Non-linearity of insulin kinetics

Mathematical modeling and analysis of insulin clearance in vivo

BACKGROUND

Analyzing the dynamics of insulin concentration in the blood is necessary for a comprehensive understanding of the effects of insulin in vivo. Insulin removal from the blood has been addressed in many studies. The results are highly variable with respect to insulin clearance and the relative contributions of hepatic and renal insulin degradation.

RESULTS

We present a dynamic mathematical model of insulin concentration in the blood and of insulin receptor activation in hepatocytes. The model describes renal and hepatic insulin degradation, pancreatic insulin secretion and nonspecific insulin binding in the liver. Hepatic insulin receptor activation by insulin binding, receptor internalization and autophosphorylation is explicitly included in the model. We present a detailed mathematical analysis of insulin degradation and insulin clearance. Stationary model analysis shows that degradation rates, relative contributions of the different tissues to total insulin degradation and insulin clearance highly depend on the insulin concentration.

CONCLUSION

This study provides a detailed dynamic model of insulin concentration in the blood and of insulin receptor activation in hepatocytes. Experimental data sets from literature are used for the model validation. We show that essential dynamic and stationary characteristics of insulin degradation are nonlinear and depend on the actual insulin concentration.

125I used for labelling of proteins in an absorption model changes the absorption rate of insulin aspart

The aim of this study is to validate the ability of the disappearance model to predict absorption rates of insulin aspart in pigs. The disappearance model is used as a screening tool to estimate absorption rates after subcutaneous injections in humans or pigs especially of insulin and insulin analogues. The disappearance model measures remaining radioactivity at the injection site and therefore radioactive labelling of the insulin analogue is necessary. The labelling is done with 125I. One of the assumptions for the disappearance model to be reliable is that absorption rates of the labelled and non-labelled molecules are comparable. In this study, we compared disappearance data with absorption calculated from plasma samples of insulin aspart. The calculated absorption is based on non-labelled insulin aspart. The absorption rate from the disappearance data was statistical significant (p = 0.0028) different from the absorption rate based on plasma samples. A control study was carried out where 125I labelled insulin aspart was compared to 127I (the natural non-radioactive isotope) insulin aspart. In this study, absorption rate from the disappearance data and absorption rate based on plasma samples were similar (p = 0.63).

CONCLUSION

Iodination of insulin aspart changes the subcutaneous absorption rate.

Comparison of the continuously calculated fractional splanchnic extraction of insulin with its fractional disappearance using a new double-tracer technique

These studies were designed to calculate the fractional disappearance rate (FDR) and splanchnic extraction of insulin in response to an exogenous (intraperitoneal) input of insulin. A double-tracer technique using insulin tritiated on both the A1 and B1 positions was introduced for the measurement of hepatic extraction. The A1 tracer, not previously characterized in vivo, was compared in terms of its kinetics with H3-B1-insulin and unlabeled insulin. The metabolic clearance rates (MCR) of the three insulins were identical, as were the decay curves of the two tracers. To measure splanchnic insulin extraction, one tracer was infused systemically to evaluate the FDR of insulin, and the second was infused into the splanchnic circulation (superior mesenteric artery) and its peripheral appearance was calculated. Splanchnic extraction was determined from the difference between this rate of appearance and the rate of infusion of the mesenteric tracer. After intraperitoneal insulin injection, insulin levels increased to peaks of 549 +/- 93 microU/mL (portal vein) and 473 +/- 99 microU/mL (inferior vena cava) and decreased to basal levels over 3 hours. The FDR decreased from 0.295 +/- 0.051 min-1 to 0.125 +/- 0.026 min-1, and splanchnic extraction decreased from 0.534 +/- 0.06 to 0.232 +/- 0.088. The latter returned to near-basal values more rapidly than did the FDR. In conclusion, the kinetics of insulin both in and out of the steady state have been shown to be nonlinear through physiological insulin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)