, and the UK Prospective Study of therapies of maturity onset diabetes (UKPS) is following patients not satisfactorily controlled on diet, randomized to different treatment modalities. These may produce some evidence within the next few years, on insulin concentrations and complications (Tattersall and Scott, 1987). Should any of this change current management of non-insulin-dependent diabetes? Despite claims of enthusiasts, special treatment regimens with intensive exercise, a particular oral agent or the addition of sulphonylureas to insulin therapy are either not generally applicable or have little theoretical basis (Martin, 1986). Current 'good practice' in Europe as put forth in a consensus document (Alberti and Gries, 1988), recognizes the need to address risk factors other than diabetes in the management of the non-insulin-dependent diabetic patient.(ABSTRACT TRUNCATED AT 400 WORDS)

The metabolic and hormonal adaptations of normal dogs to long-term exogenous sulfated insulin infusions

Hyperinsulinism frequently accompanies glucose normalization in type I diabetes but the long-term consequences of this exaggerated hormonal state are not known. To study this condition, normal dogs received constant exogenous sulfated insulin infusions for prolonged periods up to 43 weeks. During the interval and inspite of prevailing postabsorptive and fasting hypoglycemia, overt resistance to the infused insulin or loss of sensitivity did not occur. In counterring the imposed fasting hyperinsulinemia and the resulting hypoglycemia, fasting pancreatic glucagon levels rose while the fasting levels of several glucogenic precursors (lactate, pyruvate, and alanine) decreased. Fasting free fatty acid (FFA) levels were suppressed, but beta-hydroxybutyrate (beta-OHB) levels were unchanged. Body weight did not change. Most remarkably, all changes measured in the fasting levels of the hormones and metabolites reverted to normal following the cessation of exogenous sulfated insulin infusion. In addition to the hormonal and metabolite adaptations invoked by chronic exogenous hyperinsulinism in the fasting state of these normal dogs, there were interesting responses to their usual mixed meals. Of particular interest in this regard were the plasma glucose, insulin, and FFA diurnal profiles. First of all, a definite and unusual postprandial glycemic excursion occurred. Second, insulin levels were elevated some sixfold, and rather unresponsive to the meal in general. Inspite of the depressed fasting FFA levels and the absence of a postprandial rise in insulinemia, FFA showed a distinct fall after the meal. Whether the sulfated insulins infused were of the bovine or porcine species of origin made no discernible difference.(ABSTRACT TRUNCATED AT 250 WORDS)

The source of the circulating aggregate of insulin in type I diabetic patients is therapeutic insulin

Circulating insulin immunoreactivity (IRI) in type I diabetic patients (insulin-dependent diabetes mellitus [IDDM]) includes a covalent aggregate about twice the size of insulin. These studies were designed to determine the source and conditions promoting the accumulation of this material. Among 31 IDDMs, the aggregate made up 28 +/- 3.6% of the mean fasting plasma IRI. Five of these patients were restudied after 5 d of treatment with equidose intravenous insulin. The relative amount of the aggregate during subcutaneous treatment (40 +/- 8.0%) was indistinguishable (P greater than 0.7) from that at the termination of intravenous treatment (41 +/- 6.8%). To determine whether previous exposure to therapeutic insulin influenced the appearance and accumulation of the aggregate, we intravenously or subcutaneously infused insulin for 5 h in nine healthy volunteers (euglycemic clamp). At the termination of the high-dose intravenous infusion (10 mU X kg-1 X min-1), the concentration of the aggregate was 81 +/- 18 microU/ml, and it accounted for 2.9% of total IRI. At the conclusion of the other infusion protocols, the absolute amounts of aggregate were somewhat less, but they accounted for similar percentages. On polyacrylamide gel electrophoresis, the circulating aggregate was indistinguishable from a material of similar molecular weight contaminating commercial insulin. We conclude that the insulin aggregate found in the blood of IDDMs originates in commercial insulin. Its appearance is independent of the route of insulin administration. Prolonged and continuous use of insulin may increase its concentration but is not necessary for its appearance. The potential biologic and immunologic consequences of the aggregate are important matters that need to be addressed.

Sample preparation and radioimmunoassay for circulating free and antibody-bound insulin concentrations in insulin-treated diabetics: a re-evaluation of methods

Methods for blood sample preparation and radioimmunoassay of free and antibody-bound serum or plasma insulin concentrations, using polyethylene glycol (PEG) solution to precipitate bound insulin, have been evaluated and compared. Method A was rapid mixing of whole blood with PEG, followed by separation of bound insulin by centrifugation into Ficoll; method B was rapid PEG addition to whole blood, followed by centrifugation alone; method C was conventional PEG addition to thawed plasma or serum, followed by centrifugation to remove bound insulin. Method A was found to have acceptable performance and reproducibility; serum free insulin levels were not significantly different from those measured by the simpler method B. In samples from insulin-dependent diabetics, serum free insulin was significantly higher and bound insulin significantly lower in method A compared to the conventional method C. However, in samples from non-diabetics there was no significant difference between methods A and C. Spurious results were obtained by addition of PEG and assay of plasma and serum samples after either freezing and thawing or incubation at 37 degrees C for 2h, compared to PEG addition and assay of fresh plasma or serum samples. We conclude that conventional, delayed addition of PEG to plasma or serum may result in redistribution of free and bound insulin and that values more representative of in vivo blood insulin levels are obtained by rapid PEG addition methods.

Aorta and muscle metabolism in pigs with peripheral hyperinsulinaemia

Peripheral hyperinsulinaemia usually found in conventionally treated Type 1 (insulin-dependent) diabetic patients may have deleterious metabolic effects. We have used a hyperinsulinaemic model to examine intermediary metabolism in two key peripheral tissues, aorta and muscle. Nine pigs were immunized with crystalline insulin. Subsequently, they showed an insulin-binding capacity of 86.2 +/- 25.0 pmol/l and fasting total serum insulin of 3.9 +/- 3.1 nmol/l (control range 0.034-0.072 nmol/l), impaired glucose tolerance after oral glucose tolerance testing, significantly elevated levels of peripheral venous serum free insulin and C-peptide, and increased mean post-prandial free insulin/glucose ratios. The immunized pigs showed marked elevation of aorta and muscle triglycerides compared with control pigs (n = 15) but similar levels of non-esterified fatty acids. The glucose-6-phosphate-dehydrogenase, malic enzyme and 3-hydroxyacyl-CoA-dehydrogenase activities were all increased significantly (by 50%-300%) in both aorta and muscle. Phosphofructokinase was decreased in both tissues. Hexokinase was increased in muscle alone whereas pyruvate kinase was significantly decreased in aorta. Glyceraldehyde-3-phosphate dehydrogenase activity was not significantly different in aorta and muscle. Thus in insulin immunized pigs with normal beta-cell function and pronounced peripheral hyperinsulinaemia there was increased peripheral lipogenic activity. These findings have potentially important implications with regard to macrovascular disease in diabetes.

Urinary insulin excretion rate: an index of free insulinaemia in insulin-dependent diabetics

In 25 insulin-dependent diabetics, 14 managed by conventional insulin injection treatment (CIT) and 11 treated by continuous subcutaneous insulin infusion (CSII), there was a highly significant correlation between urinary insulin excretion rate (IER) per 1.73 m2 and mean serum free insulin concentration (r = 0.73, p less than 0.001), measured over a 24 h period. Urinary IER and mean daily serum free insulin levels were significantly higher in diabetics than in non-diabetics. CSII-treated patients had significantly lower mean 24 h plasma glucose levels than CIT-treated patients despite similar values of urinary IER and mean daily serum free insulin in the two groups. Urinary IER may be a useful indicator of average insulinaemia in large scale studies, avoiding the problems of multiple blood sampling and immunoassay in the presence of anti-insulin antibodies.