Skeletal muscle microvascular insulin resistance in type 2 diabetes is not improved by eight weeks of regular walking

The effect of exercise on flow-mediated dilation in people with type 2 diabetes mellitus: a systematic review and meta-analysis of randomized controlled trials

Introduction

An increasing number of studies have investigated the effect of exercise on flow-mediated dilation (FMD) in people with type 2 diabetes mellitus (T2DM), while the findings were controversial. The primary aim of this systematic review and meta-analysis was to investigate the effect of exercise on FMD in T2DM patients, and the secondary aim was to investigate the optimal type, frequency, session duration, and weekly time of exercise for T2DM patients.

Methods

Searches were conducted in PubMed, Cochrane Library, Scopus, Web of Science, Embase and EBSCO databases. The Cochrane risk of bias tool (RoB2) in randomized trial and Physiotherapy Evidence Database (PEDro) scale were used to assess the methodological quality of the included studies.

Results

From the 3636 search records initially retrieved, 13 studies met the inclusion criteria. Our meta-analysis revealed that exercise had a significant effect on improving FMD in T2DM patients [WMD, 2.18 (95% CI, 1.78-2.58), p < 0.00001, I2 = 38%], with high-intensity interval training (HIIT) being the most effective intervention type [HIIT, 2.62 (1.42-3.82); p < 0.0001; aerobic exercise, 2.20 (1.29-3.11), p < 0.00001; resistance exercise, 1.91 (0.01-3.82), p = 0.05; multicomponent training, 1.49 (0.15-2.83), p = 0.03]. In addition, a higher frequency [> 3 times, 3.06 (1.94-4.19), p < 0.00001; ≤ 3 times, 2.02 (1.59-2.45), p < 0.00001], a shorter session duration [< 60 min, 3.39 (2.07-4.71), p < 0.00001; ≥ 60 min, 1.86 (1.32-2.40), p < 0.00001], and a shorter weekly time [≤ 180 min, 2.40 (1.63-3.17), p < 0.00001; > 180 min, 2.11 (0.82-3.40), p = 0.001] were associated with larger improvements in FMD.

Conclusion

This meta-analysis provides clinicians with evidence to recommended that T2DM patients participate in exercise, especially HIIT, more than 3 times per week for less than 60 min, with a target of 180 min per week being reached by increasing the frequency of exercise.

Systematic review registration

https://www.crd.york.ac.uk/prospero, identifier CRD42023466575.

Impact of Insulin-Induced Relative Hypoglycemia on Vascular Insulin Sensitivity and Central Hemodynamics in Prediabetes

CONTEXT

Relative hypoglycemia (RH) is linked to sympathetic responses that can alter vascular function in individuals with type 2 diabetes. However, less is known about the role of RH on hemodynamics or metabolic insulin sensitivity in prediabetes.

OBJECTIVE

Determine if RH alters peripheral endothelial function or central hemodynamics to a greater extent in those with prediabetes (PD) versus normoglycemia (NG).

METHODS

Seventy adults with obesity were classified using ADA criteria as PD (n=34 (28F); HbA1c=6.02±0.1%) or NG (n=36 (30F); HbA1c=5.4±0.0%). Brachial artery endothelial function, skeletal muscle capillary perfusion, and aortic waveforms were assessed at 0 and 120min of a euglycemic clamp (40 mU/m2/min, 90 mg/dl). Plasma nitrate/nitrite and endothelin-1 (ET-1) were measured as surrogates of nitric oxide-mediated vasodilation and vasoconstriction, respectively. RH was defined as the drop in glucose (%) from fasting to clamp steady state.

RESULTS

There were no differences in age, weight, or VO2max between groups. PD had higher HbA1c (P<0.01) and a greater drop in glucose in response to insulin (14 vs. 8%; P=0.03). Further, heart rate (HR) increased in NG compared to PD (P<0.01), while forward wave (Pf) decreased in PD (P=0.04). Insulin also tended to reduce arterial stiffness (cfPWV) in NG versus PD (P=0.07), despite similar increases in pre-occlusion diameter (P=0.02), blood flow (P=0.02), and lower augmentation index (AIx75) (P≤0.05).

CONCLUSION

Compared with NG, insulin-induced RH corresponded with a blunted rise in HR and drop in Pf during insulin infusion in adults with PD, independent of changes in peripheral endothelial function.

Nutrition Interactions With Exercise Training on Endothelial Function

Exercise is advised to improve overall cardiovascular health and endothelial function. However, the role of nutrition on this exercise-induced endothelial adaptation is not clear. Here, we hypothesize that nutrients interact with exercise to influence endothelial function and chronic disease risk.

Sympathetic transduction to blood pressure during euglycemic-hyperinsulinemia in young healthy adults: role of burst amplitude

Insulin acts centrally to stimulate sympathetic vasoconstrictor outflow to skeletal muscle and peripherally to promote vasodilation. Given these divergent actions, the "net effect" of insulin on the transduction of muscle sympathetic nerve activity (MSNA) into vasoconstriction and thus, blood pressure (BP) remains unclear. We hypothesized that sympathetic transduction to BP would be attenuated during hyperinsulinemia compared with baseline. In 22 young healthy adults, MSNA (microneurography), and beat-to-beat BP (Finometer or arterial catheter) were continuously recorded, and signal-averaging was performed to quantify the mean arterial pressure (MAP) and total vascular conductance (TVC; Modelflow) responses following spontaneous bursts of MSNA at baseline and during a euglycemic-hyperinsulinemic clamp. Hyperinsulinemia significantly increased MSNA burst frequency and mean burst amplitude (baseline: 46 ± 6 au; insulin: 65 ± 16 au, P < 0.001) but did not alter MAP. The peak MAP (baseline: 3.2 ± 1.5 mmHg; insulin: 3.0 ± 1.9 mmHg, P = 0.67) and nadir TVC (P = 0.45) responses following all MSNA bursts were not different between conditions indicating preserved sympathetic transduction. However, when MSNA bursts were segregated into quartiles based on their amplitudes at baseline and compared with similar amplitude bursts during hyperinsulinemia, the peak MAP and TVC responses were blunted (e.g., largest burst quartile: MAP, baseline: Δ4.4 ± 1.7 mmHg; hyperinsulinemia: Δ3.0 ± 0.8 mmHg, P = 0.02). Notably, ∼15% of bursts during hyperinsulinemia exceeded the size of any burst at baseline, yet the MAP/TVC responses to these larger bursts (MAP, Δ4.9 ± 1.4 mmHg) did not differ from the largest baseline bursts (P = 0.47). These findings indicate that increases in MSNA burst amplitude contribute to the overall maintenance of sympathetic transduction during hyperinsulinemia.

Impact of sex and diet-induced weight loss on vascular insulin sensitivity in type 2 diabetes

Vascular insulin resistance, a major characteristic of obesity and type 2 diabetes (T2D), manifests with blunting of insulin-induced vasodilation. Although there is evidence that females are more whole body insulin sensitive than males in the healthy state, whether sex differences exist in vascular insulin sensitivity is unclear. Also uncertain is whether weight loss can reestablish vascular insulin sensitivity in T2D. The purpose of this investigation was to 1) establish if sex differences in vasodilatory responses to insulin exist in absence of disease, 2) determine whether female sex affords protection against the development of vascular insulin resistance with long-term overnutrition and obesity, and 3) examine if diet-induced weight loss can restore vascular insulin sensitivity in men and women with T2D. First, we show in healthy mice and humans that sex does not influence insulin-induced femoral artery dilation and insulin-stimulated leg blood flow, respectively. Second, we provide evidence that female mice are protected against impairments in insulin-induced dilation caused by overnutrition-induced obesity. Third, we show that men and women exhibit comparable levels of vascular insulin resistance when T2D develops but that diet-induced weight loss is effective at improving insulin-stimulated leg blood flow, particularly in women. Finally, we provide indirect evidence that these beneficial effects of weight loss may be mediated by a reduction in endothelin-1. In aggregate, the present data indicate that female sex confers protection against obesity-induced vascular insulin resistance and provide supportive evidence that, in women with T2D, vascular insulin resistance can be remediated with diet-induced weight loss.

New insights into mechanisms of endothelial insulin resistance in type 2 diabetes

Insulin resistance in the vasculature is a hallmark of type 2 diabetes (T2D), and blunting of insulin-induced vasodilation is its primary consequence. Individuals with T2D exhibit a marked impairment in insulin-induced dilation in resistance arteries across vascular beds. Importantly, reduced insulin-stimulated vasodilation and blood flow to skeletal muscle limits glucose uptake and contributes to impaired glucose control in T2D. The study of mechanisms responsible for the suppressed vasodilatory effects of insulin has been a growing topic of interest for not only its association with glucose control and extension to T2D but also its relationship with cardiovascular disease development and progression. In this mini-review, we integrate findings from recent studies by our group with the existing literature focused on the mechanisms underlying endothelial insulin resistance in T2D.

Young Women Are Protected Against Vascular Insulin Resistance Induced by Adoption of an Obesogenic Lifestyle

Vascular insulin resistance is a feature of obesity and type 2 diabetes that contributes to the genesis of vascular disease and glycemic dysregulation. Data from preclinical models indicate that vascular insulin resistance is an early event in the disease course, preceding the development of insulin resistance in metabolically active tissues. Whether this is translatable to humans requires further investigation. To this end, we examined if vascular insulin resistance develops when young healthy individuals (n = 18 men, n = 18 women) transition to an obesogenic lifestyle that would ultimately cause whole-body insulin resistance. Specifically, we hypothesized that short-term (10 days) exposure to reduced ambulatory activity (from >10 000 to <5000 steps/day) and increased consumption of sugar-sweetened beverages (6 cans/day) would be sufficient to prompt vascular insulin resistance. Furthermore, given that incidence of insulin resistance and cardiovascular disease is lower in premenopausal women than in men, we postulated that young females would be protected against vascular insulin resistance. Consistent with this hypothesis, we report that after reduced ambulation and increased ingestion of carbonated beverages high in sugar, young healthy men, but not women, exhibited a blunted leg blood flow response to insulin and suppressed skeletal muscle microvascular perfusion. These findings were associated with a decrease in plasma adropin and nitrite concentrations. This is the first evidence in humans that vascular insulin resistance can be provoked by short-term adverse lifestyle changes. It is also the first documentation of a sexual dimorphism in the development of vascular insulin resistance in association with changes in adropin levels.

Ameliorative effects of chickpea flavonoids on redox imbalance and mitochondrial complex I dysfunction in type 2 diabetic rats

Chickpeas are an important source of flavonoids in the human diet, and researchers have demonstrated that flavonoids have antidiabetic compositions in chickpeas. Because the NAD⁺/NADH redox balance is heavily perturbed in diabetes and complex I is the only site for NADH oxidation and NAD⁺ regeneration, in the present study, mitochondrial complex I was used as a target for anti-diabetes. The objective of this study was to investigate the effects of a crude chickpea flavonoid extract (CCFE) on NAD⁺/NADH redox imbalance and mitochondrial complex I dysfunction in the pancreas as well as oxidative stress in type 2 diabetes mellitus (T2DM) rats. Our results demonstrated that the degree of NAD⁺/NADH redox imbalance in the pancreas of T2DM rats was alleviated by CCFE, which is likely attributed to the inhibition of the polyol pathway and the decrease in poly ADP ribose polymerase (PARP) and sirtuin 3 (Sirt3) activities. Moreover, mitochondrial complex I dysfunction in the pancreas of T2DM rats was ameliorated by CCFE through the suppression of the activity of complex I. Furthermore, CCFE treatment could attenuate oxidative stress in T2DM rats, which was proven by the reduction in hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) as well as the upregulation of glutathione peroxidase (GSH-Px) and superoxide dismutase (SOD) in serum. CCFE treatment significantly improved dyslipidemia in T2DM rats.

Endothelial HSP72 is not reduced in type 2 diabetes nor is it a key determinant of endothelial insulin sensitivity

Impaired endothelial insulin signaling and consequent blunting of insulin-induced vasodilation is a feature of type 2 diabetes (T2D) that contributes to vascular disease and glycemic dysregulation. However, the molecular mechanisms underlying endothelial insulin resistance remain poorly known. Herein, we tested the hypothesis that endothelial insulin resistance in T2D is attributed to reduced expression of heat shock protein 72(HSP72). HSP72 is a cytoprotective chaperone protein that can be upregulated with heating and is reported to promote insulin sensitivity in metabolically active tissues, in part via inhibition of JNK activity. Accordingly, we further hypothesized that, in T2D individuals, seven days of passive heat treatment via hot water immersion to waist-level would improve leg blood flow responses to an oral glucose load (i.e., endogenous insulin stimulation) via induction of endothelial HSP72. In contrast, we found that: 1) endothelial insulin resistance in T2D mice and humans was not associated with reduced HSP72 in aortas and venous endothelial cells, respectively; 2) after passive heat treatment, improved leg blood flow responses to an oral glucose load did not parallel with increased endothelial HSP72; 3) downregulation of HSP72 (via small-interfering RNA) or upregulation of HSP72 (via heating) in cultured endothelial cells did not impair or enhance insulin signaling, respectively, nor was JNK activity altered. Collectively, these findings do not support the hypothesis that reduced HSP72 is a key driver of endothelial insulin resistance in T2D but provide novel evidence that lower-body heating may be an effective strategy for improving leg blood flow responses to glucose ingestion-induced hyperinsulinemia.

SGLT2 inhibition attenuates arterial dysfunction and decreases vascular F-actin content and expression of proteins associated with oxidative stress in aged mice

Aging of the vasculature is characterized by endothelial dysfunction and arterial stiffening, two key events in the pathogenesis of cardiovascular disease (CVD). Treatment with sodium glucose transporter 2 (SGLT2) inhibitors is now known to decrease cardiovascular morbidity and mortality in type 2 diabetes. However, whether SGLT2 inhibition attenuates vascular aging is unknown. We first confirmed in a cohort of adult subjects that aging is associated with impaired endothelial function and increased arterial stiffness and that these two variables are inversely correlated. Next, we investigated whether SGLT2 inhibition with empagliflozin (Empa) ameliorates endothelial dysfunction and reduces arterial stiffness in aged mice with confirmed vascular dysfunction. Specifically, we assessed mesenteric artery endothelial function and stiffness (via flow-mediated dilation and pressure myography mechanical responses, respectively) and aortic stiffness (in vivo via pulse wave velocity and ex vivo via atomic force microscopy) in Empa-treated (14 mg/kg/day for 6 weeks) and control 80-week-old C57BL/6 J male mice. We report that Empa-treated mice exhibited improved mesenteric endothelial function compared with control, in parallel with reduced mesenteric artery and aortic stiffness. Additionally, Empa-treated mice had greater vascular endothelial nitric oxide synthase activation, lower phosphorylated cofilin, and filamentous actin content, with downregulation of pathways involved in production of reactive oxygen species. Our findings demonstrate that Empa improves endothelial function and reduces arterial stiffness in a preclinical model of aging, making SGLT2 inhibition a potential therapeutic alternative to reduce the progression of CVD in older individuals.

Role of the arterial baroreflex in the sympathetic response to hyperinsulinemia in adult humans

Muscle sympathetic nerve activity (MSNA) increases during hyperinsulinemia, primarily attributed to central nervous system effects. Whether peripheral vasodilation induced by insulin further contributes to increased MSNA via arterial baroreflex-mediated mechanisms requires further investigation. Accordingly, we examined baroreflex modulation of the MSNA response to hyperinsulinemia. We hypothesized that rescuing peripheral resistance with coinfusion of the vasoconstrictor phenylephrine would attenuate the MSNA response to hyperinsulinemia. We further hypothesized that the insulin-mediated increase in MSNA would be recapitulated with another vasodilator (sodium nitroprusside, SNP). In 33 young healthy adults (28 M/5F), MSNA (microneurography) and arterial blood pressure (BP, Finometer/brachial catheter) were measured, and total peripheral resistance (TPR, ModelFlow) and baroreflex sensitivity were calculated at rest and during intravenous infusion of insulin (n = 20) or SNP (n = 13). A subset of participants receiving insulin (n = 7) was coinfused with phenylephrine. Insulin infusion decreased TPR (P = 0.01) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity or BP (P > 0.05). Coinfusion with phenylephrine returned TPR and MSNA to baseline, with no effect on arterial baroreflex sensitivity (P > 0.05). Similar to insulin, SNP decreased TPR (P < 0.02) and increased MSNA (P < 0.01), with no effect on arterial baroreflex sensitivity (P > 0.12). Acute hyperinsulinemia shifts the baroreflex stimulus-response curve to higher MSNA without changing sensitivity, likely due to insulin's peripheral vasodilatory effects. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.NEW & NOTEWORTHY We hypothesized that elevation in muscle sympathetic nervous system activity (MSNA) during hyperinsulinemia is mediated by its peripheral vasodilator effect on the arterial baroreflex. Using three separate protocols in humans, we observed increases in both MSNA and cardiac output during hyperinsulinemia, which we attributed to the baroreflex response to peripheral vasodilation induced by insulin. Results show that peripheral vasodilation induced by insulin contributes to increased MSNA during hyperinsulinemia.

Role of the Autonomic Nervous System in the Hemodynamic Response to Hyperinsulinemia-Implications for Obesity and Insulin Resistance

PURPOSE OF REVIEW

Herein, we summarize recent advances which provide new insights into the role of the autonomic nervous system in the control of blood flow and blood pressure during hyperinsulinemia. We also highlight remaining gaps in knowledge as it pertains to the translation of findings to relevant human chronic conditions such as obesity, insulin resistance, and type 2 diabetes.

RECENT FINDINGS

Our findings in insulin-sensitive adults show that increases in muscle sympathetic nerve activity with hyperinsulinemia do not result in greater sympathetically mediated vasoconstriction in the peripheral circulation. Both an attenuation of α-adrenergic-receptor vasoconstriction and augmented β-adrenergic vasodilation in the setting of high insulin likely explain these findings. In the absence of an increase in sympathetically mediated restraint of peripheral vasodilation during hyperinsulinemia, blood pressure is supported by increases in cardiac output in insulin-sensitive individuals. We highlight a dynamic interplay between central and peripheral mechanisms during hyperinsulinemia to increase sympathetic nervous system activity and maintain blood pressure in insulin-sensitive adults. Whether these results translate to the insulin-resistant condition and implications for long-term cardiovascular regulation warrants further exploration.

Role of Endothelin-1 Receptors in Limiting Leg Blood Flow and Glucose Uptake During Hyperinsulinemia in Type 2 Diabetes

Skeletal muscle insulin resistance is a hallmark of individuals with type 2 diabetes mellitus (T2D). In healthy individuals insulin stimulates vasodilation, which is markedly blunted in T2D; however, the mechanism(s) remain incompletely understood. Investigations in rodents indicate augmented endothelin-1 (ET-1) action as a major contributor. Human studies have been limited to young obese participants and focused exclusively on the ET-1 A (ETA) receptor. Herein, we have hypothesized that ETA receptor antagonism would improve insulin-stimulated vasodilation and glucose uptake in T2D, with further improvements observed during concurrent ETA + ET-1 B (ETB) antagonism. Arterial pressure (arterial line), leg blood flow (LBF; Doppler), and leg glucose uptake (LGU) were measured at rest, during hyperinsulinemia alone, and hyperinsulinemia with (1) femoral artery infusion of BQ-123, the selective ETA receptor antagonist (n = 10 control, n = 9 T2D) and then (2) addition of BQ-788 (selective ETB antagonist) for blockade of ETA and ETB receptors (n = 7 each). The LBF responses to hyperinsulinemia alone tended to be lower in T2D (controls: ∆161 ± 160 mL/minute; T2D: ∆58 ± 43 mL/minute, P = .08). BQ-123 during hyperinsulinemia augmented LBF to a greater extent in T2D (% change: controls: 14 ± 23%; T2D: 38 ± 21%, P = .029). LGU following BQ-123 increased similarly between groups (P = .85). Concurrent ETA + ETB antagonism did not further increase LBF or LGU in either group. Collectively, these findings suggest that during hyperinsulinemia ETA receptor activation restrains vasodilation more in T2D than controls while limiting glucose uptake similarly in both groups, with no further effect of ETB receptors (NCT04907838).

Diabetes pathogenesis and management: the endothelium comes of age

Endothelium, acting as a barrier, protects tissues against factors that provoke insulin resistance and type 2 diabetes and itself responds to the insult of insulin resistance inducers with altered function. Endothelial insulin resistance and vascular dysfunction occur early in the evolution of insulin resistance-related disease, can co-exist with and even contribute to the development of metabolic insulin resistance, and promote vascular complications in those affected. The impact of endothelial insulin resistance and vascular dysfunction varies depending on the blood vessel size and location, resulting in decreased arterial plasticity, increased atherosclerosis and vascular resistance, and decreased tissue perfusion. Women with insulin resistance and diabetes are disproportionately impacted by cardiovascular disease, likely related to differential sex-hormone endothelium effects. Thus, reducing endothelial insulin resistance and improving endothelial function in the conduit arteries may reduce atherosclerotic complications, in the resistance arteries lead to better blood pressure control, and in the microvasculature lead to less microvascular complications and more effective tissue perfusion. Multiple diabetes therapeutic modalities, including medications and exercise training, improve endothelial insulin action and vascular function. This action may delay the onset of type 2 diabetes and/or its complications, making the vascular endothelium an attractive therapeutic target for type 2 diabetes and potentially type 1 diabetes.

Mutation of the 5'-untranslated region stem-loop mRNA structure reduces type I collagen deposition and arterial stiffness in male obese mice

Arterial stiffening, a characteristic feature of obesity and type 2 diabetes, contributes to the development and progression of cardiovascular diseases (CVD). Currently, no effective prophylaxis or therapeutics is available to prevent or treat arterial stiffening. A better understanding of the molecular mechanisms underlying arterial stiffening is vital to identify newer targets and strategies to reduce CVD burden. A major contributor to arterial stiffening is increased collagen deposition. In the 5'-untranslated regions of mRNAs encoding for type I collagen, an evolutionally conserved stem-loop (SL) structure plays an essential role in its stability and post-transcriptional regulation. Here, we show that feeding a high-fat/high-sucrose (HFHS) diet for 28 wk increases adiposity, insulin resistance, and blood pressure in male wild-type littermates. Moreover, arterial stiffness, assessed in vivo via aortic pulse wave velocity, and ex vivo using atomic force microscopy in aortic explants or pressure myography in isolated femoral and mesenteric arteries, was also increased in those mice. Notably, all these indices of arterial stiffness, along with collagen type I levels in the vasculature, were reduced in HFHS-fed mice harboring a mutation in the 5'SL structure, relative to wild-type littermates. This protective vascular phenotype in 5'SL-mutant mice did not associate with a reduction in insulin resistance or blood pressure. These findings implicate the 5'SL structure as a putative therapeutic target to prevent or reverse arterial stiffening and CVD associated with obesity and type 2 diabetes.NEW & NOTEWORTHY In the 5'-untranslated (UTR) regions of mRNAs encoding for type I collagen, an evolutionally conserved SL structure plays an essential role in its stability and posttranscriptional regulation. We demonstrate that a mutation of the SL mRNA structure in the 5'-UTR decreases collagen type I deposition and arterial stiffness in obese mice. Targeting this evolutionarily conserved SL structure may hold promise in the management of arterial stiffening and CVD associated with obesity and type 2 diabetes.

Hyperinsulinemia blunts sympathetic vasoconstriction: a possible role of β-adrenergic activation

Herein we report in a sample of healthy young men (n = 14) and women (n = 12) that hyperinsulinemia induces time-dependent decreases in total peripheral resistance and its contribution to the maintenance of blood pressure. In the same participants, we observe profound vasodilatory effects of insulin in the lower limb despite concomitant activation of the sympathetic nervous system. We hypothesized that this prominent peripheral vasodilation is possibly due to the ability of the leg vasculature to escape sympathetic vasoconstriction during systemic insulin stimulation. Consistent with this notion, we demonstrate in a subset of healthy men (n = 9) and women (n = 7) that systemic infusion of insulin blunts sympathetically mediated leg vasoconstriction evoked by a cold pressor test, a well-established sympathoexcitatory stimulus. Further substantiating this observation, we show in mouse aortic rings that insulin exposure suppresses epinephrine and norepinephrine-induced vasoconstriction. Notably, we found that such insulin-suppressing effects on catecholamine-induced constriction are diminished following β-adrenergic receptor blockade. In accordance, we also reveal that insulin augments β-adrenergic-mediated vasorelaxation in isolated arteries. Collectively, these findings support the idea that sympathetic vasoconstriction can be attenuated during systemic hyperinsulinemia in the leg vasculature of both men and women and that this phenomenon may be in part mediated by potentiation of β-adrenergic vasodilation neutralizing α-adrenergic vasoconstriction.