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

Abstract

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 1) 2 min of carbon dioxide (CO₂) air breathing (high flow control), 2) 60 min of euglycemic intravenous insulin infusion (40 mU/m² body surface area/min), 3) 60 min following 160 IU of intranasal insulin, 4) 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, P < 0.001), whereas serum insulin was reduced following intranasal insulin (6.9 ± 4.5 to 4.9 ± 1.8 μIU/mL, P = 0.030). MCAv increased in response to CO₂ (60 ± 13 to 69 ± 11 cm/s, P < 0.001) but was unchanged with time control (50 ± 7 to 49 ± 8, P = 0.658) and both insulin conditions (intravenous: 61 ± 13 to 62 ± 17 cm/s, P = 0.531; intranasal: 57 ± 12 to 51 ± 15 cm/s; p = 0.061). In contrast, Ci remained at baseline levels over time (P = 0.438) and was reduced from baseline under CO₂ and both insulin conditions (CO₂, P < 0.001; intravenous, P = 0.021; intranasal, P = 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.

NEW & NOTEWORTHY 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.

INTRODUCTION

Insulin has vasoactive effects in the peripheral circulation, which are vital for skeletal muscle glucose uptake (1). Under physiological conditions, insulin binds to receptors on vascular endothelial cells within skeletal muscle and initiates vasodilation (2). Indeed, we report a 44% increase in leg blood flow during 60 min of euglycemic hyperinsulinemia (e.g., intravenous insulin infusion) in healthy young adults (3).

Though insulin’s vasodilatory effects in skeletal muscle are prominent, controversy lies in its vasoactive effects within other vascular beds. Unlike skeletal muscle, which depends on insulin for glucose uptake, the brain operates through an insulin-independent mechanism (4). Nevertheless, it is noteworthy that insulin receptors are present on cerebral endothelial cells (5). Under nonpathological conditions, insulin promotes vasodilation in isolated cerebral vessels (e.g., middle cerebral artery) (6, 7) indicating localized insulin is vasoactive; however, these effects are not observed during systemic infusion (8). In humans, discordant findings regarding the cerebral vasodilatory effects of insulin are observed. Whereas some studies indicate insulin has no effect on cerebral perfusion (9, 10), others demonstrate an increase (11, 12). Conflicting results may be attributed to methods for inducing insulin surge (e.g., euglycemic-hyperinsulinemic infusion, oral glucose tolerance test). Additionally, the use of systemic interventions may mask direct vasoactive effects of insulin within the cerebral circulation due to confounding interactions between central and peripheral hemodynamics (13, 14).

Intranasal insulin delivery has been introduced as a novel approach to localize the physiological effects of insulin to the brain. Nose-to-brain delivery effectively reaches brain tissue within 5–30 min (15) with minimal systemic effects (16). Prior research using magnetic resonance imaging reported increased cerebral blood flow within the middle cerebral artery region in older adults with type 2 diabetes after intranasal insulin administration (17). Evaluating whether insulin has similar vasoactive effects in the healthy state and whether this effect is specific to mode of administration remains unknown.

Herein, we document the effects of both systemic (intravenous) and local (intranasal) administration of insulin on indices of cerebral blood flow in young men and women. We hypothesized insulin administration augments middle cerebral artery velocity (MCAv), a surrogate measure of cerebral blood flow, as assessed using transcranial Doppler ultrasound (TCD). Given cerebral vasodilation reduces cerebrovascular compliance (Ci) (18), we further hypothesized insulin administration reduces Ci.

METHODS

Human Participants

Participants were young (≤45 yr), normo-glycemic (fasting glucose <100 mg/dL), nonsmokers without chronic diseases and taking no medications. Participants identified as: White/Non-Hispanic (66.1%), White/Hispanic (14.5%), Asian (9.7%), and Black (9.7%). Female participants were premenopausal and studied in the early follicular (days 1–7, n = 20) or placebo (n = 3) phase. Nonmenstruating females (n = 1, intrauterine device) were studied at their convenience. Participants refrained from alcohol, caffeine, and exercise for 24 h and fasted for 12 h before the study (19). Written informed consent was obtained and all experiments were approved by the Institutional Review Board at the University of Missouri (Protocols No. 2013136, No. 2016225, No. 2020286, No. 2057228) and conformed to the Declaration of Helsinki including registration in a database (NCT05244694, NCT05153395). Each protocol ran independently and concurrently, and participants were not randomly assigned. Data from these protocols have been published previously (3, 20–25), however, results presented herein are unique to the novel hypotheses raised.

Protocol 1: Effect of Hypercapnia on Cerebral Blood Flow

Hypercapnia is a vasodilatory stimulus that increases MCAv (26) and decreases Ci (18) and was applied as a high flow control. Participants (n = 23, 11 males/12 females) were admitted to the University of Missouri Physical Activity and Wellness Center (MU-PAW). Participants rested supine (n = 12) or seated with legs outstretched (n = 11) and were instrumented with an electrocardiogram to measure heart rate (Lead II; Bio Amp FE132, ADInstruments) and continuous blood pressure measurement by finger photoplethysmography (n = 12, Human NIBP Controller ML282, ADInstruments; n = 11, Finapress, Finapres Medical Systems) calibrated to upper arm blood pressure. Respiration was monitored by a piezo respiratory belt transducer (MLT1132, ADInstruments). Participants wore either a mask (n = 12) or a mouthpiece and nose clip (n = 11) connected to a nonrebreathing valve during the testing period. Inspired and expired gases (n = 12, Gemini 14-10000 Respiratory Monitor, CWE Inc; n = 11, ADInstruments, ML206) were monitored continuously.

The middle cerebral artery was insonated unilaterally (left, n = 12; right, n = 11) using a 2-MHz Doppler probe (Multigon TOC Neurovision; Elmsford, NY) adjusted over the transtemporal window. The signal was identified based on depth (range 40–55 mm), signal directionality, and high-pitched auditory response (24, 27). The probe was fixed using a headpiece to maintain signal quality (27). Optimal signal quality was determined when baseline velocities were ≥35 cm/s and data were continuous (≥2 min without signal drops) (28). Following instrumentation, participants completed 2 min of quiet rest while breathing room air, followed by inhalation of carbogen (5% carbon dioxide, 95% oxygen) for 2 min.

Protocol 2: Effect of Intravenous Insulin Infusion on Cerebral Blood Flow

Participants (n = 13, 10 males/3 females) were admitted to the Clinical Research Center at the University of Missouri. Participants rested supine for instrumentation consistent with protocol 1, with the exception that inspired and expired gases (Gemini 14-10000 Respiratory Monitor, CWE Inc.) were monitored via nasal cannula and were only available from a subset of participants (n = 3). Superficial femoral artery diameter and blood velocity were measured (Doppler ultrasound) to demonstrate the peripheral vascular response to insulin. Data were collected in duplex mode at a pulsed frequency of 3.5–5 MHz and corrected with an insonation angle of 60°. Sample volume was adjusted to encompass the entire lumen and the cursor was set mid-vessel.

An intravenous catheter was placed in both arms for insulin/glucose infusion and blood sampling. Two priming infusion rates of insulin (Humulin R U-100) were administered over 10 min, followed by a steady infusion rate of 40 mU/m² body surface area/min for 60 min (29). Blood glucose was determined every ∼5 min at bedside (YSI 2300 STAT PLUS glucose analyzer) and maintained at baseline levels by a variable dextrose infusion. Serum was obtained and stored at −80°C for analysis of insulin (ALPCO Cat. No. 80-INSHU-E10.1, Salem, NH). Primary outcome variables were assessed at baseline and at 60 min of infusion.

Protocol 3: Effect of Intranasal Insulin on Cerebral Blood Flow

Participants (n = 17, 11 males/6 females) were admitted to MU-PAW and seated in the upright position with legs outstretched. Individuals were instrumented as outlined in protocols 1 and 2, with the exception that inspired and expired gases (ADInstruments respiratory gas analyzer, ML206) were monitored while participants breathed through a mouthpiece connected to a nonrebreathing valve while wearing a nose clip.

Following a quiet baseline, 160 IU of human insulin (Humulin R, U100) was administered using a nasal drug delivery device (ViaNase; Kurve Technology; Lynwood, WA) based on methods published previously (30). Insulin was administered as 4 sprays over 5 min (40 IU/spray, alternating nostrils) (10).

An intravenous catheter was placed in the antecubital vein for blood sampling. Blood glucose (YSI 2300 STAT PLUS glucose analyzer) is reported from baseline and 60 min following insulin administration. Serum was collected at similar timepoints and stored at −20^○C for analysis of insulin (immunoassay; Quest Diagnostics Laboratories; Columbia, MO). Primary outcome variables were assessed at baseline and 60 min following insulin administration.

Protocol 4: Time Control

A subset of individuals (n = 9, 5 males/4 females) completed a separate time control condition in which participants were tested in an identical manner to protocol 3. However, unlike protocol 3, no insulin was administered, and an intravenous catheter was not placed.

Analysis

Data were collected using PowerLab data acquisition system (analog to digital converter; ADInstruments, Inc.) with a sampling rate of 1,000 Hz. Cardiovascular measurements were obtained as an average of 2 min before and during study conditions (e.g., 60-min post insulin administration). Ultrasound recordings from the leg were analyzed offline (Cardiovascular Suite, Quipu srl, Pisa, Italy). Superficial femoral artery blood flow was calculated from continuous diameter and mean blood velocity recordings. Femoral vascular conductance (FVC) was calculated as blood flow ÷ mean arterial blood pressure × 100. Mean MCAv was similarly normalized for blood pressure for measures of cerebrovascular conductance index (CVCi). Ci was calculated from a subset of participants (hypercapnia, n = 17; intravenous insulin, n = 12; intranasal insulin, n = 11; time control, n = 8) using a modified Windkessel model applied to blood pressure (finger photoplethysmography) and corresponding MCAv waveforms. Ci data were collected as an average of five waveforms occurring over the same 2-min period at baseline and during the study conditions (18). Data in which successful overlay of blood pressure and MCAv waveforms could not be achieved were excluded from the Ci analysis.

Statistical Analysis

Using previously published results (18, 24), we conducted an a priori power analysis and determined we would require n = 10 to detect a 9.2 ± 0.8 cm/s change in MCAv (24), and n = 11 to detect a 0.0001 ± 0.0001 cm/s/mmHg change in Ci (18) with power of 0.80 and α = 0.05. The evaluation of sex-related differences was not an aim of the study; therefore, data from male and female participants were pooled. The effect of intervention on main outcome variables was assessed using Student’s paired t test and normality was assessed with the Shapiro–Wilk test. Data not normally distributed were assessed using the Wilcoxon signed rank test. The response relationship between insulin-mediated perfusion [Δ = (Insulin − Baseline); % = Δ ÷ Baseline × 100] and other participant characteristics (e.g., age, body mass index, etc.) was explored using Pearson product-moment correlation. Participant demographics (age, weight, body mass index, fasting glucose, and insulin) were analyzed using one-Way ANOVA to explore differences between cohorts. P < 0.05 was considered statistically significant. Data are reported as means ± SD.

RESULTS

Protocol 1: Effect of Hypercapnia on Cerebral Blood Flow

Twenty-three adults (11 males/12 females) completed protocol 1 (Table 1). End-tidal carbon dioxide levels increased. The hypercapnia condition decreased resting heart rate and increased mean arterial blood pressure, with no change in respiratory rate (Table 2). MCAv and CVCi increased in response to hypercapnia (Fig. 1, A and B) and Ci was reduced (Fig. 1C). No correlations were observed between main outcome variables and participant demographics (P value range 0.089–0.955).

Table 1.

Participant demographics

	Hypercapnia	Intravenous Insulin	Intranasal Insulin	Time Control
Sex (M, F)	11, 12	10, 3	11, 6	5, 4
Age, yr	26 ± 7	29 ± 8	26 ± 7	27 ± 4
Height, cm	173 ± 13	178 ± 14	174 ± 14	176 ± 12
Weight, kg	75 ± 12	78 ± 16	78 ± 11	76 ± 14
Body mass index, kg/m2	25 ± 3	24 ± 2	26 ± 3	24 ± 3
Fasting glucose, mg/dL	75 ± 8	78 ± 7	76 ± 8	—
Fasting Insulin, µIU/mL	8.3 ± 4.8	6.0 ± 2.6	6.9 ± 4.5	—
HOMA-IR	1.5 ± 0.9	1.2 ± 0.5	1.3 ± 0.8	—
Systolic blood pressure, mmHg	116 ± 10	114 ± 11	116 ± 10	117 ± 8
Diastolic blood pressure, mmHg	72 ± 7	71 ± 10	72 ± 7	73 ± 8

Data are reported as means ± SD from prior to and during hypercapnia (n = 23), intravenous insulin (n = 13), intranasal insulin (n = 17), and time control (n = 9) unless otherwise specified (hypercapnia: Glucose/Insulin/HOMA-IR, n = 10–11; intranasal insulin: Insulin/HOMA-IR, n = 15). HOMA-IR: Homeostatic Model Assessment for Insulin Resistance. Participant demographics did not differ by protocol (one-way ANOVA; P value range 0.237–0.898).

Table 2.

Hemodynamic response

	Baseline	Intervention	P Value
Heart rate, beats/min
Hypercapnia	66 ± 8	63 ± 8*	<0.001
Intravenous insulin	59 ± 8	63 ± 10*	<0.001
Intranasal insulin	63 ± 8	64 ± 9	0.943
Time control	68 ± 10	66 ± 11	0.249
Mean blood pressure, mmHg
Hypercapnia	89 ± 9	93 ± 8*	<0.001
Intravenous insulin	84 ± 8	89 ± 7*	0.012
Intranasal insulin	94 ± 8	98 ± 10*	0.039
Time control	93 ± 7	96 ± 4	0.053
Respiratory rate, breaths/min
Hypercapnia	13 ± 4	12 ± 4	0.138
Intravenous insulin	15 ± 3	16 ± 3	0.255
Intranasal insulin	13 ± 4	13 ± 5	0.918
Time control	13 ± 4	13 ± 4	0.619
Femoral artery diameter, cm
Hypercapnia	—	—	—
Intravenous insulin	0.62 ± 0.08	0.62 ± 0.09	0.741
Intranasal insulin	0.56 ± 0.07	0.57 ± 0.07	0.111
Time control	0.58 ± 0.08	0.58 ± 0.09	0.653
Femoral blood flow, mL/min
Hypercapnia	—	—	—
Intravenous insulin	122 ± 70	143 ± 76*	0.048
Intranasal insulin	94 ± 23	103 ± 40	0.809
Time control	73 ± 28	77 ± 31	0.268
Femoral vascular conductance, mL/min/100 mmHg
Hypercapnia	—	—	—
Intravenous insulin	142 ± 76	159 ± 75	0.080
Intranasal insulin	101 ± 25	106 ± 39	0.463
Time control	80 ± 33	80 ± 34	0.884
Insulin, µIU/mL
Hypercapnia	—	—	—
Intravenous insulin	6.0 ± 2.6	52.7 ± 12.7*	<0.001
Intranasal insulin	6.9 ± 4.5	4.9 ± 1.8*	0.030
Time control	—	—	—
Glucose, mg/dL
Hypercapnia	—	—	—
Intravenous insulin	78 ± 7	81 ± 11	0.381
Intranasal insulin	76 ± 8	74 ± 9	0.375
Time control	—	—	—
End-tidal carbon dioxide, %
Hypercapnia	5.7 ± 0.9	6.5 ± 0.6*	<0.001
Intravenous insulin	6.4 ± 0.8	6.2 ± 0.6	0.199
Intranasal insulin	5.6 ± 0.7	5.8 ± 0.7	0.625
Time control	5.2 ± 0.6	5.5 ± 0.4	0.149

Data are reported as means ± SD from prior to and during hypercapnia (n = 23), intravenous insulin (n = 13), intranasal insulin (n = 17), and time control (n = 9) unless otherwise specified (intravenous insulin: respiratory rate, n = 12; intravenous insulin: carbon dioxide, n = 3; intranasal insulin: insulin, n = 15).

^*P < 0.05 vs. baseline determined via paired t test or Wilcoxon matched-pairs signed rank test if not normally distributed.

Figure 1.

Cerebral hemodynamics in young adults. Middle cerebral artery blood velocity (MCAv; A) and cerebrovascular conductance (CVCi; B) are reported as means ± SD, and individual data points (solid male, dashed female), prior to and during hypercapnia (n = 23), intravenous insulin (n = 13), intranasal insulin (n = 17), or time control (n = 9). Cerebrovascular compliance (Ci; C) is reported from a subset of participants (hypercapnia, n = 17; intravenous insulin, n = 12; intranasal insulin, n = 11; time control, n = 8). Data are compared within groups using a paired t test, unless not normally distributed (Shapiro–Wilk test) when comparisons were made using the Wilcoxon signed rank test (MCAv: intravenous; Ci: hypercapnia, intravenous, intranasal, time control).

Protocol 2: Effect of Intravenous Insulin on Cerebral Blood Flow

Thirteen adults (10 males/3 females) completed protocol 2 (Table 1). Intravenous insulin increased serum insulin concentrations while blood glucose was maintained (Table 2). Mean arterial blood pressure and heart rate increased in response to insulin (Table 2). Hyperinsulinemia had no effect on respiratory rate (n = 12) or end-tidal carbon dioxide (n = 3) (Table 2).

Superficial femoral artery blood flow increased in response to intravenous insulin (Table 2). In contrast, there was no change in MCAv nor CVCi during hyperinsulinemia (Fig. 1, A and B); however, Ci was reduced (Fig. 1C). No correlations were observed between main outcome variables and participant demographics (P value range 0.053–0.932).

Protocol 3: Effect of Intranasal Insulin on Cerebral Blood Flow

Seventeen adults (11 males/6 females) completed protocol 3 (Table 1). Intranasal insulin had no effect on blood glucose, whereas serum insulin was reduced following administration (Table 2). Mean arterial blood pressure increased following insulin whereas heart rate remained unchanged (Table 2). Respiratory rate and end-tidal carbon dioxide showed no change following insulin (Table 2).

Sixty minutes following intranasal insulin administration, superficial femoral artery blood flow was maintained at baseline levels (Table 2). Similarly, there was no net change in MCAv following intranasal administration (Fig. 1A), however, Ci was reduced (Fig. 1C). Changes in Ci following intranasal insulin were inversely correlated with fasting insulin (R = −0.655, P = 0.040) and HOMA-IR (R = −0.644, P = 0.045). No other correlations were observed between main outcome variables and participant demographics (P value range 0.093–0.766).

Protocol 4: Time Control

Nine young adults (5 males/4 females) completed the time control condition. No significant changes in blood pressure, respiratory rate, or end-tidal CO₂ were observed under the time control condition (Table 2). No significant changes were observed in MCAv, CVCi, or Ci (Fig. 1).

DISCUSSION

Contrary to previous animal studies and our original hypothesis, insulin administered systemically (intravenous) or locally (intranasal) had no effect on MCAv in young adults. In agreement with our hypothesis, both systemic and local insulin administration resulted in a decrease in Ci. These findings enhance our understanding of cerebrovascular responsiveness to insulin and demonstrate insulin reduces Ci in humans, regardless of form of administration.

Hyperinsulinemia has marked vasodilatory effects within the skeletal muscle of healthy adults (1); however, the magnitude of insulin-stimulated cerebrovascular dilation remains unclear. In preclinical models, insulin stimulates cerebral vasodilation (6). Indeed, in intracranial vessels, vasodilation is induced by insulin in the healthy state (8), partly mediated by nitric oxide (6). Despite these data, human studies yield mixed results. Some suggest high insulin concentrations have no effect on cerebral blood flow (9, 10), whereas others report an increase (11, 12). For instance, a study using euglycemic-hyperinsulinemic infusion and Doppler ultrasound observed increased carotid artery blood flow in a moderately-sized cohort spanning a broad age range (n = 15, 22–53 yr) (11). Conversely, a study in younger individuals (n = 8, 22–29 yr) using the same hyperinsulinemic protocol and TCD found no change in MCAv (9). Consistent with the latter, we observed no effect of hyperinsulinemia on MCAv in younger adults using TCD, despite increases in leg blood flow. Differing responses to insulin in extracranial (11) and intracranial (9) vessels may result from systemic hemodynamic changes, such as variations in blood pressure and heart rate, which can obscure insulin’s vasoactive effects (31). Preclinical studies show high doses of insulin increase femoral and carotid artery blood flow (32), but have no effect on intracranial cerebral blood flow (33). It is important to consider intracranial vessels are subject to more rigorous regulation (31). Ultimately, insulin’s effects on blood flow may vary by vascular bed and variations in systemic hemodynamics likely contribute to inconsistencies observed in human studies.

In contrast to intravenous administration, intranasal insulin offers a unique advantage by providing a direct gateway to the central nervous system (34). In preclinical models, labeled insulin delivered intranasally enters select brain regions through the olfactory and trigeminal pathways (34). In humans, intranasal delivery enables safe (10, 30) and rapid localization of insulin, increasing brain insulin within 30 min (15). Clinical trials confirm that intranasal insulin delivers a bioactive form in older adults, indicated by changes in functional connectivity (35), tissue perfusion (12, 17), and cognitive improvements (17). Indeed, intranasal insulin improves cerebral perfusion and cognitive function in older adults with type 2 diabetes (17), potentially through vasoactive mechanisms. In contrast, presently intranasal insulin had no effect on MCAv using TCD. Divergent findings may be due to the inability of TCD to capture subtle changes in blood flow due to lack of measures of vessel diameter. Thus, it remained possible that: 1) the diameter of the middle cerebral artery increased, which allowed for velocity to remain constant (36, 37), or 2) the dilatory effects of insulin are localized to select downstream vessels, which has been shown previously in the context of insulin (6, 38).

Vascular compliance plays a crucial role in regulating cerebral blood flow (31). Cerebral arteries reside in a state of vasodilation (39); further dilation compromises the load-bearing potential of elastin, increasing reliance on collagen and reducing vessel compliance (40). In agreement, Moir et al. (18) demonstrated hypercapnia-induced cerebral vasodilation reduces Ci. Other vasoactive compounds (e.g., sodium nitroglycerin) also reduce Ci via direct vasorelaxation (18). However, whether insulin exerts similar vasoactive effects within the cerebral vasculature is unclear. We hypothesized Ci would be a more sensitive index of vascular tone changes in humans than MCAv alone. Confirming previous work (18), we observed reductions in Ci during hypercapnia, but not following a control condition. In addition, both insulin administered systemically (intravenously) and locally (intranasally) reduced Ci, which is supportive of insulin-mediated vasorelaxation. Our conclusion is strengthened by preclinical work showing that insulin may exhibit differential vasoactive effects in large versus small cerebral arteries (6, 8). Taken together, we speculate hypercapnic and insulin-mediated reductions in Ci are associated with an enhanced dilatory state of the middle cerebral artery (18) and support insulin’s potential role in modulating cerebrovascular tone.

Perspectives

Current data suggest elevated fasting insulin and greater insulin resistance are linked to enhanced vasodilation following intranasal insulin administration. In obesity, cerebral hypoperfusion is prevalent (41) and may be in part attributed to low insulin concentrations within the brain (42). Low brain insulin levels have been shown to cause vasoconstriction, potentially reducing brain blood flow (7). Our findings indicate intranasal insulin-mediated vasorelaxation may be enhanced in individuals with insulin resistance, warranting further research on its effects in those with metabolic disorders.

Experimental Considerations

Strengths of the present investigation include assessing both peripheral and central effects of insulin on brain blood flow and Ci in healthy young adults combined with both high flow and time control conditions. However, there are important limitations to acknowledge. First, although both peripheral (42) and central (43) insulin administration increases brain insulin concentrations, the exact dose of insulin reaching the brain remains uncertain. Second, we tested a single, acute dose of intravenous and intranasal insulin, limiting findings to the doses studied. Third, indices of cerebral blood flow were measured at rest only from a single vessel (middle cerebral artery) at a specific timepoint (60 min). The 60-min timepoint was chosen based on prior published work supporting a peak response at that time (11, 44, 45). In review of data collected prior to 60 min, results suggest any change in MCAv was unlikely to have occurred at an earlier timepoint (data not shown). It is also important to acknowledge studies were conducted under various postural conditions. Whereas data support no influence of postural changes on MCAv or Ci (46), others have reported postural effects on MCAv (47). Given posture remained constant within individuals, it is unlikely that posture influenced present findings. However, future studies are needed to assess insulin’s effects on other vessels (e.g., posterior cerebral and internal carotid arteries), the time course of the insulin response, as well as insulin’s potential impact on varied physiological stressors (e.g., mental stress) to better understand its vasoactive effects across vascular territories and physiological conditions.

Conclusions

Contrary to our hypothesis, systemic or local insulin administration had no effect on resting MCAv in young adults. However, insulin reduced Ci regardless of the administration method. These findings enhance our understanding of the cerebrovascular response to insulin in humans. Future work should seek to translate these findings to populations with metabolic conditions such as obesity, where insulin resistance and brain hypoperfusion are prevalent (41). Understanding insulin’s role in cerebral perfusion in such populations will provide valuable insights into the mechanisms underlying cerebral blood flow regulation in the context of known metabolic disturbances.

DATA AVAILABILITY

Data will be made available upon reasonable request.

GRANTS

Work was supported by the Margaret W. Mangel Faculty Research Catalyst Fund (to J.K.L. and J.P.), University of Missouri Joy of Discovery program (to J.K.L and J.P.), American Physiological Society Beverly Petterson Bishop Award (to J.K.L.), and the NIH HL153523 (to J.K.L.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

C.M.-A., J.P., and J.K.L. conceived and designed research; B.S., J.L.H., N.J.M., A.M.G., C.M.-A., J.P., and J.K.L. performed experiments; B.S., N.J.M., A.M.G., B.J.B., and A.M.P. analyzed data; B.S., J.L.H., N.J.M., L.N.S., C.M.-A., J.P., and J.K.L. interpreted results of experiments; B.S. prepared figures; B.S., J.P., and J.K.L. drafted manuscript; B.S., J.L.H., N.J.M., A.M.G., B.J.B., A.M.P., L.N.S., C.M.-A., J.P., and J.K.L. edited and revised manuscript; B.S., J.L.H., N.J.M., A.M.G., B.J.B., A.M.P., L.N.S., C.M.-A., J.P., and J.K.L. approved final version of manuscript.