Diminished wave reflection in the aorta - A novel physiological action of insulin on large blood vessels

Epidemiological data suggest that insulin may have direct effects on large- vessel function, but thus far insulin has only been shown, after prolonged infusions, to slowly decrease peripheral vascular resistance by increasing muscle blood flow. We determined whether physiological doses of insulin aff ect function of large arteries, before any changes in peripheral blood flow , in vivo using pulse wave analysis. Nine normal men were studied on 2 occa sions: once during a 6-hour infusion of saline and once under normoglycemic hyperinsulinemic conditions (sequential 2-hour insulin infusions of 1, 2, and 5 mU/kg . min). Central aortic pressure waves were synthesized from tho se recorded in the periphery with the use of applanation tonometry and a va lidated reverse transfer function every 30 minutes. This allowed determinat ion of central aortic augmentation (the pressure difference between early a nd late systolic pressure peaks) and augmentation index (augmentation expre ssed as a percentage of pulse pressure). Both augmentation and augmentation index decreased significantly within 1 hour after administration of insuli n (P<0.001) but not saline. Systolic and diastolic blood pressure and heart rate remained unchanged for the first 2 hours. A significant increase in p eripheral (forearm) blood flow was not observed until 2.5 hours after start of the insulin infusion. These data demonstrate that insulin, in normal su bjects, rapidly decreases wave reflection in the aorta. This beneficial eff ect is consistent with increased distensibility or vasodilatation of large arteries. In contrast to the effect of insulin on peripheral blood flow, th is action of insulin is observed under conditions in which both the insulin dose and duration of insulin exposure are physiological. Resistance to thi s action of insulin could provide a mechanism linking insulin resistance an d conditions such as hypertension at the level of large arteries.