Colin N. Young

ER stress in the brain subfornical organ mediates angiotensin-dependent hypertension

Although endoplasmic reticulum (ER) stress is a pathologic mechanism in a variety of chronic diseases, it is unclear what role it plays in chronic hypertension (HTN). Dysregulation of brain mechanisms controlling arterial pressure is strongly implicated in HTN, particularly in models involving angiotensin II (Ang II). We tested the hypothesis that ER stress in the brain is causally linked to Ang II-dependent HTN. Chronic systemic infusion of low-dose Ang II in C57BL/6 mice induced slowly developing HTN, which was abolished by co-infusion of the ER stress inhibitor tauroursodeoxycholic acid (TUDCA) into the lateral cerebroventricle. Investigations of the brain regions involved revealed robust increases in ER stress biomarkers and profound ER morphological abnormalities in the circumventricular subfornical organ (SFO), a region outside the blood-brain barrier and replete with Ang II receptors. Ang II-induced HTN could be prevented in this model by selective genetic supplementation of the ER chaperone 78-kDa glucose-regulated protein (GRP78) in the SFO. These data demonstrate that Ang II-dependent HTN is mediated by ER stress in the brain, particularly the SFO. To our knowledge, this is the first report that ER stress, notably brain ER stress, plays a key role in chronic HTN. Taken together, these findings may have broad implications for the pathophysiology of this disease.

Central sympathetic overactivity: Maladies and mechanisms

There is growing evidence to suggest that many disease states are accompanied by chronic elevations in sympathetic nerve activity. The present review will specifically focus on central sympathetic overactivity and highlight three main areas of interest: 1) the pathological consequences of excessive sympathetic nerve activity; 2) the potential role of centrally derived nitric oxide in the genesis of neural dysregulation in disease; and 3) the promise of several novel therapeutic strategies targeting central sympathetic overactivity. The findings from both animal and human studies will be discussed and integrated in an attempt to provide a concise update on current work and ideas in these important areas.

Increased muscle sympathetic nerve activity acutely alters conduit artery shear rate patterns

Escalating evidence indicates that disturbed flow patterns, characterized by the presence of retrograde and oscillatory shear stress, induce a proatherogenic endothelial cell phenotype; however, the mechanisms underlying oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde and oscillatory shear. Fourteen healthy men (25 +/- 1 yr) performed three sympathoexcitatory maneuvers: graded lower body negative pressure (LBNP) from 0 to -40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA (microneurography; peroneal nerve), arterial blood pressure (finger photoplethysmography), and brachial artery velocity and diameter (duplex Doppler ultrasound) in the contralateral arm were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline (P < 0.05). Retrograde shear (-3.96 +/- 1.2 baseline vs. -8.15 +/- 1.8 s(-1), -40 LBNP, P < 0.05) and oscillatory shear index (0.09 +/- 0.02 baseline vs. 0.20 +/- 0.02 arbitrary units, -40 LBNP, P < 0.05) were progressively augmented during graded LBNP. In contrast, during CPT and PEI, in which MSNA and blood pressure were concomitantly increased (P < 0.05), minimal or no changes in retrograde and oscillatory shear were noted. These data suggest that acute elevations in MSNA are associated with an increase in conduit artery retrograde and oscillatory shear, an effect that may be influenced by concurrent increases in arterial blood pressure. Future studies should examine the complex interaction between MSNA, arterial blood pressure, and other potential modulatory factors of shear rate patterns.

Autonomic control of heart rate by metabolically sensitive skeletal muscle afferents in humans.

Isolated activation of metabolically sensitive skeletal muscle afferents (muscle metaboreflex) using post‐exercise ischaemia (PEI) following handgrip partially maintains exercise‐induced increases in arterial blood pressure (BP) and muscle sympathetic nerve activity (SNA), while heart rate (HR) declines towards resting values. Although masking of metaboreflex‐mediated increases in cardiac SNA by parasympathetic reactivation during PEI has been suggested, this has not been directly tested in humans. In nine male subjects (23 ± 5 years) the muscle metaboreflex was activated by PEI following moderate (PEI‐M) and high (PEI‐H) intensity isometric handgrip performed at 25% and 40% maximum voluntary contraction, under control (no drug), parasympathetic blockade (glycopyrrolate) and β‐adrenergic blockade (metoprolol or propranalol) conditions, while beat‐to‐beat HR and BP were continuously measured. During control PEI‐M, HR was slightly elevated from rest (+3 ± 2 beats min−1); however, this HR elevation was abolished with β‐adrenergic blockade (P < 0.05 vs. control) but augmented with parasympathetic blockade (+8 ± 2 beats min−1, P < 0.05 vs. control and β‐adrenergic blockade). The HR elevation during control PEI‐H (+9 ± 3 beats min−1) was greater than with PEI‐M (P < 0.05), and was also attenuated with β‐adrenergic blockade (+4 ± 2 beats min−1, P < 0.05 vs. control), but was unchanged with parasympathetic blockade (+9 ± 2 beats min−1, P > 0.05 vs. control). BP was similarly increased from rest during PEI‐M and further elevated during PEI‐H (P < 0.05) in all conditions. Collectively, these findings suggest that the muscle metaboreflex increases cardiac SNA during PEI in humans; however, it requires a robust muscle metaboreflex activation to offset the influence of cardiac parasympathetic reactivation on heart rate.

Increased brachial artery retrograde shear rate at exercise onset is abolished during prolonged cycling: role of thermoregulatory vasodilation

Acute leg exercise increases brachial artery retrograde shear rate (SR), while chronic exercise improves vasomotor function. These combined observations are perplexing given the proatherogenic impacts of retrograde shear stress on the vascular endothelium and may be the result of brief protocols used to study acute exercise responses. Therefore, we hypothesized that brachial artery retrograde SR increases initially but subsequently decreases in magnitude during prolonged leg cycling. Additionally, we tested the role of cutaneous vasodilation in the elimination of increased retrograde SR during prolonged exercise. Brachial artery diameter and velocity profiles and forearm skin blood flow and temperature were measured at rest and during 50 min of steady-state, semirecumbent leg cycling (120 W) in 14 males. Exercise decreased forearm vascular conductance (FVC) and increased retrograde SR at 5 min (both P < 0.05), but subsequently forearm and cutaneous vascular conductance (CVC) rose while retrograde SR returned toward baseline values. The elimination of increased retrograde SR was related to the increase in FVC (r(2) = 0.58; P < 0.05) and CVC (r(2) = 0.32; P < 0.05). Moreover, when the forearm was cooled via a water-perfused suit between minutes 30 and 40 to blunt cutaneous vasodilation attending exercise, FVC was reduced and the magnitude of retrograde SR was increased from -49.7 ± 13.6 to -78.4 ± 16.5 s(-1) (P < 0.05). Importantly, these responses resolved with removal of cooling during the final 10 min of exercise (retrograde SR: -46.9 ± 12.5 s(-1)). We conclude that increased brachial artery retrograde SR at the onset of leg cycling subsequently returns toward baseline values due in part to thermoregulatory cutaneous vasodilation during prolonged exercise.

Insulin enhances the gain of arterial baroreflex control of muscle sympathetic nerve activity in humans

Recent animal studies indicate that insulin increases arterial baroreflex control of lumbar sympathetic nerve activity; however, the extent to which these findings can be extrapolated to humans is unknown. To begin to address this, muscle sympathetic nerve activity (MSNA) and arterial blood pressure were measured in 19 healthy subjects (27 ± 1 years) before, and for 120 min following, two common methodologies used to evoke sustained increases in plasma insulin: a mixed meal and a hyperinsulinaemic euglycaemic clamp. Weighted linear regression analysis between MSNA and diastolic blood pressure was used to determine the gain (i.e. sensitivity) of arterial baroreflex control of MSNA. Plasma insulin was significantly elevated within 30 min following meal intake (Δ34 ± 6 uIU ml−1; P < 0.05) and remained above baseline for up to 120 min. Similarly, after meal intake, arterial baroreflex‐MSNA gain for burst incidence and total MSNA was increased and remained elevated for the duration of the protocol (e.g. burst incidence gain: −3.29 ± 0.54 baseline vs.−5.64 ± 0.67 bursts (100 heart beats)−1 mmHg−1 at 120 min; P < 0.05). During the hyperinsulinaemic euglycaemic clamp, in which insulin was elevated to postprandial concentrations (Δ42 ± 6 μIU ml−1; P < 0.05), while glucose was maintained constant, arterial baroreflex‐MSNA gain was similarly enhanced (e.g. burst incidence gain: −2.44 ± 0.29 baseline vs.−4.74 ± 0.71 bursts (100 heart beats)−1 mmHg−1 at 120 min; P < 0.05). Importantly, during time control experiments, with sustained fasting insulin concentrations, the arterial baroreflex‐MSNA gain remained unchanged. These findings demonstrate, for the first time in healthy humans, that increases in plasma insulin enhance the gain of arterial baroreflex control of MSNA.

Autonomic adjustments to exercise in humans.

Autonomic nervous system adjustments to the heart and blood vessels are necessary for mediating the cardiovascular responses required to meet the metabolic demands of working skeletal muscle during exercise. These demands are met by precise exercise intensity-dependent alterations in sympathetic and parasympathetic nerve activity. The purpose of this review is to examine the contributions of the sympathetic and parasympathetic nervous systems in mediating specific cardiovascular and hemodynamic responses to exercise. These changes in autonomic outflow are regulated by several neural mechanisms working in concert, including central command (a feed forward mechanism originating from higher brain centers), the exercise pressor reflex (a feed-back mechanism originating from skeletal muscle), the arterial baroreflex (a negative feed-back mechanism originating from the carotid sinus and aortic arch), and cardiopulmonary baroreceptors (a feed-back mechanism from stretch receptors located in the heart and lungs). In addition, arterial chemoreceptors and phrenic afferents from respiratory muscles (i.e., respiratory metaboreflex) are also capable of modulating the autonomic responses to exercise. Our goal is to provide a detailed review of the parasympathetic and sympathetic changes that occur with exercise distinguishing between the onset of exercise and steady-state conditions, when appropriate. In addition, studies demonstrating the contributions of each of the aforementioned neural mechanisms to the autonomic changes and ensuing cardiac and/or vascular responses will be covered.

Angiotensin II-dependent hypertension requires cyclooxygenase 1-derived prostaglandin E2 and EP1 receptor signaling in the subfornical organ of the brain.

Cyclooxygenase (COX)-derived prostanoids have long been implicated in blood pressure (BP) regulation. Recently prostaglandin E2 (PGE2) and its receptor EP1 (EP1R) have emerged as key players in angiotensin II (Ang II)–dependent hypertension (HTN) and related end-organ damage. However, the enzymatic source of PGE2, that is, COX-1 or COX-2, and its site(s) of action are not known. The subfornical organ (SFO) is a key forebrain region that mediates systemic Ang II–dependent HTN via reactive oxygen species (ROS). We tested the hypothesis that cross-talk between PGE2/EP1R and ROS signaling in the SFO is required for Ang II HTN. Radiotelemetric assessment of blood pressure revealed that HTN induced by infusion of systemic “slow-pressor” doses of Ang II was abolished in mice with null mutations in EP1R or COX-1 but not COX-2. Slow-pressor Ang II–evoked HTN and ROS formation in the SFO were prevented when the EP1R antagonist SC-51089 was infused directly into brains of wild-type mice, and Ang-II-induced ROS production was blunted in cells dissociated from SFO of EP1R−/− and COX-1−/− but not COX-2−/− mice. In addition, slow-pressor Ang II infusion caused a ≈3-fold increase in PGE2 levels in the SFO but not in other brain regions. Finally, genetic reconstitution of EP1R selectively in the SFO of EP1R-null mice was sufficient to rescue slow-pressor Ang II–elicited HTN and ROS formation in the SFO of this model. Thus, COX 1–derived PGE2 signaling through EP1R in the SFO is required for the ROS-mediated HTN induced by systemic infusion of Ang II and suggests that EP1R in the SFO may provide a novel target for antihypertensive therapy.

Regulation of middle cerebral artery blood velocity during dynamic exercise in humans: influence of aging

Although cerebral autoregulation (CA) appears well maintained during mild to moderate intensity dynamic exercise in young subjects, it is presently unclear how aging influences the regulation of cerebral blood flow during physical activity. Therefore, to address this question, middle cerebral artery blood velocity (MCAV), mean arterial pressure (MAP), and the partial pressure of arterial carbon dioxide (Pa(CO(2))) were assessed at rest and during steady-state cycling at 30% and 50% heart rate reserve (HRR) in 9 young (24 +/- 3 yr; mean +/- SD) and 10 older middle-aged (57 +/- 7 yr) subjects. Transfer function analysis between changes in MAP and mean MCAV (MCAV(mean)) in the low-frequency (LF) range were used to assess dynamic CA. No age-group differences were found in Pa(CO(2)) at rest or during cycling. Exercise-induced increases in MAP were greater in older subjects, while changes in MCAV(mean) were similar between groups. The cerebral vascular conductance index (MCAV(mean)/MAP) was not different at rest (young 0.66 +/- 0.04 cm x s(-1) x mmHg(-1) vs. older 0.67 +/- 0.03 cm x s(-1) x mmHg(-1); mean +/- SE) or during 30% HRR cycling between groups but was reduced in older subjects during 50% HRR cycling (young 0.67 +/- 0.03 cm x s(-1) x mmHg(-1) vs. older 0.56 +/- 0.02 cm x s(-1) x mmHg(-1); P < 0.05). LF transfer function gain and phase between MAP and MCAV(mean) was not different between groups at rest (LF gain: young 0.95 +/- 0.05 cm x s(-1) x mmHg(-1) vs. older 0.88 +/- 0.06 cm x s(-1) x mmHg(-1); P > 0.05) or during exercise (LF gain: young 0.80 +/- 0.05 cm x s(-1) x mmHg(-1) vs. older 0.72 +/- 0.07 cm x s(-1) x mmHg(-1) at 50% HRR; P > 0.05). We conclude that despite greater increases in MAP, the regulation of MCAV(mean) is well maintained during dynamic exercise in healthy older middle-aged subjects.

Endothelin 1–Dependent Neurovascular Dysfunction in Chronic Intermittent Hypoxia

Obstructive sleep apnea, a condition resulting in chronic intermittent hypoxia (CIH), is an independent risk factor for stroke and dementia, but the mechanisms of the effect are unknown. We tested the hypothesis that CIH increases cerebrovascular risk by altering critical mechanisms regulating cerebral blood flow thereby lowering cerebrovascular reserves. Male C57Bl6/J mice were subjected to CIH (10% O2 for 90 seconds/room air for 90 seconds; during sleep hours) or sham treatment for 35 days. Somatosensory cortex blood flow was assessed by laser Doppler flowmetry in anesthetized mice equipped with a cranial window. CIH increased mean arterial pressure (from 74±2 to 83±3 mm Hg; P<0.05) and attenuated the blood flow increase produced by neural activity (whisker stimulation; −39±2%; P<0.05) or neocortical application of endothelium-dependent vasodilators (acetylcholine response: −41±3%; P<0.05). The cerebrovascular dysfunction was associated with oxidative stress in cerebral resistance arterioles and was abrogated by free radical scavenging or NADPH oxidase inhibition. Furthermore, cerebrovascular dysfunction and free radical increase were not observed in mice lacking the NOX2 subunit of NADPH oxidase. CIH markedly increased endothelin 1 in cerebral blood vessels, whereas cerebrovascular dysfunction and oxidative stress were abrogated by neocortical application of the endothelin type A receptor antagonist BQ123. These data demonstrate for the first time that CIH alters key regulatory mechanisms of the cerebral circulation through endothelin 1 and NADPH oxidase–derived radicals. The ensuing cerebrovascular dysfunction may increase stroke risk in patients with sleep apnea by reducing cerebrovascular reserves and increasing the brains susceptibility to cerebral ischemia.