Effects of arousal and sleep state on systemic and pulmonary hemodynamics in obstructive apnea.

During obstructive sleep apnea (OSA), systemic (Psa) and pulmonary (Ppa) arterial pressures acutely increase after apnea termination, whereas left and right ventricular stroke volumes (SV) reach a nadir. In a canine model (n = 6), we examined the effects of arousal, parasympathetic blockade (atropine 1 mg/kg iv), and sleep state on cardiovascular responses to OSA. In the absence of arousal, SV remained constant after apnea termination, compared with a 4.4 +/- 1.7% decrease after apnea with arousal (P < 0.025). The rise in transmural Ppa was independent of arousal (4.5 +/- 1.0 vs. 4.1 +/- 1.2 mmHg with and without arousal, respectively), whereas Psa increased more after apnea termination in apneas with arousal compared with apneas without arousal. Parasympathetic blockade abolished the arousal-induced increase in Psa, indicating that arousal is associated with a vagal withdrawal of the parasympathetic tone to the heart. Rapid-eye-movement (REM) sleep blunted the increase in Psa (pre- to end-apnea: 5.6 +/- 2.3 mmHg vs. 10.3 +/- 1.6 mmHg, REM vs. non-REM, respectively, P < 0.025), but not transmural Ppa, during an obstructive apnea. We conclude that arousal and sleep state both have differential effects on the systemic and pulmonary circulation in OSA, indicating that, in patients with underlying cardiovascular disease, the hemodynamic consequences of OSA may be different for the right or the left side of the circulation.

Neural and local effects of hypoxia on cardiovascular responses to obstructive apnea.

Obstructive sleep apnea (OSA) acutely increases systemic (Psa) and pulmonary (Ppa) arterial pressures and decreases ventricular stroke volume (SV). In this study, we used a canine model of OSA (n = 6) to examine the role of hypoxia and the autonomic nervous system (ANS) in mediating these cardiovascular responses. Hyperoxia (40% oxygen) completely blocked any increase in Ppa in response to obstructive apnea but only attenuated the increase in Psa. In contrast, after blockade of the ANS (20 mg/kg iv hexamethonium), obstructive apnea produced a decrease in Psa (-5.9 mmHg; P < 0.05) but no change in Ppa, and the fall in SV was abolished. Both the fall in Psa and the rise in Ppa that persisted after ANS blockade were abolished when apneas were induced during hyperoxia. We conclude that 1) hypoxia can account for all of the Ppa and the majority of the Psa response to obstructive apnea, 2) the ANS increases Psa but not Ppa in obstructive apnea, 3) the local effects of hypoxia associated with obstructive apnea cause vasodilation in the systemic vasculature and vasoconstriction in the pulmonary vasculature, and 4) a rise in Psa acts as an afterload to the heart and decreases SV over the course of the apnea.

Leptin prevents respiratory depression in obesity.

Human obesity leads to an increase in respiratory demands. As obesity becomes more pronounced some individuals are unable to compensate, leading to elevated arterial carbon dioxide levels (PaCO2), alveolar hypoventilation, and increased cardiorespiratory morbidity and mortality (Pickwickian syndrome). The mechanisms that link obesity and hypoventilation are unknown, but thought to involve depression of central respiratory control mechanisms. Here we report that obese C57BL/6J-Lepob mice, which lack circulating leptin, also exhibit respiratory depression and elevated PaCO2 (> 10 mm Hg; p < 0. 0001). A role for leptin in restoring ventilation in these obese, mutant mice was investigated. Three days of leptin infusion (30 microg/d) markedly increased minute ventilation (V E) across all sleep/wake states, but particularly during rapid eye movement (REM) sleep when respiration was otherwise profoundly depressed. The effect of leptin was independent of food intake, weight, and CO2 production, indicating a reversal of hypoventilation by stimulation of central respiratory control centers. Furthermore, leptin replacement in mutant mice increased CO2 chemosensitivity during non-rapid eye movement (NREM) (4.0 +/- 0.5 to 5.6 +/- 0.4 ml/min/%CO2; p < 0.01) and REM (-0.1 +/- 0.5 to 3.0 +/- 0.8 ml/min/%CO2; p < 0.01) sleep. We also demonstrate in wild-type mice that ventilation is appropriately compensated when obesity is diet-induced and endogenous leptin levels are raised more than tenfold. These results suggest that leptin can prevent respiratory depression in obesity, but a deficiency in central nervous system (CNS) leptin levels or activity may induce hypoventilation and the Pickwickian syndrome in some obese subjects. O'Donnell CP, Schaub CD, Haines AS, Berkowitz DE, Tankersley CG, Schwartz AR, Smith PL. Leptin prevents respiratory depression in obesity.

Effect of sleep/wake state on arterial blood pressure in genetically identical mice.

Genetic determinants may contribute to the large variability in arterial blood pressure responses to changes in sleep/wake state in humans. In this study, we developed techniques to examine the relationship between sleep/wake state and mean arterial pressure (MAP) in unrestrained, genetically identical mice (C57BL/6J; n = 9). The left common carotid artery was catheterized, and arterial blood gases were analyzed 24-48 h postsurgery to verify normal respiratory and metabolic function. The animals were then allowed to cycle naturally through sleep/wake states over a 3- to 4-h period while continuous polysomnography and arterial pressure measurements were made. The MAP decreased from quiet wakefulness to non-rapid-eye-movement sleep (9.8 +/- 1.3 mmHg; P < 0.001) and further decreased from non-rapid-eye-movement to rapid-eye-movement sleep (9.7 +/- 1.8 mmHg; P < 0.001). We conclude that the inbred strain of C57BL/6J mice exhibits significant and consistent changes in MAP related to sleep/wake state. Future studies can compare responses in this strain of mice with those in other inbred or transgenic mice to determine whether specific genes regulate arterial blood pressure responses to sleep/wake state.

Systemic and pulmonary hemodynamic responses to normal and obstructed breathing during sleep.

We examined the hemodynamic responses to normal breathing and induced upper airway obstructions during sleep in a canine model of obstructive sleep apnea. During normal breathing, cardiac output decreased (12.9 +/- 3.5%, P < 0.025) from wakefulness to non-rapid-eye-movement sleep (NREM) but did not change from NREM to rapid-eye-movement (REM) sleep. There was a decrease (P < 0.05) in systemic (7.2 +/- 2.1 mmHg) and pulmonary (2.0 +/- 0.6 mmHg) arterial pressures from wakefulness to NREM sleep. In contrast, systemic (8.1 +/- 1.0 mmHg, P < 0.025), but not pulmonary, arterial pressures decreased from NREM to REM sleep. During repetitive airway obstructions (56.0 +/- 4.7 events/h) in NREM sleep, cardiac output (17.9 +/- 3.1%) and heart rate (16.2 +/- 2.5%) increased (P < 0.05), without a change in stroke volume, compared with normal breathing during NREM sleep. During single obstructive events, left (7.8 +/- 3.0%, P < 0.05) and right (7.1 +/- 0.7%, P < 0.01) ventricular outputs decreased during the apneic period. However, left (20.7 +/- 1.6%, P < 0.01) and right (24.0 +/- 4.2%, P < 0.05) ventricular outputs increased in the post-apneic period because of an increase in heart rate. Thus 1) the systemic, but not the pulmonary, circulation vasodilates during REM sleep with normal breathing; 2) heart rate, rather than stroke volume, is the dominant factor modulating ventricular output in response to apnea; and 3) left and right ventricular outputs oscillate markedly and in phase throughout the apnea cycle.

Phosphotyrosine-dependent interaction of SHC and insulin receptor substrate 1 with the NPEY motif of the insulin receptor via a novel non-SH2 domain.

The SHC proteins have been implicated in insulin receptor (IR) signaling. In this study, we used the sensitive two-hybrid assay of protein-protein interaction to demonstrate that SHC interacts directly with the IR. The interaction is mediated by SHC amino acids 1 to 238 and is therefore independent of the Src homology 2 domain. The interaction is dependent upon IR autophosphorylation, since the interaction is eliminated by mutation of the IR ATP-binding site. In addition, mutational analysis of the Asn-Pro-Glu-Tyr (NPEY) motif within the juxtamembrane domain of the IR showed the importance of the Asn, Pro, and Tyr residues to both SHC and IR substrate 1 (IRS-1) binding. We conclude that SHC interacts directly with the IR and that phosphorylation of Tyr-960 within the IR juxtamembrane domain is necessary for efficient interaction. This interaction is highly reminiscent of that of IRS-1 with the IR, and we show that the SHC IR-binding domain can substitute for that of IRS-1 in yeast and COS cells. We identify a homologous region within the IR-binding domains of SHC and IRS-1, which we term the SAIN (SHC and IRS-1 NPXY-binding) domain, which may explain the basis of these interactions. The SAIN domain appears to represent a novel motif which is able to interact with autophosphorylated receptors such as the IR.