Insulin effects on blood flow:

Endothelial function and arterial stiffness in uncomplicated type 1 diabetes and healthy controls and the impact of insulin on these parameters during an euglycemic clamp

BACKGROUND

In addition to its role in glucose metabolism, insulin has shown to exert numerous vascular effects, and an impaired vascular function of insulin is assumed to be a major contributor in the development of vascular complications. Arterial augmentation (AP) and the augmentation index (Aix) are surrogate parameters of arterial stiffness and are commonly used as predictors for cardiovascular risk. The aim of this study is to investigate the effect of insulin on arterial stiffness and parameters of endothelial function in patients with type 1 diabetes and healthy control subjects.

METHODS

Fourteen patients with type 1 diabetes (six male, eight female) with a mean age of 36.6 +/- 11.8 years and 14 healthy subjects (seven male, seven female) with a mean age of 27.3 +/- 5.5 years were randomized to an euglygemic clamp with either a low (0.25 mU/kg/min) or a high (1.0 mU/kg/min) insulin dose on two different days. The mean HbA1c in the diabetic subjects was 7.3 +/- 0.7%. In these subjects, arterial stiffness was measured by pulse wave analysis (SphygmoCor, AtCor Medical, Australia). AP was calculated as the difference between the second and the first systolic shoulders of the central pressure wave curve, and the Aix was expressed as the percentage of AP from total pulse pressure. As parameters of endothelial function, cyclic guanosine monophosphate, nitrotyrosine, and asymmetric dimethylarginine were determined at baseline and after 120 minutes.

RESULTS

Patients with type 1 diabetes showed increased values for AP with 3.5 +/- 3.1 mm Hg and Aix with 12.5 +/- 12.5% compared to healthy controls with -0.7 +/- 2.6 mm Hg for AP and -4.2 +/- 10.6% for Aix. This difference was statistically significant (p < 0.01). During the euglycemic clamp, insulin improved, but did not normalize the increased values for AP and Aix in patients with type 1 diabetes. Concerning parameters of endothelial function, patients with type 1 diabetes showed statistically significant increased values for nitrotyrosine compared to healthy controls at baseline [low insulin: diabetes mellitus (DM) 1993.12 +/- 1330.85 nmol/liter vs healthy controls 803.7 +/- 726.91; high insulin DM: 2208.02 +/- 1736.57 nmol/liter vs healthy controls: 750.83 +/- 426.03 nmol/liter] (p < 0.05).

CONCLUSION

Patients with type 1 diabetes mellitus revealed an increased arterial stiffness measured as augmentation and augmentation index and increased nitrotyrosine levels as a marker of oxidative stress compared to healthy control subjects at baseline. Application of insulin improves the arterial elastic properties, but was not able to normalize the vascular function in patients with type 1 diabetes.

Impact of insulin on microvascular blood flow and endothelial cell function in the postprandial state in patients with Type 1 diabetes

The aim of the present study was to investigate postprandial microvascular blood flow following a standardized test meal in nondiabetic subjects and in patients with Type 1 diabetes after regular insulin or insulin lispro. In this open-label, randomised cross-over study, 20 nondiabetic participants and 20 patients with Type 1 diabetes were enrolled. To valuate the postprandial time course of skin microvascular blood flow, laser Doppler flux (LDF) readings were obtained at baseline and every 30 min following a standardized test meal. Furthermore, the microvascular response to acetylcholine (Ach) was measured, and blood was collected for the measurement of serum insulin and blood glucose levels. Patients with Type 1 diabetes received single doses of regular insulin or insulin lispro, respectively, in a randomised sequence, while in nondiabetics, no insulin substitution was performed. In nondiabetic participants, skin microvascular blood flow showed an early increase in LDF by median 6.0 arbitrary units (AU; interquartile range: 1.8-14.0 AU) within the first postprandial hour. The microvascular response to Ach also increased with a median response of 26.0 (19.0-49.3) AU at 30 min pp and 50.0 (31.7-65.1) AU at 60 min pp. In patients with Type 1 diabetes, the time course of postprandial LDF measurements observed after the administration of insulin lispro was nearly similar to the one observed in nondiabetic controls and differed from that after subcutaneous regular insulin treatment. The postprandial microvascular response to Ach was stronger following insulin lispro compared with regular insulin [30 min pp: 26.0 (19.0-49.3) vs. 20.9 (9.7-26.1) AU, P=.0001]. Postprandial microvascular blood flow is disturbed in patients with Type 1 diabetes. Improvement of postprandial metabolic control was found to improve postprandial microvascular function.