Joyner, Michael J.

Dietary sodium loading decreases β‐adrenergic receptor mediated vasodilation in hypertensive men, and paradoxically increases this dilation in normotensive men; no information exists in women. The purpose of this investigation was to test the hypothesis that dietary sodium loading and restriction influences β‐receptor mediated vasodilation in a sex‐dependent manner. Healthy normotensive subjects consumed a high (400 mmol/day, n=36, mean age±SEM 23±1 yr, 23 women) or low (10 mmol/day, n=30, 30±1 yr, 14 women) sodium diet for 5 days prior to study. Forearm blood flow (FBF, plethysmography) responses to brachial artery administration of isoproterenol (ISO: 1, 3, 6, 12 ng/100 ml tissue/min) were measured before and after endothelial NO synthase inhibition with N G ‐monomethyl‐L‐arginine (L‐NMMA). Consistent with prior reports, the FBF response to ISO was greater in men on high sodium compared to men on low sodium (p=0.03, ISO‐by‐sodium interaction). The highest response to ISO was 16.6±1.7 and 11.0±1.1 ml/100 ml tissue/min (high vs. low, p=0.01). After LNMMA the men's response to ISO did not differ based on sodium. In contrast, in women dietary sodium did not affect the response to ISO before or after LNMMA. We conclude that dietary sodium alters β‐adrenergic receptor mediated vasodilation in men, which appears to be dependent on NO. This is the first study to suggest that these mechanisms of vasodilation are resistant to dietary sodium in women. Support: HL‐089331, CTSA RR‐024150

Cardio‐vagal tone (CVT) and heart rate variability (HRV) are reduced during hypoglycemia. Carotid body (CB) peripheral chemoreceptors are multi‐modal; therefore we hypothesized a correlation between CB activity and CVT during hypoglycemia. Seven healthy adults (4M/3F) underwent two 180 min hyperinsulemic (2mU/kg FFM/min), hypoglycemic (3.33 mmol/L) clamps one week apart, randomized to normoxia (arterial P O2 (P a O 2 ) 111 ± 6.3 mmHg) or hyperoxia (P a O 2 345 ± 80.6 mmHg) to desensitize the CB. CVT was assessed via the high frequency (HF) component of HRV from 5 min periods of 5‐lead ECG recording under baseline (euglycemia) and hypoglycemia. HF values decreased significantly from baseline during hypoglycemia, however this reduction was attenuated under hyperoxia (Normoxia (N): −69% vs. Hyperoxia (H): −41%, p=0.04). Heart rate increased significantly during hypoglycemia (N: 59.7 bpm ± 2.4 vs. 71.9 ± 4.0; H: 60.6 bpm ± 1.9 vs. 68.5 ± 3, p<0.001), however, this increase was not different between normoxia and hyperoxia, despite differences in CVT and HRV. These data indicate that the CB chemoreceptors may be partially responsible for the reduction in CVT associated with hypoglycemia, and they provide evidence for a role of the CB chemoreceptors in parasympathetic tone to the heart. Future studies are required to determine whether inhibition of the CB may serve to improve CVT function during hypoglycemia. T32 DK07352, F32 DK84624‐01 A1, CTSA 1 UL 1 RR024150 , R01DK 29953

Aortic wave reflection increases with age, and is influenced by numerous factors, including sympathetic nerve activity. The purpose of this study was to examine the effect of autonomic blockade using trimethaphan camsylate on indices of aortic wave reflection in young (23±4 yr) and older postmenopausal (57±3 yr) women and determine if aging alters these interactions. Arterial tonometry was used to non‐invasively measure arterial waveforms from the radial artery before and during trimethaphan infusion. Aortic augmentation index (AI x ), augmented pressure, and left ventricular wasted energy (E w ) were higher in older women compared to young women at baseline (p<0.05). Mean arterial pressure was similar between young (93±5 mmHg) and older women (91±2 mmHg) at baseline. AI x (Δ −12±5%), augmented pressure (Δ −11±4 mmHg), and E w (Δ −2364±1020 dyne·cm 2 ·sec) decreased after autonomic blockade in older women (p<0.05 for all). In addition, the age‐related differences in AI x and aortic wave reflection persisted during autonomic blockade (p<0.05). In conclusion, autonomic blockade reduces augmentation index and aortic wave reflection in older women. Our results suggest that sympathetic activity contributes to the higher aortic wave reflection observed in older women.

We tested the hypothesis that elevated sympathetic tone is responsible for lower peak vasoldilation following single muscle contractions in older adults. Younger (n=8; 5M/3F, age 27 ± 2) and older (n=10; 5M/5F, age 69 ± 2) adults performed single forearm contractions at 10, 20 and 40% of maximum during 1) control, 2) sympathetic activation via lower body negative pressure (LBNP; ‐ 20mmHg), and 3) intra‐arterial infusion of phentolamine (α‐adrenergic antagonist). Brachial artery diameter and velocities were measured via Doppler ultrasound and forearm vascular conductance (FVC) was calculated from blood flow (ml/min) and blood pressure (mmHg). Peak vasodilator responses (ΔFVC from baseline) were attenuated in older adults for all intensities ( P < 0.05). LBNP reduced the ΔFVC at 10% (95 ± 18 vs. 60 ± 11), 20% (128 ± 9 vs. 91 ± 18) and 40% (192 ± 21 vs. 133 ± 23; P < 0.01 vs. control) in younger subjects, but not older adults (61 ± 6 vs. 58 ± 10; 99 ± 10 vs. 101 ± 17; 141 ± 17 vs. 129 ± 19, respectively; P = 0.53–0.97). With phentolamine ΔFVC was enhanced in older adults at each contraction intensity (91 ± 12, 136 ± 22, and 181 ± 22, respectively; P < 0.01 vs. control), but not younger adults (87 ± 9, 145 ± 12, and 192 ± 24, respectively; P = 0.31–1.0 vs. control). Our data indicate that overlying α‐adrenergic vasoconstriction plays a significant role in the age‐related decreases in skeletal muscle contraction‐induced rapid vasodilation.

We tested the hypothesis that hemodynamic responses would be similar during lower body negative pressure (LBNP) and hemorrhage (HEM). We measured central venous pressure (CVP), mean arterial pressure (MAP), and heart rate (HR) in 6 men during LBNP and HEM. The LBNP protocol consisted of 5‐minute stages at 0, ‐15, ‐30, and ‐45 mmHg of pressure. After a 45 minute break, the HEM protocol began and included 5 minutes at baseline and following three stages of 333 mL of HEM (1 L total). HR during LBNP increased from baseline to ‐45 mmHg (61 ± 3 vs. 86 ± 10 bpm, P < 0.01) while HR was unchanged after 1 L HEM (59 ± 4 vs. 66 ± 5 bpm, P = 0.07). HR was higher during LBNP at ‐30 mmHg vs. HEM at 667 mL (71 ± 7 vs. 63 ± 4 bpm, P = 0.03) and at the final stage (P < 0.01). From baseline, MAP tended to be reduced to a similar degree during ‐45 mmHg LBNP (90.9 ± 3.0 vs. 82.2 ± 4.0 mmHg) and 1 L HEM (89.5 ± 2.1 vs. 80.8 ± 4.0 mmHg) (P = 0.06). From baseline, CVP declined similarly during −45 mmHg LBNP (7.9 ± 1.1 vs. 0.1 ± 1.2 mmHg, P < 0.01) vs. 1 L HEM (5.8 ± 1.1 vs. 1.1 ± 1.4 mmHg, P < 0.01). Using the amalgamated r 2 across each level of LBNP and HEM, HR ( r 2 = 0.98, P < 0.01) and CVP were significantly correlated ( r 2 = 0.98, P < 0.01), but MAP was not ( r 2 = 0.53, P = 0.27). Although LBNP models the changes in MAP and CVP observed in HEM, differences in the HR response to LBNP and HEM require further exploration. Funding: US Army MRMC Combat Casualty Care Research Program Grant # W81XWH‐11–1‐0823.

Omega‐3 fatty acids found in fish oil have been suggested to protect against cardiovascular disease, yet underlying mechanisms remain unclear. Given the well documented links between mental stress (MS) and cardiovascular risk, we hypothesized that fish oil would blunt blood pressure (BP), heart rate (HR), and muscle sympathetic nerve activity (MSNA) responsiveness to MS. Neural and cardiovascular responses to a 5 min mental arithmetic protocol were recorded in 67 non‐hypertensive subjects before and after 8 wks of fish oil (9g/day; n=33) or placebo (olive oil; 9g/day; n=34). Fish oil blunted HR reactivity to MS (group × condition × time interaction, P=0.012), but did not alter BP reactivity (interactions, P>;0.05). Fish oil blunted total MSNA reactivity to MS (interaction, P=0.039), but did not alter MSNA burst frequency and burst incidence reactivity (interactions, P>;0.05). Finally, fish oil significantly blunted calf vascular conductance (CVC) reactivity to MS (interaction, P=0.013), but did not alter forearm vascular conductance reactivity (interaction, P>;0.05). In conclusion, 8 wks of fish oil supplementation significantly attenuated HR and total MSNA reactivity to MS, and elicited a paradoxical blunting of CVC. These findings support and extend the growing evidence that fish oil may have positive health benefits regarding neural cardiovascular control in humans. Supported by NIH (HL‐088689).

Resting energy expenditure (REE) equations multiplied by a predetermined physical activity factor (PAF) are commonly used to determine energy requirements However, estimated vs. actual REE and PAF among post‐gastric‐bypass surgery (GBS), obese (Ob) and lean (L) adults have not been compared. We hypothesized that GBS, Ob and L adults will have: 1) actual REE lower than estimated and 2) actual PAF lower than estimated. A group of 13 GB, 15 Ob and 11 L adults completed REE measurements by indirect calorimetry following standard clinical procedures. The Harris‐Benedict equation (HB) and pre‐determined PAF were used to estimate energy requirements. Physical activity energy expenditure (PAEE) was determined with accelerometry and PAF with the equation: (Actual REE + accelerometry PAEE)/Actual REE. Two‐way ANOVAs were used to detect differences by group and method, and agreement between methods using Bland & Altman plots. Group differences in actual REE (1631±174, 1830±290, 1441±151 kcal/day, respectively; P<0.01) and HB REE (1600±131, 1970±274, 1542±225 kcal/day, respectively, P<0.01) were observed, with no differences by method. Actual PAEE (269±70, 365±98, 228±112 kcal/day, respectively, P=0.002) yielded PAF ranging from 1.1 to 1.3, while HB PAF ranged from 1.2 to 1.7; causing higher predicted vs. actual energy requirement in all groups. While HB REE appeared adequate, predetermined PAF are not recommended.

The thermic effect of food (TEF) is partially mediated by β‐adrenergic receptors (βAR) and is an important component of daily energy expenditure. Greater sympathetic activity has been associated with decreased BAR responsiveness. We hypothesized that muscle sympathetic nerve activity (MSNA) would be inversely correlated with the βAR component of TEF (TEF β ). We measured MSNA (microneurography) and energy expenditure (EE) (indirect calorimetry) before and for 5 h after a mixed meal (40% daily EE). Measurements were made during systemic βAR blockade with propranolol (BB) and saline (NS) infusion on separate days in 10 healthy subjects (2 female, age 28±2 y, BMI 24.4±0.6 kg/m 2 ). Resting MSNA was 39±3 bursts/100 heartbeats (hb). Within 1 h after the meal, MSNA significantly increased during both NS (12±3 bursts/100 hb) and BB (18±4) but was not significantly different between the two days, (P=0.375). The maximum increase in REE after the meal was 269±29 (NS) and 198±22 (BB) kcal/24 h (P<0.05). TEF β (TEF β = AUC TEF NS – AUC TEF BB ) was 48±20% of total TEF. TEF β was positively correlated with resting MSNA (R 2 =0.70, P<0.05). Contrary to our hypothesis, we found that greater levels of MSNA were associated with greater TEF β . These results indicated that resting MSNA may be an important contributor to daily energy expenditure. Studies are needed to determine if greater MSNA is associated with a resistance to weight gain. The project described was supported by the Mayo Foundation for Medical Education and Research and NIH Grant Numbers K23 DK82424, P30 DK50456, and UL1 RR024150.

β‐adrenergic receptor activation causes a substantial portion of hypoxic vasodilation at rest and during forearm exercise. Recent evidence suggests that β‐receptors are either more sensitive or upregulated in young women vs. men. Therefore, we examined whether sex influences hypoxic vasodilation in 31 subjects (15F/16M; 26± 1 yr). We also examined whether potential sex related differences existed in a group of older adults (6F/5M; 61± 2 yr). All subjects performed forearm exercise at 10% and 20% of maximum under normoxic and hypoxic (80% arterial O2 saturation) conditions. Forearm vascular conductance (FVC; ml/min/100 mmHg) was calculated from blood flow (ml/min) and blood pressure (mmHg). At rest young females demonstrated a greater vasodilator response to hypoxia compared to men (39 ± 12% vs. 13 ± 6%, P < 0.05). The compensatory vasodilator response to hypoxia exercise (% change in ΔFVC compared to respective normoxic exercise trial) was also greater in young womenat 10% (46 ± 8% vs. 29 ± 5%, P < 0.05) and 20% exercise (38 ± 6% vs. 23 ± 4%, P < 0.05). Interestingly the compensatory vasodilator responses between older women and men were similar at 10% (28 ± 6% vs. 28 ± 10%, P = 0.99) and 20% exercise (13 ± 6% vs. 15 ± 7%, P = 0.84), respectively. Our data suggest that the compensatory vasodilator response to hypoxic exercise is greater in young women compared to men. However, sex specific differences appear to be lost with aging.

We tested the hypothesis that reduced nitric oxide (NO) bioavailability contributes to the attenuated peak and total vasodilation following single muscle contractions in older adults. Young (n=8; 25 ± 2 yr) and older (n=9; 67 ± 2 yr) adults performed single forearm contractions at 10, 20 and 40% of maximum during saline infusion (control) and NO synthase (NOS) inhibition via L‐NMMA. Brachial artery diameter and velocities were measured using Doppler ultrasound and forearm vascular conductance (FVC) was calculated from blood flow (ml/min) and blood pressure (mmHg). Peak and total vasodilator responses (ΔFVC from baseline) were attenuated in older adults at all intensities (P < 0.05). NOS inhibition reduced the peak ΔFVC at 10% (95 ± 13 vs. 59 ± 10), 20% (134 ± 11 vs. 90 ± 12) and 40% (222 ± 30 vs. 143 ± 24) in young subjects (P < 0.05 for all) and in older adults at 10% (61 ± 5 vs. 48 ± 7, P < 0.05) and 20% (90 ± 8 vs. 70 ± 8, P < 0.05), but not 40% (129 ± 13 vs. 105 ± 11, P = 0.12). The relative (%) reduction in peak ΔFVC due to NOS inhibition was greater in young vs. older adults at 20% (−36 ± 5 vs. −22 ± 5%, P < 0.05) and 40% (−35 ± 7 vs. −17 ± 7%, P < 0.05). The reduction in the total vasodilator response (area under the curve) with NOS inhibition was also greater in young vs. older adults at 20 and 40%. Our data suggest that contraction‐induced rapid vasodilation is mediated in part by NO; however, it appears the NO contribution in the response is greater in young adults.