Effect of insulin on indices of cerebral blood flow and cerebrovascular compliance in young adults

Insulin has important vasodilatory effects in the peripheral circulation, but less is known about insulin's role in cerebrovascular control. Herein, we hypothesized both systemic (intravenous) and local (intranasal) insulin administration would increase indices of cerebral blood flow and reduce cerebrovascular compliance (Ci) in young adults. Participants were assigned to one of four separate protocols. Middle cerebral artery blood velocity (MCAv, transcranial Doppler ultrasound) and blood pressure (BP, finger photoplethysmography) were measured at baseline and at <i>1</i>) 2 min of carbon dioxide (CO₂) air breathing (high flow control), <i>2</i>) 60 min of euglycemic intravenous insulin infusion (40 mU/m² body surface area/min), <i>3</i>) 60 min following 160 IU of intranasal insulin, <i>4</i>) 60 minutes of time control. Ci was calculated (modified Windkessel model). Intravenous insulin increased serum insulin (6.0 ± 2.6 to 52.7 ± 12.7 μIU/mL, <i>P</i> < 0.001), whereas serum insulin was reduced following intranasal insulin (6.9 ± 4.5 to 4.9 ± 1.8 μIU/mL, <i>P</i> = 0.030). MCAv increased in response to CO₂ (60 ± 13 to 69 ± 11 cm/s, <i>P</i> < 0.001) but was unchanged with time control (50 ± 7 to 49 ± 8, <i>P</i> = 0.658) and both insulin conditions (intravenous: 61 ± 13 to 62 ± 17 cm/s, <i>P</i> = 0.531; intranasal: 57 ± 12 to 51 ± 15 cm/s; <i>p</i> = 0.061). In contrast, Ci remained at baseline levels over time (<i>P</i> = 0.438) and was reduced from baseline under CO₂ and both insulin conditions (CO₂, <i>P</i> < 0.001; intravenous, <i>P</i> = 0.021; intranasal, <i>P</i> = 0.001). Contrary to our hypothesis, there was no effect of systemic or local insulin administration on resting MCAv in young adults; however, both systemic and local insulin administration reduced Ci. These findings advance our understanding of the cerebrovascular response to acute insulin exposure.<b>NEW & NOTEWORTHY</b> Insulin has important vasodilatory effects in the peripheral circulation, but less is known about the role of insulin in cerebrovascular control. Contrary to our hypothesis, there was no effect of systemic (intravenous) nor local (intranasal) insulin administration on middle cerebral artery blood velocity; however, both systemic and local insulin administration reduced cerebrovascular compliance. Our findings advance our understanding of the cerebrovascular response to insulin and may have implications in the context of known metabolic disturbances.