No effect of fitness on brachial or forearm vascular function during acute inflammation in young adults

Abstract

Acute inflammation is associated with increased risk of cardiovascular events and impaired vasodilatory capacity. Vasodilatory capacity can be measured in different segments of the arterial tree; however, it is unknown if the effects of acute inflammation are vascular segment-specific or if inflammation-induced dysfunction can be attenuated by factors that modulate cardiovascular risk, such as high cardiorespiratory fitness. The purpose of this study was to determine the effect of acute inflammation and fitness on conduit artery, resistance artery, and microvascular function in healthy, young adults. Vascular function was assessed at baseline and 24 h after a typhoid vaccination in 11 low-fit (5 male, 24 yr of age, 34.5 ± 2.9 ml·kg⁻¹·min⁻¹ peak O₂ uptake (V̇o_2peak)] and 12 high-fit (7 male, 27 yr of age, 56.4 ± 9.7 ml·kg⁻¹·min⁻¹ V̇o_2peak) young adults. Vascular assessments included flow-mediated dilation (FMD) of the brachial artery, forearm reactive hyperemia (RH) via venous occlusion plethysmography, and near-infrared spectroscopy (NIRS) during a 5-min arterial occlusion. Acute inflammation was evident with increases in IL-6 and C-reactive protein (P < 0.01), and mean arterial pressure did not change (P = 0.33). FMD was lower in the high-fit group, yet it was reduced in both groups at 24 h, even after controlling for shear (P < 0.05). No effect of acute inflammation was observed for RH or NIRS (P > 0.05). Acute inflammation had nonuniform effects on vascular function throughout the arterial tree in young adults, and fitness did not alter the vascular response. This suggests that cardiorespiratory fitness may not protect the vasculature during acute inflammation in young adults in the absence of age- or disease-related decline in vascular function.

INTRODUCTION

Acute inflammation, occurring during bouts of illness, is associated with an increase in cardiovascular events (38, 40). Over half of the cardiovascular events occur within the first 24 h (7) and may stem from alterations in vascular function. During acute inflammation, vasodilatory capacity is reduced (6, 16) partially due to increases in oxidative stress and reductions in nitric oxide bioavailability (6, 26, 31). This blunted vasodilation may contribute to the increased risk of cardiovascular events during acute inflammation.

Vasodilatory capacity of the vessel is often determined in different segments of the arterial tree (conduit arteries, resistance arteries, and microvasculature) in humans by assessment of vessel reactivity to different vasoactive stimuli [reactive hyperemia (RH) and drug infusions]. During acute inflammation, both conduit artery vasodilatory capacity, as measured by flow-mediated dilation (FMD) of the brachial artery (16, 22), and resistance artery reactivity to vasodilatory drugs (6, 21, 31) are reduced in young adults. Microvascular reactivity is also reduced during severe inflammation, such as sepsis (10, 29); however, no previous studies have investigated the effects of modest acute inflammation, similar to those during bouts of illness, on the microvasculature. Investigating each segment of the arterial tree would provide insight into whether the vascular dysfunction is uniform throughout the arterial tree or if a segment, such as the microvasculature, is able to maintain functionality despite reductions in upstream vascular function. To our knowledge, only one study has simultaneously assessed the effects of acute inflammation on the different segments of the arterial tree (16); thus it is unknown if the effects of acute inflammation are vascular segment-specific.

Furthermore, given the elevated cardiovascular risk associated with acute inflammation, it is important to determine if the inflammation-induced vascular dysfunction can be attenuated or prevented by factors that modulate cardiovascular risk. High cardiorespiratory fitness is associated with reduced risk of cardiovascular disease and all-cause mortality (4, 39), and fit individuals have lower levels of systemic inflammation (19, 20). During acute inflammation, fitness has only been investigated for conduit artery function, in which moderate fitness does not alter the conduit artery response in young adults (35). This may be because the effect of high fitness on FMD in young adults is controversial (5, 14); however, microvascular reactivity is greater in high-fit young adults than their low-fit counterparts (25). Thus it is undetermined if high fitness protects the vasculature during an acute bout of inflammation.

Assessing more than one site of the arterial tree during acute inflammation would provide a comprehensive overview of vascular reactivity and help determine if acute inflammation has uniform effects. Additionally, determining whether higher fitness alters the response to acute inflammation may provide the basis for future behavioral interventions to reduce cardiovascular burden during acute inflammation. Therefore, the purpose of this study was to determine the effect of acute inflammation and fitness on conduit artery, resistance artery, and microvascular function in healthy, young adults. We hypothesized that acute inflammation, induced via a typhoid vaccination, would reduce vascular function in all three segments of the arterial tree; however, high fitness would attenuate the response in the microvasculature.

METHODS

Subjects

Healthy, young (18–35 yr of age) adults were recruited from the Chicago area for participation in the study. Participants were enrolled in the study if they were either sedentary (defined as <30 min of moderately intense physical activity per day) or aerobically trained (defined as ≥30 min of moderately intense aerobic activity per day, ≥4 days/wk for at least the past 3 mo), assessed using self-reported physical activity as a first screening of cardiorespiratory fitness. High- and low-fit groups were then defined as peak O₂ uptake (V̇o_2peak) at the ≥75th and <75th percentile, respectively, according to the American College of Sports Medicine age- and sex-specific percentiles for cardiorespiratory fitness (1). Exclusion criteria included smoking; antioxidant or vitamin supplementation; anti-inflammatory medication within the previous 2 wk; body mass index >35 kg/m²; any known cardiovascular, metabolic, or inflammatory disease; current use of blood pressure medication or other drugs influencing cardiovascular outcomes; typhoid vaccination within the previous 2 yr or a prior adverse reaction; illness within 2 wk before testing; and pregnancy.

All participants provided written informed consent before participation. The study was approved by the Institutional Review Board at the University of Illinois at Chicago (IRB no. 2017-0650) and conformed to the guidelines set forth by the Declaration of Helsinki.

Study Design

The study was completed in two study visits. All participants arrived at the laboratory postprandial for ≥10 h and had refrained from caffeine and alcohol consumption and physical activity for ≥24 h before each testing visit. Female participants were tested during the first 7 days of their menstrual cycle or during their placebo week if taking oral contraceptives (n = 3: 1 low-fit, 2 high-fit).

During visit 1, a fasting blood sample was collected, and then the participants rested quietly in a temperature-controlled room for 10 min in the supine position. Thereafter, resting blood pressure and baseline vascular measurements were obtained. All vascular measurements were obtained simultaneously by coordinating the release of the hyperemic cuffs on each arm between the two study personnel. Conduit artery and microvascular function were assessed on the right arm, while resistance artery function was assessed on the left arm. After baseline vascular measurements were obtained, participants completed a maximal exercise test to determine V̇o_2peak. A registered nurse then injected the Salmonella typhi polysaccharide vaccine (Typhim Vi, Sanofi Pasteur SA) into the nondominant arm to induce acute inflammation. Acute inflammation is often induced in cardiovascular research by use of either influenza or typhoid vaccination (16, 22, 35). Vaccinations provide a safe and controlled inflammatory response; the typhoid vaccination was shown to increase high-sensitivity C-reactive protein (CRP) up to ≥32 h postvaccination (45). After 24 h (at visit 2), the fasting blood sample and vascular measurements were repeated in the same conditions and at the same time of day to avoid diurnal variation.

Measurements

Anthropometrics.

Height and weight were measured and used to calculate body mass index (BMI) as weight (kg) ÷ height (m²). Dual-energy X-ray absorptiometry (Lunar iDXA, GE, Boston, MA) was used to determine body fat percentage, arm fat mass, and arm lean mass.

Cardiorespiratory fitness.

V̇o_2peak (ml·kg⁻¹·min⁻¹) was assessed using an incremental treadmill test during analysis of expired gases (TrueOne, Parvo Medics, Sandy, UT). Because of varying fitness levels by study design, participants chose a comfortable walking or jogging pace to perform a 3-min warm-up at 0% incline. During the test, work rate increased by 2% grade every 2 min up to 12%, after which speed was increased 0.5 mph every minute until volitional exhaustion. The test was terminated and considered a maximal effort when three of the following five criteria were met: 1) respiratory exchange ratio ≥1.10; 2) peak heart rate (HR) within 10 beats/min of age-predicted HR; 3) plateau (increase of ≤150 ml) in O₂ uptake with an increase in workload; 4) final rating of perceived exertion ≥17 on the Borg scale; and 5) volitional exhaustion.

Brachial blood pressure.

Resting brachial blood pressure was measured in the right arm following 10 min of rest in the supine position using an automated ambulatory blood pressure monitor (Mobil-O-Graph 24 PWA, IEM, Stolberg, Germany). All measurements were made in duplicate. If values differed by >5 mmHg, another measurement was obtained until the two values were within 5 mmHg.

Conduit artery function.

Brachial artery FMD was assessed by measuring the change in arterial diameter during RH following a forearm cuff occlusion, according to standard guidelines (42). The right brachial artery was imaged via ultrasound (Hitachi-Aloka Alpha 7, Tokyo, Japan) using a high-frequency linear-array probe 5–10 cm proximal to a rapid-release blood pressure cuff. Arterial diameters and Doppler velocity were measured simultaneously and recorded for the entire 1-min baseline, 5-min occlusion at 250 mmHg, and 3-min recovery. Data were analyzed with automatic edge detection software (FMD Studio Cardiovascular Suite, QUIPU, Pisa, Italy). FMD was defined as the maximum percent change in diameter compared with baseline. To characterize the RH stimulus, peak blood flow, peak blood velocity, and shear rate area under the curve (AUC) to maximum diameter were determined. Given the role of shear stress on FMD, FMD was controlled for shear by dividing FMD by shear AUC (42). Three participants (2 low-fit and 1 high-fit) were excluded from the analyses due to a low-quality FMD recording. Our laboratory coefficients of variation for FMD and FMD/shear AUC are 9.7% and 8.2%, respectively.

Resistance artery function.

Vasodilatory capacity of forearm resistance arteries was assessed using RH and strain-gauge plethysmography (EC-6, DE Hokanson, Bellevue, WA). Because of the methodology of the measurement, the forearm blood flow (FBF) response likely also includes the microvasculature. A strain gauge was placed over the widest aspect of the forearm, and resting FBF was assessed as the mean of six stable measurements. A 5-min upper arm occlusion was performed, and changes in forearm volume were measured following cuff release in thirteen 15-s cycles (7 s of occlusion and 8 s of deflation). Peak FBF was recorded as the highest reading following cuff release. RH was subsequently calculated as peak FBF − resting FBF. AUC was determined using all 13 FBF values as a measure of total RH. One high-fit participant was excluded from the analyses because of equipment malfunction. Our laboratory coefficients of variation for RH and AUC are 5.95% and 2.5%, respectively.

Microvascular function.

Near-infrared spectroscopy (NIRS) is used during vascular occlusion tests as a noninvasive proxy of microvascular function (3, 13). NIRS detects hemoglobin in the smallest arterioles, capillaries, and venules of the microcirculation, as >1-mm-diameter vessels maximally absorb NIRS light due to the sufficient hemoglobin present (23). Thus the NIRS test is solely reflective of microvascular function. Microvascular reactivity and oxygenation were assessed by placement of the NIRS probe (OxiplexTS, ISS, Champaign, IL) on the right forearm flexor muscles below the FMD occlusion cuff. During the FMD occlusion, tissue saturation index (TSI, a calculated parameter of oxygenation) was simultaneously recorded during the 1-min baseline, 5-min arterial occlusion, and 3-min RH. The TSI was subsequently used for all analyses. The TSI occlusion slope and magnitude of decline are indexes of muscle oxidative capacity (3, 12); TSI reperfusion slope, time to peak, and reperfusion magnitude are indicative of microvascular reactivity and the ability to accommodate increases in blood flow (3, 11); and peak hyperemic response was calculated as the peak TSI – baseline TSI and is indicative of RH (13). Our laboratory coefficients of variation for the occlusion slope, reperfusion slope, and peak hyperemic response are 9.2%, 5.3%, and 13.4%, respectively.

Inflammatory markers.

Venous blood samples were collected following a ≥10-h fast in a serum separation tube and allowed to clot for 30 min. Samples were then centrifuged at 4°C for 15 min at 2,000 g and stored at −80°C until analysis. Interleukin-6 (IL-6) and CRP were assessed as markers of systemic inflammation in duplicate using commercially available high-sensitivity enzyme-linked immunosorbent assay kits (HS600B, R & D Systems, Minneapolis, MN; Crystal Chem, Elk Grove Village, IL). All samples were analyzed using a single plate, with intra-assay coefficients of variation between duplicates of 4.1% for IL-6 and 4.3% for CRP.

Statistical Analysis

Values are means ± SD. Data were assessed for normality and appropriately transformed when necessary to satisfy statistical assumptions. All data are reported as raw values for ease of interpretation. Baseline differences between groups were analyzed by independent t tests or χ² tests. The inflammatory response was analyzed by a 2 × 2 (group × time) repeated-measures ANOVA on absolute values. The inflammatory response was also analyzed with an independent t test on absolute and percent change values. A 2 × 2 (group × time) repeated-measures ANOVA (low-fit vs. high-fit, pre- vs. postvaccination) was used to identify effects in all vascular outcome variables. Secondary analyses were conducted for FMD and NIRS by inclusion of the change in baseline diameter and arm fat mass, respectively, as covariates in the model. Adipose tissue thickness can impact total-hemoglobin signal derived from NIRS (8); thus we conducted NIRS analysis covarying for arm fat mass as a proxy of adipose tissue thickness. To further investigate the influence of baseline diameter, FMD was allometrically scaled, as previously described by Atkinson and Batterham (2). Data analysis was carried out using SPSS (version 24, SPSS, Chicago, IL), and an a priori significance level was set at P < 0.05.

RESULTS

Of the 81 individuals screened for eligibility, 29 were excluded based on study criteria [intrauterine device (IUD; n = 4), moderate physical activity (n = 8), previous vaccination (n = 2), smoking (n = 8) and medications (n = 7)], 24 chose not to participate because of the time commitment and scheduling, and 5 were unwilling to receive the vaccine. Twenty-three young adults (11 low-fit and 12 high-fit) participated in the study. V̇o_2peak was at the ≥75th percentile in all individuals in the high-fit group and the <50th percentile in all individuals in the low-fit group, except one, whose V̇o_2peak was at the 60th percentile. The high-fit group was slightly older (P = 0.04) and had less body fat (P < 0.01) and greater cardiorespiratory fitness (P < 0.01) than the low-fit group (Table 1). Acute inflammation was induced in both groups, as evident by the increases in IL-6 (P < 0.01, from 1.41 ± 0.46 to 2.6 ± 1.36 pg/ml in low-fit and from 0.90 ± 0.49 to 2.22 ± 1.35 pg/ml in high-fit) and CRP (P < 0.01, from 1.66 ± 2.05 to 3.50 ± 3.69 mg/l in low-fit and from 0.7 ± 2.01 to 2.31 ± 3.95 mg/l in high-fit; Fig. 1). The magnitude of the inflammatory response did not differ between groups when assessed as either absolute (P = 0.80 for IL-6, P = 0.79 for CRP) or percent (P = 0.11 for IL-6, P = 0.06 for CRP) change. During acute inflammation, there was no change in mean arterial pressure in either group (from 86 ± 9 to 86 ± 7 mmHg in low-fit and from 86 ± 9 to 85 ± 7 mmHg in high-fit, P = 0.33).

Table 1.

Descriptive characteristics

	Low-Fit (n = 11)	High-Fit (n = 12)	P Value
Male/female	5/6	7/5	0.54
Age, yr	24 ± 5	27 ± 4	0.04
Height, cm	170.2 ± 7.1	172.0 ± 11.6	0.67
Weight, kg	67.0 ± 17.5	67.1 ± 10.8	0.99
Body mass index, kg/m2	22.8 ± 4.3	22.6 ± 1.9	0.84
Body fat, %	32.2 ± 5.2	19.7 ± 6.7	<0.01
V̇o2peak, ml·kg−1·min−1	34.5 ± 2.9	56.4 ± 9.7	<0.01

Values are means ± SD. V̇o_2peak, peak O₂ uptake.

Fig. 1.

Inflammatory response to vaccination in low-fit (n = 8) and high-fit (n = 11) participants. Thick line indicative of group mean change. IL-6, interleukin-6; CRP, C-reactive protein.

FMD was reduced in both groups during acute inflammation (P = 0.01; Fig. 2), as well as absolute diameter change (P = 0.01, from 0.22 ± 0.08 to 0.16 ± 0.04 mm in low-fit and from 0.18 ± 0.09 to 0.14 ± 0.07 mm in high-fit). Brachial vasodilation during acute inflammation approached significance (P = 0.07), with baseline diameter increasing from 3.37 ± 0.44 to 3.42 ± 0.43 mm and from 4.11 ± 0.80 to 4.19 ± 0.79 mm in low- and high-fit individuals, respectively. After covariance for the change in baseline diameter, the effect of inflammation on FMD remained (P = 0.02). Furthermore, the effect of inflammation on FMD also remained after controlling for shear stress [shear AUC to peak: from 19,557 ± 7,663 to 20,615 ± 4,739 for low-fit and from 14,508 ± 6,096 to 13,617 ± 5,687 for high-fit; FMD/shear P = 0.03]. Neither inflammation nor fitness influenced peak blood flow or peak blood velocity during RH (P > 0.05). Overall, FMD was lower in high-fit individuals (P = 0.01), and baseline brachial diameters were larger (P = 0.02) and shear was less in high- than low-fit individuals (P = 0.03). After controlling for shear stress, the group differences in FMD were abrogated (P = 0.90). Moreover, the effect of inflammation on the reduction in FMD remained with allometric scaling (P = 0.045), but the group difference in FMD was eliminated (P = 0.07).

Fig. 2.

Conduit artery function at baseline and during acute inflammation in low- and high-fit participants. Thick line indicative of group mean change. FMD, flow-mediated dilation; AUC, area under the curve; Max, maximum diameter.

Resistance artery vasodilatory function was not altered during acute inflammation as assessed by RH (P = 0.25) or AUC (P = 0.64), nor did it differ between groups (P > 0.05; Table 2). Baseline FBF was also not different between groups (0.96), nor did baseline FBF change with inflammation (P = 0.92).

Table 2.

Resistance artery and microvascular function at baseline and during acute inflammation in low- and high-fit participants

	Low-Fit		High-Fit		P Value [Effect size (η2)]
	Baseline	24 h	Baseline	24 h	Inflammation	Fitness	Interaction
Resistance artery
Baseline FBF, ml·min−1·100 ml tissue−1	2.6 ± 0.7	2.5 ± 0.6	2.4 ± 1.4	2.6 ± 0.9	0.92 (0.001)	0.96 (0.000)	0.50 (0.024)
RH, ml min−1·100 ml tissue−1	21.5 ± 4.9	20.2 ± 6.7	25.6 ± 6.3	24.7 ± 5.2	0.25 (0.066)	0.08 (0.148)	0.79 (0.004)
RH AUC, ml min−1·100 ml tissue−1·s	62 ± 17	63 ± 14	66 ± 32	70 ± 15	0.64 (0.011)	0.49 (0.025)	0.75 (0.005)
Microvasculature
TSI baseline, %	72 ± 4	73 ± 3	73 ± 4	73 ± 3	0.95 (0.000)	0.60 (0.013)	0.30 (0.052)
Occlusion slope, %/s	−0.075 ± 0.019	−0.076 ± 0.014	−0.085 ± 0.022	−0.087 ± 0.028	0.61 (0.013)	0.24 (0.064)	0.89 (0.001)
TSI minimum, %	51 ± 6	51 ± 4	48 ± 6	45 ± 8	0.20 (0.011)	0.08 (0.137)	0.08 (0.139)
TSI peak, %	86 ± 4	86 ± 3	86 ± 3	85 ± 2	0.93 (0.000)	0.59 (0.014)	0.70 (0.007)
Time to peak, s	37 ± 8	38 ± 6	32 ± 4	31 ± 3	0.83 (0.002)	0.01 (0.280)	0.51 (0.021)
Magnitude of decline, %	−21 ± 6	−22 ± 4	−25 ± 6	−28 ± 7	0.08 (0.141)	0.045 (0.179)	0.17 (0.089)
Reperfusion magnitude, %	35 ± 7	35 ± 5	37 ± 7	40 ± 9	0.14 (0.103)	0.20 (0.076)	0.08 (0.135)
Reperfusion slope, %/s	1.72 ± 0.4	1.69 ± 0.34	2.01 ± 0.51	2.00 ± 0.47	0.75 (0.005)	0.09 (0.129)	0.86 (0.002)
Peak hyperemic response, %	14 ± 4	14 ± 3	12 ± 3	13 ± 3	1.00 (0.000)	0.31 (0.049)	0.39 (0.036)

Values are means ± SD. AUC, area under the curve; FBF, forearm blood flow, RH, reactive hyperemia; TSI, tissue saturation index.

Microvascular reactivity and oxidative capacity were also not influenced by acute inflammation (P > 0.05; Table 2). However, time to peak was faster (P = 0.01) and magnitude of decline overall was greater (P = 0.045) in high-fit participants during the vascular occlusion test. No significant group differences were observed in arm lean mass, but high-fit participants had less arm fat mass than low-fit participants (P < 0.01). After controlling for arm fat mass, the group difference in the magnitude of decline was eliminated (P = 0.83).

DISCUSSION

Acute inflammation reduced vasodilatory capacity of the large, conduit arteries, with no changes in resistance arteries or the microvasculature. High cardiorespiratory fitness did not alter the vascular response to acute inflammation in these young, healthy adults. Contrary to our hypotheses, acute inflammation has nonuniform effects on vascular function throughout the arterial tree in young adults, and high fitness does not protect against the effects of acute inflammation in the conduit arteries.

Vascular Function

We noted reductions in conduit artery vasodilatory capacity in young adults during acute inflammation. The reduction in FMD is in agreement with a number of studies that evaluated vasodilatory capacity of large conduit arteries in young adults and consistently showed impairments during acute inflammation (16, 22). FMD is dependent on shear stress, as increased shear stress following a 5-min occlusion produces a greater increase in vessel diameter (36, 42). Importantly, the reductions in vasodilatory capacity remained after controlling for the shear stimulus. Thus our results suggest a true alteration in endothelial function during acute inflammation that cannot be explained by changes in diameter or shear stress alone.

The reductions in vasodilatory capacity likely manifest from alterations in endothelium-dependent nitric oxide bioavailability during acute inflammation, as endothelium-independent vasodilation appears to remain intact (16). Reductions in endothelial function may be due to increases in oxidative stress, as higher levels of reactive oxygen species reduce nitric oxide bioavailability (15), leading to endothelial dysfunction. Indeed, administration of an antioxidant during acute inflammation partially or completely restores resistance artery vasodilatory capacity (6, 26, 31). The imbalance of antioxidant capacity and oxidative stress during acute inflammation is further supported by studies that have shown reductions in total antioxidant capacity (6) and increases in oxidized low-density lipoprotein (22).

Unlike the large conduit arteries, we observed no significant decrement in resistance artery or microvascular function during acute inflammation. We observed no significant effect of inflammation on any FBF outcomes, which capture a degree of resistance artery and microvascular function. This finding is inconsistent with previous literature assessing FBF; however, our study used a physiological stimulus (RH) to assess vasodilatory capacity, whereas all previous studies used vasoactive drug infusions (6, 16, 31). One explanation for these discrepant results may be the vasodilatory mechanisms involved. Vasoactive drugs such as acetylcholine and bradykinin are specific to endothelium-dependent nitric oxide-mediated vasodilation. Nitric oxide, however, does not play a significant role in peak blood flow responses in resistance arteries and plays only a modest role in maintaining vasodilation during the mid-to-late phase of RH (41). The contribution of endothelium-derived hyperpolarizing factor (EDHF) to vasodilation increases as vessel size decreases and is upregulated during states of reduced nitric oxide availability (9, 30). Increased reliance on EDHF in resistance vessels may prove beneficial and protective during bouts of acute inflammation when nitric oxide bioavailability is reduced. Additionally, we observed no change in peak brachial blood flow or velocity during RH, a proxy of resistance artery function, during acute inflammation (17, 44). Thus, greater reliance on EDHF or hydrogen peroxide may explain why no reduction in vasodilatory capacity assessed via FBF RH was observed in the current study.

Microvascular reactivity assessed via NIRS was also maintained during acute inflammation in the present study, suggesting maintenance of oxidative capacity, tissue reperfusion, and vascular recruitment. This response is different from that observed during sepsis, which produces reductions in baseline O₂ saturation and impairments in tissue flow reperfusion and vascular recruitment (10, 28, 37). The vaccine model induces a much more modest inflammatory stimulus than that seen during sepsis or other substantial inflammatory challenges. Thus the degree of inflammation may play a role in the microvascular response, whereby the microvasculature is resilient to mild inflammation but impaired by severe inflammation. Additionally, although the reperfusion slope was shown to significantly correlate with FMD given the similarity in their endothelium-dependent vasodilatory pathway involvement (24), the mechanisms governing control of vasodilation of the microvasculature are not fully understood. Like the resistance arteries, it is possible that several different endothelium-independent vasodilators play a role in the vasodilatory pathway in the microvasculature. Alternate vasodilatory pathways may explain why no alterations in microvascular function were seen during acute inflammation.

Alternatively, differential responses across segments of the arterial tree may be related to the time course of inflammation and its effects on the vasculature. Previous studies reported decrements in resistance vessel function at 8 h postvaccination (6, 16), whereas resistance vessel function was undisturbed in our 24-h protocol. Although the vascular response to inflammation may be sensitive to time, it is difficult to hypothesize why the vasculature may respond at different times when exposed to a comparable dose of systemic inflammation. Future studies would be necessary to investigate the time course of each segmental response.

Cardiorespiratory Fitness

Cardiorespiratory fitness did not influence vascular function during acute inflammation in any aspect of the arterial tree interrogated herein. In our previous work we also saw no role of fitness on FMD in younger adults during acute inflammation; however, fitness appeared to be protective in older, moderately fit adults (35). The sample in our previous study was a secondary analysis of only moderately fit individuals. Thus the current study aimed to conduct a more robust investigation of the effects of fitness on the vascular response to acute inflammation through a priori recruitment of high- and low-fit adults. Moreover, the present study investigated conduit, resistance, and microvascular function compared with only conduit artery function in our previous work. As we also observed no effects of fitness among young, healthy adults in the present study, we suspect that the protective effects of cardiorespiratory fitness may not be apparent in the absence of age-related reductions in vascular function (27).

The similar increases in inflammatory markers in both groups in the current study could be argued as the explanation for the similar responses; however, our previous study also showed similar increases in inflammatory markers in all groups (older and young, low and moderate fitness). Alternatively, high fitness may influence the recovery time following acute inflammation. We did not collect data at various time points; thus a study incorporating more assessments may offer additional information about the effect of fitness on the vasculature during acute inflammation. Despite no significant changes during acute inflammation, fitness did alter vascular function between groups. FMD was lower in our high-fit than unfit individuals. Although fitness would be expected to protect and improve FMD, ours is not the first study to show the opposite response in highly trained young adults (14). Exercise training subjects the vascular wall to repeated bouts of increased shear stress and transmural pressure, leading to conduit artery remodeling by increasing vessel diameter and decreasing wall thickness (18, 34). In support of this, brachial artery diameter was larger in our high-fit than unfit individuals. Larger brachial artery diameter inherently reduces shear stress and FMD (14, 43) and likely explains our overall group differences. Indeed, after correction for shear stress, the group differences in FMD were eliminated, suggesting that group differences in FMD were driven by differing shear stimuli.

Fitness did not alter resistance artery or microvascular function assessed by venous occlusion plethysmography RH but had some influence on microvascular reactivity assessed by NIRS. During the NIRS vascular occlusion test, we observed a larger magnitude of decline and shorter time to peak in the high-fit individuals. These data may suggest that high-fit individuals have higher muscle oxidative capacity and more rapid recruitment and opening of the microvasculature to accommodate the increased blood flow during RH. These results are in line with the study by McLay et al. (24), who showed greater microvascular reactivity measured by the reperfusion slope and a trend toward greater muscle oxidative capacity in trained individuals. Together, these data may suggest that high fitness in young adults is beneficial for greater microvascular function.

Overall, our study had a limited sample to assess the effects of fitness on our vascular outcomes. Post hoc power analyses to detect an interaction between inflammation and fitness based on our calculated effect sizes (Table 2) suggest that ≥27 participants per group would be required for an interaction effect for microvascular function (reperfusion magnitude, TSI minimum). Although a sample size of 54 participants is feasible, most vascular outcomes had effect sizes <0.05, which would require a sample size of ≥150 participants to detect an interaction. Thus, while fitness may statistically alter the vascular response to acute inflammation in a larger sample, the magnitude of the effect of fitness appears minimal based on our sample of young, healthy adults.

Limitations

The study does present with some limitations. There may be differences in body composition in the populations recruited. Our groups were well matched on height, weight, and BMI: only one individual had a BMI >30 kg/m², and all other BMIs were <28 kg/m². Although our high-fit group had less body fat than our unfit group, the baseline inflammatory levels were not different between groups, nor was the inflammatory response (Fig. 1). An additional consideration is that the vaccination was administered following a maximal exercise test, which may influence the inflammatory response. In this study the vaccine was used as a model to induce systemic inflammation, and although the maximal exercise test was completed 24 h before visit 2, systemic inflammation was still evident, with increases in IL-6 and CRP. Previous research has also shown that, following 45 min of exercise, the immune response to a vaccine is not altered compared with a sham group (32). The present study also did not include a sham control group for testing the effect of the vaccine; however, it has been shown that a vaccine is effective at eliciting an inflammatory response and changes in vascular function compared with a sham group (33, 45). Finally, these findings are specific to young adults and would likely differ with the inclusion of older adults, who already exhibit impaired vascular function and have greater cardiovascular risk during acute inflammation. Future studies with larger sample sizes are required to evaluate the effects of acute inflammation on consecutive segments of the arterial tree and in healthy older adults and those with chronic health conditions.

Perspectives and Significance

The development of endothelial dysfunction in the conduit arteries matched with a cytokine response suggests that the typhoid vaccination leads to a state with a higher cardiovascular risk profile. As the typhoid vaccination is used as a model of acute systemic inflammation, it may provide insight into the increased risk of cardiovascular events during bouts of acute systemic inflammation, such as that seen during a seasonal cold or high stress. On the contrary, young, healthy adults maintained resistance artery and microvascular function in response to physiological stimuli during low levels of systemic inflammation. Thus, future work is required to evaluate the effects of inflammation on consecutive segments of the arterial tree with aging and disease and how all aspects of the arterial tree influence cardiovascular risk with acute inflammation.

In conclusion, acute inflammation had nonuniform effects on vascular function throughout the arterial tree in young adults, as conduit artery endothelial function was reduced, but resistance artery and microvascular function were preserved. Furthermore, fitness did not alter the vascular response to acute inflammation, regardless of the level along the arterial tree. This suggests that cardiorespiratory fitness may not protect the vasculature during acute inflammation in young adults in the absence of age- or disease-related decline in vascular function.

GRANTS

W. K. Lefferts is currently supported by the National Heart, Lung, And Blood Institute of the National Institutes of Health under Award Number T32HL134634. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

DISCLOSURES

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

AUTHOR CONTRIBUTIONS

E.C.S. and B.F. conceived and designed research; E.C.S., T.I.H., W.K.L., and N.R. performed experiments; E.C.S. analyzed data; E.C.S., T.I.H., W.K.L., N.R., T.B., and B.F. interpreted results of experiments; E.C.S. prepared figures; E.C.S. drafted manuscript; E.C.S., T.I.H., W.K.L., N.R., T.B., and B.F. edited and revised manuscript; E.C.S., T.I.H., W.K.L., N.R., T.B., and B.F. approved final version of manuscript.