John M. Dopp

Impaired Vascular Regulation in Patients with Obstructive Sleep Apnea: Effects of Continuous Positive Airway Pressure Treatment

RATIONALE Impaired endothelium-dependent vasodilation has been documented in patients with sleep apnea. This impairment may result in blood flow dysregulation during apnea-induced fluctuations in arterial blood gases. OBJECTIVES To test the hypothesis that hypoxic and hypercapnic vasodilation in the forearm and cerebral circulation are impaired in patients with sleep apnea. METHODS We exposed 20 patients with moderate to severe sleep apnea and 20 control subjects, to isocapnic hypoxia and hyperoxic hypercapnia. A subset of 14 patients was restudied after treatment with continuous positive airway pressure. MEASUREMENTS AND MAIN RESULTS Cerebral flow velocity (transcranial Doppler), forearm blood flow (venous occlusion plethysmography), arterial pressure (automated sphygmomanometry), oxygen saturation (pulse oximetry), ventilation (pneumotachograph), and end-tidal oxygen and carbon dioxide tensions (expired gas analysis) were measured during three levels of hypoxia and two levels of hypercapnia. Cerebral vasodilator responses to hypoxia (-0.65 +/- 0.44 vs. -1.02 +/- 0.72 [mean +/- SD] units/% saturation; P = 0.03) and hypercapnia (2.01 +/- 0.88 vs. 2.57 +/- 0.89 units/mm Hg; P = 0.03) were smaller in patients versus control subjects. Hypoxic vasodilation in the forearm was also attenuated (-0.05 +/- 0.09 vs. -0.10 +/- 0.09 unit/% saturation; P = 0.04). Hypercapnia did not elicit forearm vasodilation in either group. Twelve weeks of continuous positive airway pressure treatment enhanced hypoxic vasodilation in the cerebral circulation (-0.83 +/- 0.32 vs. -0.46 +/- 0.29 units/% saturation; P = 0.01) and forearm (-0.19 +/- 0.15 vs. -0.02 +/- 0.08 units/% saturation; P = 0.003), and hypercapnic vasodilation in the brain showed a trend toward improvement (2.24 +/- 0.78 vs. 1.76 +/- 0.64 units/mm Hg; P = 0.06). CONCLUSIONS Vasodilator responses to chemical stimuli in the cerebral circulation and the forearm are impaired in many patients with obstructive sleep apnea. Some of these impairments can be improved with continuous positive airway pressure.

Sleep Disordered Breathing and Obesity: Pathophysiology, Complications and Treatment

Sleep-disordered breathing (SDB) is a medical condition that has increasingly recognized adverse health effects. Obesity is the primary risk factor for the development of SDB and contributes to cardiovascular and metabolic abnormalities in this population. However, accumulating evidence suggests that SDB may be related to the development of these abnormalities independent of obesity. Periodic apneas and hypopneas during sleep result in intermittent hypoxemia, arousals, and sleep disturbances. These pathophysiologic characteristics of SDB are likely mechanisms underlying cardiovascular and metabolic abnormalities including hypertension and other cardiovascular diseases, altered adipokines, inflammatory cytokines, insulin resistance, and glucose intolerance. Treatment of SDB with continuous positive airway pressure reverses some but not all of these abnormalities; however, studies to date have demonstrated inconsistent findings. Weight loss strategies, including diet, exercise, medications, and bariatric surgery, have been evaluated as a treatment strategy for SDB. In preliminary studies, dietary intervention and exercise reduced severity of SDB. One study demonstrated improvements in SDB severity using the weight-reducing medication sibutramine. In morbidly obese subjects, bariatric surgery effectively induces weight loss and improvement in SDB severity and symptoms, but long-term benefits remain uncertain. Large randomized trials are required to determine the utility of these strategies as long-term approaches to improving SDB and reducing associated complications.

Impaired skeletal muscle and skin microcirculatory function in human obesity.

Background Obesity is associated with exaggerated blood pressure and systemic vascular resistance responses to mental stress. Objective To test the hypothesis that skin and muscle microvascular dilatation in response to mental stress is blunted in obesity. Design and methods Blood pressure, heart rate and forearm and skin blood flow responses to mental stress were compared in 23 obese and 23 age- and sex-matched lean normotensive individuals. Results Blood pressure was almost identical in both obese (mean 94 ± 1 mmHg) and lean (93 ± 2 mmHg) individuals. The increase in blood pressure during mental stress was similar in obese and lean individuals (2.0 ± 0.9% compared with 3.1 ± 4.0%;P = 0.8). Forearm vascular resistance decreased during mental stress in both groups, but this decrease was significantly blunted in obese individuals compared with controls (decreases of 14 ± 4% and 26 ± 3%;P < 0.01). Skin microcirculatory dilatation was also significantly blunted in obese individuals compared with controls (decreases of 5 ± 2 and 17 ± 4%;P = 0.02). Conclusions Normotensive obese individuals exhibit markedly impaired muscle and skin microcirculatory responses to mental stress. The increased propensity of obese individuals to develop hypertension under conditions of chronic psychosocial stress may underlie obesity-related hypertension and cardiovascular disease.

Xanthine oxidase inhibition attenuates endothelial dysfunction caused by chronic intermittent hypoxia in rats.

Background: Xanthine oxidase is a major source of superoxide in the vascular endothelium. Previous work in humans demonstrated improved conduit artery function following xanthine oxidase inhibition in patients with obstructive sleep apnea. Objectives: To determine whether impairments in endothelium-dependent vasodilation produced by exposure to chronic intermittent hypoxia are prevented by in vivo treatment with allopurinol, a xanthine oxidase inhibitor. Methods: Sprague-Dawley rats received allopurinol (65 mg/kg/day) or vehicle via oral gavage. Half of each group was exposed to intermittent hypoxia (FIO2 = 0.10 for 1 min, 15×/h, 12 h/day) and the other half to normoxia. After 14 days, gracilis arteries were isolated, cannulated with micropipettes, and perfused and superfused with physiological salt solution. Diameters were measured before and after exposure to acetylcholine (10–6M) and nitroprusside (10–4M). Results: In vehicle-treated rats, intermittent hypoxia impaired acetylcholine-induced vasodilation compared to normoxia (+4 ± 4 vs. +21 ± 6 µm, p = 0.01). Allopurinol attenuated this impairment (+26 ± 6 vs. +34 ± 9 µm for intermittent hypoxia and normoxia groups treated with allopurinol, p = 0.55). In contrast, nitroprusside-induced vasodilation was similar in all rats (p = 0.43). Neither allopurinol nor intermittent hypoxia affected vessel morphometry or systemic markers of oxidative stress. Urinary uric acid concentrations were reduced in allopurinol- versus vehicle-treated rats (p = 0.02). Conclusions: These data confirm previous findings that exposure to intermittent hypoxia impairs endothelium-dependent vasodilation in skeletal muscle resistance arteries and extend them by demonstrating that this impairment can be prevented with allopurinol. Thus, xanthine oxidase appears to play a key role in mediating intermittent hypoxia-induced vascular dysfunction.

Humoral immune responses to influenza vaccination in patients with obstructive sleep apnea.

Study Objective. To compare humoral immune responses to influenza vaccination in subjects with obstructive sleep apnea (OSA) and in healthy volunteers (control subjects).

Sildenafil Increases Sympathetically Mediated Vascular Tone in Humans

BACKGROUND Sildenafil, a selective phosphodiesterase-type-5 (PDE-5) inhibitor, produces vasodilation that improves erectile dysfunction and pulmonary hypertension. Sildenafil could also cause baroreflex sympathetic activation that would enhance vascular tone and oppose direct vasodilation. We tested the hypothesis that sildenafil administration increases sympathetically mediated vascular tone in healthy middle-aged men. METHODS We randomized 9 healthy, middle-aged, male volunteers (mean age 45±2 years) in a double-blind, crossover fashion to receive a single oral dose of sildenafil 100mg or placebo on 2 separate study days. Hemodynamics and forearm blood flow responses were measured at baseline, at 30 and 45 minutes after study drug administration, and then during intra-arterial infusions of vasoactive drugs. After sildenafil and placebo administration, intrabrachial medications were infused to test forearm alpha receptor sensitivity (norepinephrine), cyclic-AMP-mediated vasodilation (isoproterenol), and sympathetically mediated vascular tone (phentolamine) (adenosine was a control vasodilator). Blood samples were taken before and 60 minutes after study drug administration and at the end of the intrabrachial infusions for measurement of plasma norepinephrine concentrations. RESULTS Forearm vascular responses to norepinephrine, isoproterenol, and adenosine were not different after placebo and sildenafil administration. Percentage reduction in forearm vascular resistance during phentolamine was significantly lower after sildenafil than placebo (-73% ± 3% vs -63% ± 3%; P = 0.0002). Sildenafil significantly increased plasma norepinephrine compared with placebo 60 minutes after study drug administration and at the end of the study session (P = 0.02). CONCLUSIONS Sildenafil increased sympathetically mediated vascular tone in middle-aged healthy men. Alpha-adrenergic-mediated vasoconstriction may offset vasodilation during PDE-5 inhibition and may explain the significant hypotension observed in patients taking alpha-blockers with sildenafil.

Influence of Intermittent Hypoxia on Myocardial and Hepatic P‐glycoprotein Expression in a Rodent Model

Study Objective. Patients with obstructive sleep apnea who receive drug therapy for cardiovascular disease may experience resistant hypertension, arrhythmias, or more severe heart failure, and many of the drugs used to treat these conditions are substrates for P‐glycoprotein (P‐gp) transporters. Therefore, we sought to determine if intermittent hypoxia, which mimics obstructive sleep apnea, would upregulate myocardial and hepatic P‐gp expression and Abcb1a and Abcb1b messenger RNA (mRNA) expression (genes that encode for P‐gp) in an animal model.

Oxidative stress augments chemoreflex sensitivity in rats exposed to chronic intermittent hypoxia.

Chronic exposure to intermittent hypoxia (CIH) elicits plasticity of the carotid sinus and phrenic nerves via reactive oxygen species (ROS). To determine whether CIH-induced alterations in ventilation, metabolism, and heart rate are also dependent on ROS, we measured responses to acute hypoxia in conscious rats after 14 and 21 d of either CIH or normoxia (NORM), with or without concomitant administration of allopurinol (xanthine oxidase inhibitor), combined allopurinol plus losartan (angiotensin II type 1 receptor antagonist), or apocynin (NADPH oxidase inhibitor). Carotid body nitrotyrosine production was measured by immunohistochemistry. CIH produced an increase in the ventilatory response to acute hypoxia that was virtually eliminated by all three pharmacologic interventions. CIH caused a robust increase in carotid body nitrotyrosine production that was greatly attenuated by allopurinol plus losartan and by apocynin but unaffected by allopurinol. CIH caused a decrease in metabolic rate and a reduction in hypoxic bradycardia. Both of these effects were prevented by allopurinol, allopurinol plus losartan, and apocynin.

Chronic intermittent hypoxia alters ventilatory and metabolic responses to acute hypoxia in rats

We determined the effects of chronic exposure to intermittent hypoxia (CIH) on chemoreflex control of ventilation in conscious animals. Adult male Sprague-Dawley rats were exposed to CIH [nadir oxygen saturation (SpO2), 75%; 15 events/h; 10 h/day] or normoxia (NORM) for 21 days. We assessed the following responses to acute, graded hypoxia before and after exposures: ventilation (V̇e, via barometric plethysmography), V̇o2 and V̇co2 (analysis of expired air), heart rate (HR), and SpO2 (pulse oximetry via neck collar). We quantified hypoxia-induced chemoreceptor sensitivity by calculating the stimulus-response relationship between SpO2 and the ventilatory equivalent for V̇co2 (linear regression). An additional aim was to determine whether CIH causes proliferation of carotid body glomus cells (using bromodeoxyuridine). CIH exposure increased the slope of the V̇e/V̇co2/SpO2 relationship and caused hyperventilation in normoxia. Bromodeoxyuridine staining was comparable in CIH and NORM. Thus our CIH paradigm augmented hypoxic chemosensitivity without causing glomus cell proliferation.

Lipopolysaccharide Increases Cell Surface P-glycoprotein That Exhibits Diminished Activity in Intestinal Epithelial Cells

Increasingly, it is recognized that commensal microflora regulate epithelial cell processes through the dynamic interaction of pathogen-associated molecular patterns and host pattern recognition receptors such as Toll-like receptor 4 (TLR4). We therefore investigated the effects of bacterial lipopolysaccharide (LPS) on intestinal P-glycoprotein (P-gp) expression and function. Human SW480 (P-gp+/TLR4+) and Caco-2 (P-gp+/TLR4–) cells were treated with medium control or LPS (100 ng/ml) for 24 h prior to study. P-gp function was assessed by measuring the intracellular concentration of rhodamine 123 (Rh123). To confirm P-gp-specific effects, breast cancer resistance protein (BCRP/ABCG2) and multidrug resistance-associated protein 2 (MRP-2/ABCC2) were also analyzed. Treatment of SW480 cells with LPS led to diminished P-gp activity, which could be prevented with polymyxin B (control: 207 ± 16 versus LPS: 402 ± 22 versus LPS + polymyxin B: 238 ± 26 pmoles Rh123/mg protein, p < 0.05 control versus LPS). These effects could be blocked by using polymyxin B and were not seen in the P-gp+/TLR4– Caco-2 cell line (control: 771 ± 28 versus LPS: 775 ± 59 pmoles Rh123/mg protein). Total cellular levels of P-gp did not change in LPS-treated SW480 cells; however, a significant increase in cell surface P-gp was detected. No change in activity, total protein, or apically located MRP-2 was detected following LPS treatment. Sequence analysis confirmed wild-type status of SW480 cells. These data suggest that activation of TLR4 in intestinal epithelial cells leads to an increase in plasma membrane P-gp that demonstrates a diminished capacity to transport substrate.