Curry, Timothy B.

We previously reported that augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes beta‐adrenergic mechanisms. Furthermore, hypoxic vasodilation at rest is known to include beta‐adrenergic mediated nitric oxide (NO) production. Therefore, we tested the hypothesis that NO contributes to augmented vasodilation with hypoxic exercise. Twelve subjects (6M/6F; 26 ± 2 years) breathed hypoxic gas to titrate arterial O 2 saturation to 80% while remaining normocapnic via a rebreath system. Subjects performed incremental forearm exercise (10% and 20% of maximum) during saline infusion (control) and NO synthase inhibition ( N G ‐monomethyl‐L‐arginine; L‐NMMA) under normoxic and hypoxic conditions. Forearm vascular conductance (FVC; ml/min/100mmHg) was calculated from blood flow (ml/min) and blood pressure (mmHg). During the control condition, the rise in FVC (Δfrom normoxic baseline) due to hypoxic exercise was 211 ± 23 and 367 ± 34 (10% and 20% respectively). Infusion of L‐NMMA during hypoxic exercise attenuated ΔFVC at 10% (174 ± 20; P < 0.001) and 20% (317 ± 32; P < 0.01) compared to control condition. Our data indicate that NO contributes to the augmented vasodilation observed during hypoxic exercise. Supported by NIH HL‐46493 (MJJ) and AR‐55819 (DPC) and by CTSA RR‐024150.

Physical activity (PA) is inversely related with body fat, insulin resistance (IR), and leptin. However, these associations are not clear in patients after gastric bypass surgery (GBS). We hypothesized an inverse relation between PA, BMI, IR, and leptin; and a positive relation between leptin, BMI, and IR in a group of 9 patients after GBS, 8 obese (Ob), and 7 non‐obese (N‐Ob) controls (age=39.0±5.7, 27.7±6.9, 28.0±10.2 yr, respectively). Subjects wore an accelerometer from 6‐7 days, and underwent measurements of height, weight, DEXA, blood glucose, insulin and leptin. IR was estimated using HOMA. Between group differences in PA, BMI, IR, and leptin were determined with ANOVA; and the relation between these variables was determined with simple linear regressions. Between group differences were observed for BMI (Ob=38.7±1.6, GBS=30.4±1.5, N‐Ob=23.8±1.7 kg/m2), IR (Ob=3.0±0.5, GBS=1.1±0.5, N‐Ob=0.7±0.5), and leptin (Ob=55.9, GBS=28.7±4.5, N‐Ob=5.1±5.6 ng/ml). PA was not different between groups. Inverse associations were observed between PA and BMI (r= ‐ 0.55), and leptin (r= ‐ 0.56); and positive associations were observed between leptin and BMI (r= 0.92), and IR (r= 0.63). PA appears to be associated with the control of obesity and IR possibly via reductions in leptin, a mechanism that requires further study. Funded by NIH CTSA UL1‐RR24150, NIH CTSA KL2‐ RR024151

The sensitivity of the baroreflex control of muscle sympathetic nerve activity (MSNA) represents the responsiveness of MSNA to changes in blood pressure, whereas the baroreflex threshold diagram evaluates the likelihood of a burst occurrence over a range of resting blood pressure values. We hypothesized that the slope of the threshold diagram would show a significant positive correlation with sympathetic baroreflex sensitivity (BRS). We compared BRS calculated by the Modified Oxford technique (ModOx) with the slope and midpoint (T50) values from threshold analysis (threshold) in 10 participants (9M, age 26 yrs ± 3[mean ± SEM]). MSNA (peroneal microneurography) and blood pressure (brachial catheter) were measured at rest and during vasoactive drug boluses (ModOx). A threshold diagram was created for each subject during rest by associating DBP to the % occurrence of a burst over a 4 min period prior to the ModOx drugs. The midpoint (T50) of this diagram was defined as the DBP at which 50% of the diastoles were associated with a burst. ModOx‐derived BRS positively correlated with the slope of the threshold diagram (r = 0.64, P<0.05), but not with the T50 (r =0.11, P > 0.2). Our results suggest that the slope of the threshold diagram may be a better index of ModOx‐derived BRS values compared to more commonly used "spontaneous" baroreflex techniques. NIH HL‐083947

The key vascular signals regulating muscle blood flow during exercise are unclear. We tested the hypothesis that in young adults, leg blood flow (LBF) during exercise would be reduced independently by pharmacological inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX). In 13 young adults, we measured heart rate (ECG), blood pressure (femoral artery and vein), blood gases, and venous oxygen saturation (indwelling oximeter) during steady‐state dynamic (60/min) knee extension exercise. LBF was determined by Doppler ultrasound of the femoral artery. Exercise (20W) increased leg vascular conductance (LVC) from 4± 1 to 33± 3 ml/min/100mmHg. The NOS inhibitor L‐NAME reduced LVC by ∼10% to 30± 1. Infusion of the COX inhibitor, ketoralac (n=9), increased LVC from 30± 1 to 34± 2. L‐NAME infusion reduced venous oxygen saturation from 48 to 38%, suggesting greater extraction to maintain oxygen consumption. Control intra‐arterial infusions of acetylcholine (ACH) and nitroprusside (NTP) tested inhibitor efficacy. The change in LVC to two doses of ACh was blunted 53 and 15%, by L‐NAME+ketorolac infusion. In contrast, the LVC responses to NTP were unaltered. These data implicate a modest role for NO‐mediated leg exercise hyperemia in adults and a potentially modest LBF restraint by vasoconstrictor prostaglandins. Funded by the NIH HL‐46493 and the Minnesota Obesity Center.

Dehydration in humans decreases orthostatic tolerance and alters sympathetic neural control of the circulation, but the mechanisms involved are unclear. Our goal was to evaluate the role of angiotensin II (AngII) in altering the sympathetic control mechanisms in dehydration. We hypothesized that dehydration would increase resting muscle sympathetic nerve activity (MSNA) and increase sensitivity of baroreflex control of MSNA and that these changes are mediated in part by AngII via AT1 receptors. We measured resting MSNA and baroreflex control of MSNA in healthy young adults under two conditions: euhydration and dehydration after 24 hr fluid restriction. We continuously measured heart rate (ECG), beat‐to‐beat blood pressure (Finometer), central venous pressure (peripherally inserted central catheter) and MSNA (peroneal microneurography) in both conditions. In each condition, measurements were taken before and after 50 mg losartan (AT1 antagonist). Dehydration was associated with a ~52 % increase in resting MSNA, but losartan did not consistently influence resting MSNA. Contrary to our hypothesis, baroreflex sensitivity increased ~39 % following losartan administration during dehydration. Our results confirm the sympathoexcitatory influence of dehydration, but the role of AngII in this process requires further evaluation. Supported by AHA 0750036Z and NIH CTSA UL1‐RR24150.

The physiological changes that occur after gastric bypass surgery (GBS) are not well understood but changes in activation of the sympathetic nervous system may be involved. We hypothesized that GBS would be associated with greater sympathetically mediated resting energy expenditure as measured by a greater decrease in REE after beta‐blockade (beta‐REE). Subjects who had undergone GBS (average reduction in BMI of 31.6% from peak, N=7) along with lean (BMI<25 kg/m 2 , N=5) and obese (BMI=35–45 kg/m 2 , N=4) subjects underwent measurements of REE before and after IV propranolol (0.25 mg/kg bolus followed by an infusion of 0.004 mg/kg/min). The decrease in REE (mean±SD) after beta‐blockade tended to be greater in lean (102±58 kcal) compared with obese (−22±100 kcal) and GBS (26±55 kcal) subjects (P=0.06). The decrease in REE after beta‐blockade negatively correlated with the % body fat (r=0.54, P<0.05) and VO 2max (r=0.67, P=0.01) of the subjects, but not with resting catecholamine or leptin levels. We did not find that subjects who had undergone GBS had a significant difference in the amount of their REE that is dependent on sympathetic stimulation. Beta‐REE appears to be related to fitness and also to fatness and the fact that GBS subjects had high body fat even after significant weight loss may explain our results. Support: NIH HL46493, NIH CTSA UL1‐RR24150, Department of Anesthesiology.

The vascular effects of prostacyclin and contribution of endothelial derived nitric oxide (NO) to prostacyclin mediated dilation are unknown in aging. A pilot study was performed in healthy older subjects (n=6, 61–73 years old) and younger controls (n=6, 19–45 years old) that were matched for BMI. Subjects underwent brachial artery catheterization and received escalating doses of epoprostenol (PGI 2 2.5, 5, 10 ng/100ml forearm tissue/min) followed by sodium nitroprusside (SNP 0.25, 0.5, 1 mcg/100ml forearm tissue/min) after PGI 2 washout. SNP and PGI 2 were repeated after NO synthase inhibition with N G ‐Monomethyl‐L arginine acetate (L‐NMMA 1mg/min after 50mg load). Using venous occlusion plethysmography, forearm vascular conductance (FVC) was calculated using mean arterial pressure (MAP), as FVC= (FBF/MAP X 100). Percent change in FVC from baseline after PGI 2 was lower in the aging individuals (47, 160, 227 % change FVC) compared to younger controls (85, 218, 327 %change FVC) (p=0.039), L‐NMMA reduced baseline FVC similarly, and the dilator response to PGI 2 was no longer significant between groups. SNP effects were not significant between groups and unaffected by L‐NMMA. This data suggests a reduced effect to prostacyclin in the aging individual may be due to a reduction in the contribution of endothelial derived NO. Support provided by HL‐46493 and UL1‐RR24150.

Heart failure (HF) patients have reduced cardiac reserve and increased work of breathing (WOB)., Increased demand for locomotor muscle blood flow (i.e. activities of daily living) may result in competition between respiratory and locomotor vascular beds. We hypothesized that HF patients would demonstrate improved locomotor blood flow with respiratory muscle unloading during moderate activity. We studied 10 HF patients (EF=30±8%) and 10 controls during two 10 min. cycling sessions at 60% VO 2 max. Session 1 (S1): 5 min of normal breathing (NB) and 5 min respiratory muscle unloading with a ventilator. Session 2 (S2): 5 min NB and 5 min of respiratory muscle loading with inspiratory resistance. Measurements included: leg blood flow (LBF), cardiac output (CO), and inspiratory pleural pressure time integral as an index of the WOB. During S1, WOB was reduced similarly in both groups (HF: 50±60%; CTL: 50±56%, p<0.05 for both). In HF, CO was reduced (9.6±1.4 vs 8.2±2.1 L/min, p<0.05) and LBF increased (4.9±2.6 vs 6.2±2.9 L/min, p<0.01). In CTL, there was no change in CO (14.7±3.1 vs 14.8±5.0 L/min) or LBF (10.9±5.6 vs 10.3±5.2 L/min). During S2, WOB was increased similarly in both groups (HF: 111±75%, CTL: 110±85%, p<0.01 for both). In HF, there was no change in CO (10.0±1.3 vs 10.3±2.5 L/min) or LBF (5.0±1.8 vs 4.9±1.4 L/min). In CTL, CO increased (15.4±4.3 vs 16.9±4.7 L/min, p<0.01) with no change in LBF (10.7±5.3 vs 10.3±5.6 L/min). These data suggest that the respiratory muscles of HF patients preferentially recruit blood flow away from the locomotor muscles during moderate intensity, whole body exercise.

Stewart, Glenn M., Troy J. Cross, Michael J. Joyner, Steven C. Chase, Timothy Curry, Josh Lehrer-Graiwer, Kobina Dufu, Nicholas E. Vlahakis, and Bruce D. Johnson. Impact of pharmacologically left shifting the oxygen–hemoglobin dissociation curve on arterial blood gases and pulmonary gas exchange during maximal exercise in hypoxia. High Alt Med Biol . 22:249–262, 2021. Introduction: Physiological and pathological conditions, which reduce the loading of oxygen onto hemoglobin (Hb), can impair exercise capacity and cause debilitating symptoms. Accordingly, this study examined the impact of pharmacologically left shifting the oxygen–hemoglobin dissociation curve (ODC) on arterial oxygen saturation (SaO 2 ) and exercise capacity. Methods: Eight healthy subjects completed a maximal incremental exercise test in hypoxia (F I O 2 : 0.125) and normoxia (F I O 2 : 0.21) before (Day 1) and after (Day 15) daily ingestion of 900 mg of voxelotor (an oxygen/Hb affinity modulator). Pulmonary gas exchange and arterial blood gases were assessed throughout exercise and at peak. Data for a 1,500 mg daily drug dose are reported in a limited cohort ( n = 3). Results: Fourteen days of drug administration left shifted the ODC (p50 measured under standard conditions, Day 1: 28.0 ± 2.1 mmHg vs. Day 15: 26.1 ± 1.8 mmHg, p < 0.05). Throughout incremental exercise in hypoxia, SaO 2 was systematically higher after drug (peak exercise SaO 2 on Day 1: 71 ± 2 vs. Day 15: 81% ± 2%, p < 0.001), whereas oxygen extraction (Ca-vO 2 diff) and consumption (VO 2 ) were similar (peak exercise Ca-vO 2 diff on Day 1: 11.5 ± 1.7 vs. Day 15: 11.0 ± 1.8 ml/100 ml blood, p = 0.417; peak VO 2 on Day 1: 2.59 ± 0.39 vs. Day 15: 2.47 ± 0.43 l/min, p = 0.127). Throughout incremental exercise in normoxia, SaO 2 was systematically higher after drug, whereas peak VO 2 was reduced (peak exercise SaO 2 on Day 1: 93.9 ± 1.8 vs. Day 15: 95.8% ± 1.0%, p = 0.008; peak VO 2 on Day 1: 3.62 ± 0.55 vs. Day 15: 3.26 ± 52 l/min, p = 0.001). Conclusion: Pharmacologically increasing the affinity of Hb for oxygen improved SaO 2 during hypoxia without impacting exercise capacity; however, left shifting the ODC in healthy individuals appears detrimental to exercise capacity in normoxia. Left shifting the ODC to different magnitudes and under more chronic forms of hypoxia warrants further study.

Patients with sleep apnea exhibit ventilatory instability and increased carotid chemoreceptor activity. Whether ventilatory instability is directly related to altered chemoreceptor activity has not been examined in humans. Therefore we tested the hypothesis that acute activation of the carotid chemoreflex with hypoxia (10% O 2 ) would increase breathing variability (assessed by coefficient of variation of tidal volume {%CV}) and blunting the peripheral chemoreflex with IV dopamine would attenuate the response. To test this hypothesis, nine healthy subjects (3 M/6 F) were studied on two separate study days: 1) saline or 2) low‐dose dopamine infusion. Individualized dopamine dose (1‐4 mg/kg/min), that maximally blunted chemoreflex sensitivity, was identified during a separate screening visit. Respiratory rate (RR), tidal volume (TV), end tidal CO 2 (ETCO 2 )and O 2 (ETO 2 ) were measured. Baseline measures were similar between days. Saline infusion did not affect RR, TV, end tidal gases, or %CV. However, dopamine infusion significantly increased %CV (25 ± 5 to 32 ± 6 %; p<0.05) and ETCO 2 (0.3 ± 0.1 %; p<0.0001) without affecting RR, or TV. Hypoxia increased %CV during saline infusion (18 ± 3 to 29 ± 3%; p<0.05), and this effect was blunted during dopamine infusion (31 ± 6 to 38 ± 6%; p>0.05). These data indicate hypoxia increases respiratory variability and this effect is mediated by the carotid chemoreceptors. Furthermore, the effect of increased chemoreceptor drive on ventilatory instability, such as that seen in sleep apnea, may be blunted by low‐dose dopamine in humans. These data provide a potential therapeutic target for improving ventilatory instability in human disease. Funding:NIH DK090541