Charles L. Stebbins

Effects of Dietary Decosahexaenoic Acid (DHA) on eNOS in Human Coronary Artery Endothelial Cells

Endothelial dysfunction occurs in heart disease and may reduce functional capacity via attenuations in peripheral blood flow. Dietary decosahexaenoic acid (DHA) may improve this dysfunction, but the mechanism is unknown. This study determined if DHA enhances expression and activity of eNOS in cultured human coronary artery endothelial cells (HCAEC). HCAEC from 4 donors were treated with 5 nM, 50 nM, or 1 μM DHA for 7 days to model chronic DHA exposure. A trend for increased expression of endothelial nitric oxide synthase (eNOS) and phospho-eNOS was observed with 5 and 50 nM DHA. DHA also enhanced expression of 2 proteins instrumental in activation of eNOS: phospho-Akt (5 and 50 nM) and HSP90 (50 nM and 1 μM). Vascular endothelial growth factor—induced activation of Akt increased NOx in treated (50 nM DHA) versus untreated HCAEC (9.2 ± 1.0 vs 3.3 ± 1.1 μmol/μg protein/μL). Findings suggest that DHA enhances eNOS and Akt activity, augments HSP90 expression, and increases NO bioavailability in response to Akt kinase activation.

THE POWER ATHLETE

A number of normal daily and athletic activities require isometric or static exercise. Sports such as weight lifting and other high-resistance activities are used by power athletes to gain strength and skeletal muscle bulk. Static exercise, the predominant activity used in power training, significantly increases blood pressure, heart rate, myocardial contractility, and cardiac output. These changes occur in response to central neural irradiation, called central command, as well as a reflex originating from statically contracting muscle. Studies have demonstrated that blood pressure appears to be the regulated variable, presumably because the increased pressure provides blood flow into muscles whose arterial inflow is reduced as a result of increases in intramuscular pressure created by contraction. Thus, static exercise is characterized by a pressure load on the heart and can be differentiated from the hemodynamic response to dynamic (isotonic) exercise, which involves a volume load to the heart. Physical training with static exercise (i.e., power training) leads to concentric cardiac (particularly left ventricular) hypertrophy, whereas training with dynamic exercise leads to eccentric hypertrophy. The magnitude of cardiac hypertrophy is much less in athletes training with static than dynamic exercise. Neither systolic nor diastolic function is altered by the hypertrophic process associated with static exercise training. Many of the energy requirements for static exercise, particularly during more severe levels of exercise, are met by anaerobic glycolysis because the contracting muscle becomes comes deprived of blood flow. Power athletes, training with repetitive static exercise, derive little benefit from an increase in oxygen transport capacity, so that maximal oxygen consumption is increased only minimally or not at all. Peripheral cardiovascular adaptations also can occur in response to training with static exercise. Although the studies are controversial, these adaptations include modest decreases in resting blood pressure, reduced increases in blood pressure and sympathetic nerve activity during a given workload, enhanced baroreflex function, increases in muscle capillary-to-fiber ratio, possible improvements in lipid and lipoprotein profiles, and increases in glucose and insulin responsiveness. Some of these adaptations can occur in cardiac or hypertensive patients with no concomitant cardiovascular complications. In both healthy individuals and those with cardiovascular disease, the manner in which resistance training is performed may dictate the extent to which these adjustments take place. Specifically, training that involves frequent repetitions of moderate weight (and hence contains dynamic components) seems to produce the most beneficial results.

Effects of nitric oxide synthase inhibition on vascular conductance during high speed treadmill exercise in rats

To determine the functional role of nitric oxide (NO) in regulating vascular conductance during high intensity dynamic exercise in skeletal muscles composed of all major fibre types, female Wistar rats (277 ± 4 g; n= 7) were run on a motor‐driven treadmill at a speed and gradient (60 m min‐1, 10% gradient) established to yield maximal oxygen uptake (Vo2,max). Vascular conductance (ml min‐1 (100g)‐1 mmHg‐1), defined as blood flow normalised to mean arterial pressure (MAP), was determined using radiolabelled microspheres during exercise before and after NO synthase (NOS) inhibition with NG‐nitro‐L‐arginine methyl ester (l‐NAME; 10 mg kg‐1, i.a.). The administration of l‐NAME increased MAP from pre‐l‐NAME baseline values, demonstrating that NOS activity is reduced. The administration of l‐NAME also reduced vascular conductance in 20 of the 28 individual hindlimb muscles or muscle parts examined during high speed treadmill exercise. These reductions in vascular conductance correlated linearly with the estimated sum of the percentage of slow twitch oxidative (SO) and fast twitch oxidative glycolytic (FOG) types of fibres in each muscle (Δconductance = ‐0.0082(%SO +%FOG) ‐ 0.0105; r= 0.66; P < 0.001). However, if the reduction in vascular conductance found in the individual hindquarter muscles or muscle parts was expressed as a percentage decrease from the pre‐l‐NAME value (%Δ= (pre‐l‐NAME conductance — post‐l‐NAME conductance)/pre‐l‐NAME conductance × 100), then the reduction in vascular conductance was similar in all muscles examined (average %Δ= ‐23 ± 2%). These results suggest that NO contributes substantially to the regulation of vascular conductance within and among muscles of the rat hindquarter during high intensity exercise. When expressed in absolute terms, the results suggest that the contribution of NO to the regulation of vascular conductance during high intensity exercise is greater in muscles that possess a high oxidative capacity. In contrast, if results are expressed in relative terms, then the contribution of NO to the regulation of vascular conductance during high intensity exercise is similar across the different locomotor muscles located in the rat hindlimb and independent of the fibre type composition.

Effects of angiotensin II receptor blockade during exercise : Comparison of losartan and saralasin

Previous studies indicate that angiotensin II (ANG II) plays a minor role in the hemodynamic responses during dynamic exercise. However, nonspecific effects associated with methods used to block its production [e.g., angiotensin-converting enzyme (ACE) inhibitors] or receptors (e.g., saralasin) may have contributed to these findings. Losartan is a nonpeptide ANG II receptor antagonist that is devoid of such nonspecific effects. We hypothesized that the contribution of ANG II to the cardiovascular response to dynamic exercise is characterized more precisely with losartan than with saralasin. On separate days, 6 miniswine performed treadmill running at 80% of their maximal heart rate (HR) reserve (HRR) in the presence of vehicle (0.9% saline), saralasin (10 or 20 micrograms/kg/min intraleft arterially, i.a.), or losartan (15 or 20 mg/kg i.a.). Cardiac output (CO), HR, and myocardial contractility were similar among all exercise conditions. As compared with the vehicle, losartan decreased mean arterial pressure (MAP) and systemic vascular resistance (SVR) during exercise, whereas no differences occurred between the vehicle and saralasin conditions. Both receptor antagonists increased blood flow and/or decreased vascular resistance during exercise in the myocardium, stomach, small intestine, and colon. As compared with that during treadmill running with vehicle infusion, renal blood flow (RBF) was increased by losartan and decreased by saralasin. We conclude that the contribution of ANG II to the cardiovascular response to dynamic exercise is demonstrated more clearly with losartan than with saralasin.

Augmentation of the exercise pressor reflex in prehypertension: roles of the muscle metaboreflex and mechanoreflex

This study investigated the hemodynamic mechanisms underlying the exaggerated blood pressure response to muscle contraction in prehypertensive humans and the potential role of skeletal muscle metabo- and mechanoreceptors in this response. To accomplish this, changes in peak mean arterial blood pressure (ΔMAP), cardiac output, and total peripheral resistance (ΔTPR) were compared between prehypertensive (n = 23) and normotensive (n = 19) male subjects during 2 min of static contraction (at 50% of maximal tension), 2 min of postexercise muscle ischemia (metaboreflex), and 1 min of passive dorsiflexion of the foot (tendon stretch, mechanoreceptor reflex). These variables were assessed before and during the interventions. Percentage increases from baseline in MAP and TPR in response to the exercise pressor reflex were augmented in the prehypertensives, compared with the normotensives (44% ± 5% vs. 33% ± 4% and 34% ± 15% vs. 2% ± 8%, respectively) (p < 0.05). Metaboreflex-induced increases in MAP and TPR were also augmented in the prehypertensives (28% ± 5% vs. 14% ± 4% and 36% ± 12% vs. 14% ± 9%, respectively) (p < 0.05). In response to the mechanoreflex, no differences in the percentage increase in MAP or TPR were seen between groups. The results indicate that the reflex pressor response to static contraction is augmented in prehypertension and suggest that this phenomenon is due, at least in part, to enhanced activation of metaboreceptors.

Taekwondo training and fitness in female adolescents.

Abstract In this study, we determined the specificity of a low frequency taekwondo training programme on physical fitness levels in adolescent females who receive limited physical education instruction (i.e. 2 days per week). Major components of physical fitness assessed were: skeletal muscle fitness (hand grip strength, bent arm hang, standing long jump, and isokinetic strength), flexibility (sit-and-reach test), speed and agility (10 × 5-m shuttle run), and cardiovascular fitness ([Vdot]O 2max and 20-m shuttle run). Changes in body composition were also assessed (dual X-ray absorptiometry, DXA). Participants were divided into two groups, a taekwondo training group (n = 21), which trained 50 min a day, 2 days per week for 12 weeks, and a control group (n = 10). Taekwondo training improved isokinetic strength, standing long jump, and sit-and-reach performance. Body fat mass and percent body fat were reduced. No changes in grip strength, bent arm hang time, speed and agility, or cardiorespiratory fitness were observed. Results indicate that low frequency taekwondo training in adolescent females produces beneficial changes in skeletal muscle fitness, flexibility, and body composition in a relatively short period of time. Consequently, this specific type of training can be useful to female adolescents in structured school environments where physical education classes are limited and there is little free time for physical activity.

Effects of Chronic Dietary Nitrate Supplementation on the Hemodynamic Response to Dynamic Exercise

While acute treatment with beetroot juice (BRJ) containing nitrate (NO3 (-)) can lower systolic blood pressure (SBP), afterload, and myocardial O2 demand during submaximal exercise, effects of chronic supplementation with BRJ (containing a relatively low dose of NO3 (-), 400 mg) on cardiac output (CO), SBP, total peripheral resistance (TPR), and the work of the heart in response to dynamic exercise are not known. Thus, in 14 healthy males (22 ± 1 yr), we compared effects of 15 days of both BRJ and nitrate-depleted beetroot juice (NDBRJ) supplementation on plasma concentrations of NOx (NO3 (-)/NO2 (-)), SBP, diastolic blood pressure (DBP), mean arterial pressure (MAP), CO, TPR, and rate pressure product (RPP) at rest and during progressive cycling exercise. Endothelial function was also assessed via flow-mediated dilation (FMD). BRJ supplementation increased plasma NOx from 83.8 ± 13.8 to 167.6 ± 13.2 μM. Compared with NDBRJ, BRJ reduced SBP, DBP, MAP, and TPR at rest and during exercise (P < 0.05). In addition, RPP was decreased during exercise, while CO was increased, but only at rest and the 30% workload (P < 0.05). BRJ enhanced FMD-induced increases in brachial artery diameter (pre: 12.3 ± 1.6%; post: 17.8 ± 1.9%). We conclude that 1) chronic supplementation with BRJ lowers blood pressure and vascular resistance at rest and during exercise and attenuates RPP during exercise and 2) these effects may be due, in part, to enhanced endothelium-induced vasodilation in contracting skeletal muscle. Findings suggest that BRJ can act as a dietary nutraceutical capable of enhancing O2 delivery and reducing work of the heart, such that exercise can be performed at a given workload for a longer period of time before the onset of fatigue.

Regional blood flow responses to acute ANG II infusion: Effects of nitric oxide synthase inhibition

We hypothesized that nitric oxide (NO) opposes regional vasoconstriction caused by acute angiotensin II (ANG II) infusion in conscious rats. Mean arterial pressure (MAP), blood flow, and vascular conductance (regional blood flow/ MAP; ml/min/100 g/mm Hg) were measured and/or calculated before and at 2 min of ANG II infusion (0.05 or 1 microg/kg/min, i.a.) in the absence and presence of NO synthase (NOS) inhibition [N(G)-nitro-L-arginine methyl ester (L-NAME), 0.25 or 1 mg/kg, i.a.]. ANG II reduced stomach and hindlimb conductance only after NOS inhibition. For example, whereas 0.05 microg/kg/min ANG II did not attenuate conductance in the stomach (i.e., 1.04+/-0.08 to 0.93+/-0.12 ml/min/100 g/mm Hg), this variable was reduced (i.e., 0.57+/-0.14 to 0.34-/+0.05 ml/min/100 g/mm Hg; p < 0.05) when ANG II was infused after 0.25 mg/kg L-NAME. In addition, whereas hindlimb conductance was similar before and after administering 1 microg/kg/min ANG II (i.e., 0.13+/-0.01 and 0.09+/-0.02, respectively), this variable was reduced (i.e., 0.07+/-0.01 and 0.02+/-0.00, respectively; p < 0.05) when ANG II was infused after 1 mg/kg L-NAME. These findings indicate that NO opposes ANG II-induced vasoconstriction in the stomach and hindlimb. In contrast, whereas both doses of ANG II decreased (p < 0.05) vascular conductance in the kidneys and small and large intestine regardless of whether NOS inhibition was present, absolute vascular conductance was lower (p < 0.05) after L-NAME. For example, 1 microg/kg ANG II reduced renal conductance from 3.34+/-0.31 to 1.22+/-0.14 (p < 0.05). After 1 mg/kg L-NAME, renal conductance decreased from 1.39+/-0.18 to 0.72+/-0.16 (p < 0.05) during ANG II administration. Therefore the constrictor effects of NOS inhibition and ANG II are additive in these circulations. Taken together, our results indicate that the ability of NO to oppose ANG II-induced constriction is not homogeneous among regional circulations.

Hemodynamic and regional blood flow responses to nicotine at rest and during exercise

We hypothesized that nicotine compromises cardiovascular responses to dynamic exercise. Hemodynamic variables were measured in conscious miniswine before and at 2 min of nicotine infusion (20 micrograms.kg-1.min-1; i.a.; N = 6) during resting conditions. Mean arterial pressure elevations (MAP; 14%) and plasma nicotine concentrations (49 +/- 7 ng.ml-1) were similar to those elicited by cigarette smoking in humans. In addition, nicotine increased systemic vascular resistance (SVR; 56%), the heart rate x systolic blood pressure product (RPP; 11%), and regional vascular resistance in the left-ventricular, renal, and splanchnic circulations, while cardiac output decreased (CO; 23%) and skeletal muscle blood flow and vascular resistance were unaffected. Plasma norepinephrine and epinephrine increased by approximately 30% and 90%, respectively. On separate days, the same hemodynamic responses were measured before and at 20 min of treadmill running during vehicle or nicotine infusion for the last 2 min of exercise (N = 10). Nicotine increased MAP (6%), SVR (14%), and RPP (3%), and elevated vascular resistance in the proximal colon and pancreas. Moreover, compared to exercise + vehicle, norepinephrine and epinephrine increased by approximately 13% and 24%, respectively, during exercise + nicotine infusion. These findings suggest that the detrimental effects of nicotine observed at rest are minimized during exercise. Nicotines effects may be reduced during exercise by competition from local vasodilators in the heart and active musculature, and/or by differing activation of sympathetic nerve activity.

Central and peripheral effects of angiotensin II on the cardiovascular response to exercise

The authors tested the hypothesis that angiotensin II modulates cardiovascular responses to dynamic exercise via peripheral and central effects on the autonomic nervous system. Ten subjects performed three identical exercise tests during treatment with placebo, valsartan (an angiotensin II type 1 receptor blocker), or enalapril (an angiotensin-converting enzyme inhibitor). With placebo, plasma concentrations of angiotensin II, norepinephrine, and epinephrine were elevated during cycling at 80% of heart rate reserve (HRR). Enalapril attenuated increases in heart rate, mean arterial pressure (MAP), and catecholamines during cycling, whereas valsartan only attenuated MAP and rate-pressure product above 60% HRR, and norepinephrine. The different responses provoked by the two drug treatments suggest that angiotensin-converting enzyme inhibition affects cardiovascular responses to exercise by mechanisms unrelated to production of angiotensin II. Indices of autonomic function during dynamic exercise were not changed by either drug. Attenuation of norepinephrine release during exercise by valsartan suggests that angiotensin II facilitates the release of norepinephrine from sympathetic postganglionic neurons. Angiotensin II, therefore, contributes to the pressor response to exercise by inducing peripheral vasoconstriction and facilitation of norepinephrine release from postganglionic sympathetic nerve endings that are unrelated to central activation of the autonomic nervous system.