Splenic contraction-induced reversible increase in hemoglobin concentration in intermittent hypoxia

Larger spleens and greater splenic contraction during exercise may be an adaptive characteristic of Nepali Sherpa at high-altitude

OBJECTIVES

The Sherpa ethnic group living at altitude in Nepal may have experienced natural selection in response to chronic hypoxia. We have previously shown that Sherpa in Kathmandu (1400 m) possess larger spleens and a greater apnea-induced splenic contraction compared to lowland Nepalis. This may be significant for exercise capacity at altitude as the human spleen responds to stress-induced catecholamine secretion by an immediate contraction, which results in transiently elevated hemoglobin concentration ([Hb]).

METHODS

To investigate splenic contraction in response to exercise at high-altitude (4300 m; Pb = ~450 Torr), we recruited 63 acclimatized Sherpa (29F) and 14 Nepali non-Sherpa (7F). Spleen volume was measured before and after maximal exercise on a cycle ergometer by ultrasonography, along with [Hb] and oxygen saturation (SpO2).

RESULTS

Resting spleen volume was larger in the Sherpa compared with Nepali non-Sherpa (237 ± 62 vs. 165 ± 34 mL, p < .001), as was the exercise-induced splenic contraction (Δspleen volume, 91 ± 40 vs. 38 ± 32 mL, p < .001). From rest to exercise, [Hb] increased (1.2 to 1.4 g.dl-1), SpO2 decreased (~9%) and calculated arterial oxygen content (CaO2) remained stable, but there were no significant differences between groups. In Sherpa, both resting spleen volume and the Δspleen volume were modest positive predictors of the change (Δ) in [Hb] and CaO2 with exercise (p-values from .026 to .037 and R2 values from 0.059 to 0.067 for the predictor variable).

CONCLUSIONS

Larger spleens and greater splenic contraction may be an adaptive characteristic of Nepali Sherpa to increase CaO2 during exercise at altitude, but the direct link between spleen size/function and hypoxia tolerance remains unclear.

The cardiosplenic axis: the prognostic role of the spleen in heart failure

Despite the number of available methods to predict prognosis in patients with heart failure, prognosis remains poor, likely because of marked patient heterogeneity and varied heart failure etiologies. Thus, identification of novel prognostic indicators to stratify risk in patients with heart failure is of paramount importance. The spleen is emerging as a potential novel prognostic indicator for heart failure. In this article, we provide an overview of the current prognostic tools used for heart failure. We then introduce the spleen as a potential novel prognostic indicator, before outlining the structure and function of the spleen and introducing the concept of the cardiosplenic axis. This is followed by a focused discussion on the function of the spleen in the immune response and in hemodynamics, as well as a review of what is known about the usefulness of the spleen as an indicator of heart failure. Expert insight into the most effective spleen-related measurement indices for the prognostication of patients with heart failure is provided, and suggestions on how these could be measured in clinical practice are considered. In future, studies in humans will be required to draw definitive links between specific splenic measurements and different heart failure manifestations, as well as to determine whether splenic prognostic measurements differ between heart failure classes and etiologies. These contributions will provide a step forward in our understanding of the usefulness of the spleen as a prognostic predictor in heart failure.

Rapid and reversible modulation of blood haemoglobin content during diel cycles of hypoxia in killifish (Fundulus heteroclitus)

We investigated whether fish can make dynamic haematological adjustments to support aerobic metabolism during repeated cycles of hypoxia-reoxygenation. Killifish were acclimated to normoxia, constant hypoxia (2 kPa O₂), or intermittent cycles of nocturnal hypoxia (12 h of normoxia: 12 h of 2 kPa O₂ hypoxia) for 28 days. Normoxia-acclimated fish were sampled in the daytime in normoxia and after exposure to a single bout of nocturnal hypoxia. Each hypoxia acclimation group were sampled at the PO₂ experienced during acclimation during both the day and night. All acclimation groups had increased blood haemoglobin content and haematocrit and reduced spleen mass during nocturnal hypoxia compared to normoxic controls. Blood haemoglobin content was negatively correlated with spleen mass at both the individual and group level. Fish acclimated to intermittent hypoxia rapidly reversed these changes during diurnal reoxygenation. The concentrations of haemoglobin, ATP, and GTP within erythrocytes did not vary substantially between groups. We also measured resting O₂ consumption rate (MO₂) and maximum MO₂ (induced by an exhaustive chase) in hypoxia in each acclimation group. Fish acclimated to intermittent hypoxia maintained higher resting MO₂ than other groups in hypoxia, comparable to the resting MO₂ of normoxia-acclimated controls measured in normoxia. Differences in resting MO₂ in hypoxia did not result from variation in O₂ transport capacity, because maximal MO₂ in hypoxia always exceeded resting MO₂. Therefore, reversible modulation of blood haemoglobin content along with metabolic adjustments help killifish cope with intermittent cycles of hypoxia in the estuarine environment.

Spleen contraction elevates hemoglobin concentration at high altitude during rest and exercise

PURPOSE

Hypoxia and exercise are known to separately trigger spleen contraction, leading to release of stored erythrocytes. We studied spleen volume and hemoglobin concentration (Hb) during rest and exercise at three altitudes.

METHODS

Eleven healthy lowlanders did a 5-min modified Harvard step test at 1370, 3700 and 4200 m altitude. Spleen volume was measured via ultrasonic imaging and capillary Hb with Hemocue during rest and after the step test, and arterial oxygen saturation (SaO₂), heart rate (HR), expiratory CO₂ (ETCO₂) and respiratory rate (RR) across the test.

RESULTS

Resting spleen volume was reduced with increasing altitude and further reduced with exercise at all altitudes. Mean (SE) baseline spleen volume at 1370 m was 252 (20) mL and after exercise, it was 199 (15) mL (P < 0.01). At 3700 m, baseline spleen volume was 231 (22) mL and after exercise 166 (12) mL (P < 0.05). At 4200 m baseline volume was 210 (23) mL and after exercise 172 (20) mL (P < 0.05). After 10 min, spleen volume increased to baseline at all altitudes (NS). Baseline Hb increased with altitude from 138.9 (6.1) g/L at 1370 m, to 141.2 (4.1) at 3700 m and 152.4 (4.0) at 4200 m (P < 0.01). At all altitudes Hb increased from baseline during exercise to 146.8 (5.7) g/L at 1370 m, 150.4 (3.8) g/L at 3700 m and 157.3 (3.8) g/L at 4200 m (all P < 0.05 from baseline). Hb had returned to baseline after 10 min rest at all altitudes (NS). The spleen-derived Hb elevation during exercise was smaller at 4200 m compared to 3700 m (P < 0.05). Cardiorespiratory variables were also affected by altitude during both rest and exercise.

CONCLUSIONS

The spleen contracts and mobilizes stored red blood cells during rest at high altitude and contracts further during exercise, to increase oxygen delivery to tissues during acute hypoxia. The attenuated Hb response to exercise at the highest altitude is likely due to the greater recruitment of the spleen reserve during rest, and that maximal spleen contraction is reached with exercise.

Therapeutic effects of Qishen Yiqi Dropping Pill on myocardial injury induced by chronic hypoxia in rats

The present study was designed to determine the effects of a traditional Chinese medicine, called Qishen Yiqi Dropping Pill on chronic hypoxia-induced myocardial injury. To establish a rat chronic hypoxia model to be used in the evaluation of the therapeutic effects of the Qishen Yiqi Dropping Pill, Sprague-Dawley (SD) rats were randomly divided into three groups: the control, model, and treatment groups (n = 10 per group). The animals were housed in a plexiglass container. The control animals were under normal oxygen concentration and the model and treatment groups were exposed to air and nitrogen for 5 weeks. The rats in the treatment group were orally administered the Qishen Yiqi Dropping pill (35 mg·kg(-1)·d(-1)) for 5 weeks. After the treatment, the cardiac function and morphology were analyzed, and the expression levels of hypoxia-inducible factor 1α (HIF-1α) were determined using Western blotting. Our results indicated that the cardiac function was impaired, cell apoptosis was enhanced, and HIF-1α expression was up-regulated in the model group, compared to the control group. These changes were ameliorated by the treatment with the Qishen Yiqi Dropping Pill. In conclusion, Qishen Yiqi Dropping pill can ameliorate myocardial injury induced by chronic hypoxia, improve cardiac function, and decrease myocardial cell apoptosis, which may provide a basis for its clinical use for the treatment of chronic cardiovascular diseases.

Reduced Noradrenergic Signaling in the Spleen Capsule in the Absence of CB1 and CB2 Cannabinoid Receptors

The spleen is a visceral organ that contracts during hypoxia to expel erythrocytes and immune cells into the circulation. Spleen contraction is under the control of noradrenergic sympathetic innervation. The activity of noradrenergic neurons terminating in the spleen capsule is regulated by α2-adrenergic receptors (AR). Interactions between endogenous cannabinoid signaling and noradrenergic signaling in other organ systems suggest endocannabinoids might also regulate spleen contraction. Spleens from mice congenitally lacking both CB₁ and CB₂ cannabinoid receptors (Cnr1 ^-/- /Cnr2 ^-/- mice) were used to explore the role of endocannabinoids in spleen contraction. Spleen contraction in response to exogenous norepinephrine (NE) was found to be significantly lower in Cnr1 ^-/- /Cnr2 ^-/- mouse spleens, likely due to decreased expression of capsular α1AR. The majority of splenic Cnr1 mRNA expression is by cells of the spleen capsule, suggestive of post-synaptic CB₁ receptor signaling. Thus, these studies demonstrate a role for CB₁ and/or CB₂ in noradrenergic splenic contraction.

Acute stressor exposure modifies plasma exosome-associated heat shock protein 72 (Hsp72) and microRNA (miR-142-5p and miR-203)

Exosomes, biologically active nanoparticles (40-100 nm) released by hematopoietic and non-hematopoietic cells, contain a variety of proteins and small, non-coding RNA known as microRNA (miRNA). Exposure to various pathogens and disease states modifies the composition and function of exosomes, but there are no studies examining in vivo exosomal changes evoked by the acute stress response. The present study reveals that exposing male Fisher 344 rats to an acute stressor modulates the protein and miRNA profile of circulating plasma exosomes, specifically increasing surface heat shock protein 72 (Hsp72) and decreasing miR-142-5p and -203. The selected miRNAs and Hsp72 are associated with immunomodulatory functions and are likely a critical component of stress-evoked modulation of immunity. Further, we demonstrate that some of these stress-induced modifications in plasma exosomes are mediated by sympathetic nervous system (SNS) activation of alpha-1 adrenergic receptors (ADRs), since drug-mediated blockade of the receptors significantly attenuates the stress-induced modifications of exosomal Hsp72 and miR-142-5p. Together, these findings demonstrate that activation of the acute stress response modifies the proteomic and miRNA profile of exosomes released into the circulation.

Single dose acute toxicity testing for N,N-bis(2-mercaptoethyl)-N',N' diethylethylenediamine in beagles

N,N-Bis(2-mercaptoethyl)-N',N'-diethylenediamine (BMEDA) is used in the preparation of the (188)Re-BMEDA-liposome as a chelator to deliver rhenium 188 into liposomes. Although the safety of the (188)Re-BMEDA-liposome had been established, the use of BMEDA in preparing the liposome is of interest; however, an assessment of its safety is warranted. In this present work, we report on the acute toxicity study of BMEDA in beagles to identify doses causing no adverse effect and doses causing life-threatening toxicity. In a single dose 14-day systemic toxicity study conducted in beagles, BMEDA was without compound-related adverse effects at doses of up to 2mg/kg in a series of clinical observations and clinical pathology examinations. The results of these studies could aid in choosing doses for repeat-dose studies and in the selection of starting doses for Phase 1 human studies.

Commensal bacteria and MAMPs are necessary for stress-induced increases in IL-1β and IL-18 but not IL-6, IL-10 or MCP-1

Regular interactions between commensal bacteria and the enteric mucosal immune environment are necessary for normal immunity. Alterations of the commensal bacterial communities or mucosal barrier can disrupt immune function. Chronic stress interferes with bacterial community structure (specifically, α-diversity) and the integrity of the intestinal barrier. These interferences can contribute to chronic stress-induced increases in systemic IL-6 and TNF-α. Chronic stress, however, produces many physiological changes that could indirectly influence immune activity. In addition to IL-6 and TNF-α, exposure to acute stressors upregulates a plethora of inflammatory proteins, each having unique synthesis and release mechanisms. We therefore tested the hypothesis that acute stress-induced inflammatory protein responses are dependent on the commensal bacteria, and more specifically, lipopolysaccharide (LPS) shed from Gram-negative intestinal commensal bacteria. We present evidence that both reducing commensal bacteria using antibiotics and neutralizing LPS using endotoxin inhibitor (EI) attenuates increases in some (inflammasome dependent, IL-1 and IL-18), but not all (inflammasome independent, IL-6, IL-10, and MCP-1) inflammatory proteins in the blood of male F344 rats exposed to an acute tail shock stressor. Acute stress did not impact α- or β- diversity measured using 16S rRNA diversity analyses, but selectively reduced the relative abundance of Prevotella. These findings indicate that commensal bacteria contribute to acute stress-induced inflammatory protein responses, and support the presence of LPS-mediated signaling in stress-evoked cytokine and chemokine production. The selectivity of the commensal bacteria in stress-evoked IL-1β and IL-18 responses may implicate the inflammasome in this response.

Optoacoustic monitoring of cerebral venous blood oxygenation through extracerebral blood

There is strong clinical evidence that controlling cerebral venous oxygenation (oxyhemoglobin saturation) is critically important for patients with severe traumatic brain injury as well as for patients undergoing cardiac surgery. However, the only available method for cerebral venous blood oxygenation monitoring is invasive and requires catheterization of the internal jugular vein. We designed and built a novel optoacoustic monitor of cerebral venous oxygenation as measured in the superior sagittal sinus (SSS), the large midline cerebral vein. To the best of our knowledge, optical monitoring of cerebral venous blood oxygenation through overlying extracerebral blood is reported for the first time in this paper. The system was capable of detecting SSS signals in vivo at 700, 800, and 1064 nm through the thick (5-6 mm) sheep skull containing the circulating blood. The high (submillimeter) in-depth resolution of the system provided identification of the SSS peaks in the optoacoustic signals. The SSS peak amplitude closely followed the actual SSS blood oxygenation measured invasively using catheterization, blood sampling, and "gold standard" CO-Oximetry. Our data indicate the system may provide accurate measurement of the SSS blood oxygenation in patients with extracerebral blood over the SSS.

Diving bradycardia: a mechanism of defence against hypoxic damage

A feature of all air-breathing vertebrates, diving bradycardia is triggered by apnoea and accentuated by immersion of the face or whole body in cold water. Very little is known about the afferents of diving bradycardia, whereas the efferent part of the reflex circuit is constituted by the cardiac vagal fibres. Diving bradycardia is associated with vasoconstriction of selected vascular beds and a reduction in cardiac output. The diving response appears to be more pronounced in mammals than in birds. In humans, the bradycardic response to diving varies greatly from person to person; the reduction in heart rate generally ranges from 15 to 40%, but a small proportion of healthy individuals can develop bradycardia below 20 beats/min. During prolonged dives, bradycardia becomes more pronounced because of activation of the peripheral chemoreceptors by a reduction in the arterial partial pressure of oxygen (O2), responsible for slowing of heart rate. The vasoconstriction is associated with a redistribution of the blood flow, which saves O2 for the O2-sensitive organs, such as the heart and brain. The results of several investigations carried out both in animals and in humans show that the diving response has an O2-conserving effect, both during exercise and at rest, thus lengthening the time to the onset of serious hypoxic damage. The diving response can therefore be regarded as an important defence mechanism for the organism.

The spleen is required for 5-HT1A receptor agonist-mediated increases in mean circulatory filling pressure during hemorrhagic shock in the rat

The 5-HT(1A) receptor agonist, 8- OH-DPAT, increases whole body venous tone (mean circulatory filling pressure; MCFP), and attenuates metabolic acidosis in a rat model of unresuscitated hemorrhagic shock. To determine whether improved acid-base balance was associated with sympathetic activation and venous constriction, MCFP, sympathetic activity (SA), and blood gases were compared in hemorrhaged rats following administration of 5-HT(1A) receptor agonist 8-OH-DPAT, the arterial vasoconstrictor arginine vasopressin (AVP), or saline. To further determine whether protection of acid-base balance was dependent on splenic contraction and blood mobilization, central venous pressure (CVP), MCFP, and blood gases were determined during hemorrhage and subsequent 8-OH-DPAT-administration in rats subjected to real or sham splenectomy. Subjects were hemorrhaged to an arterial pressure of 50 mmHg for 25 min and subsequently were treated with 8-OH-DPAT (30 nmol/kg iv), AVP titrated to match the pressor effect of 8-OH-DPAT (approximately 2 ng/min iv), or infusion of normal saline. 8-OH-DPAT increased MAP, CVP, MCFP, and SA, and decreased lactate accumulation. AVP did not affect CVP or SA, but raised MCFP slightly to a level intermediate between 8-OH-DPAT- and saline-treated rats. Infusion of AVP also produced a modest protection against metabolic acidosis. Splenectomy prevented the rise in CVP, MCFP, and protection against metabolic acidosis produced by 8-OH-DPAT but had no effect on the immediate pressor response to the drug. Together, the data indicate that 8-OH-DPAT produces a pattern of cardiovascular responses consistent with a sympathetic-mediated venoconstriction that is, in part, responsible for the drug's beneficial effect on acid-base balance. Moreover, blood mobilization stimulated by the spleen is required for the beneficial effects of 8-OH-DPAT.

Impaired transient elevation of blood hemoglobin in response to acute hypoxia in neonates with asplenia

BACKGROUND

It has been shown that acute hypoxia induces the transient elevation of blood hemoglobin concentration ([Hb]) as a consequence of sympathetic-mediated splenic contraction to maintain the supply of oxygen, and splenectomy abolishes this phenomenon. The purpose of the present paper was to determine, retrospectively, whether the ability of transient elevation of [Hb] against acute hypoxia would be impaired in neonates with asplenia.

METHOD

Eleven neonates who underwent surgery from 1998 to 2003 were enrolled in this retrospective study. They were divided into two groups: (i) five patients with asplenia syndrome with cyanotic congenital heart disease (asplenia group); and (ii) six patients with hypoplastic left heart syndrome who needed nitrogen gas inhalation therapy (HLHS group). In the asplenia group simultaneous data of arterial oxygen saturation (Sao(2)) and [Hb] were obtained before and after the temporary unexpected decrease of percutaneous arterial oxygen saturation. In the HLHS group they were obtained before and after nitrogen gas administration therapy. The arterial oxygen content (Cao(2)) changes and the ratio of Cao(2) change (Cao(2) after hypoxia divided by Cao(2) before hypoxia) were also calculated.

RESULTS

In the asplenia group [Hb] was unchanged (12.9 +/- 1.6 g/dL to 12.8 +/- 1.4, n.s.) and Cao(2) was decreased (14.5 +/- 1.6 mL/dL to 11.9 +/- 1.1, P = 0.018). In the HLHS group [Hb] was increased (14.6 +/- 1.3 g/dL to 15.4 +/- 1.5, P = 0.028), but Cao(2) was changed (18.2 +/- 2.2 mL/dL to 16.7 +/- 3.0, P = 0.043). The ratio of Cao(2) change for the HLHS group was significantly different from that of the asplenia group (0.92 +/- 0.10, 0.83 +/- 0.10, respectively, P = 0.02).

CONCLUSIONS

Patients with asplenia syndrome have some disadvantage regarding this protective mechanism against acute hypoxia.

Short-term effects of normobaric hypoxia on the human spleen

Spleen contraction resulting in an increase in circulating erythrocytes has been shown to occur during apnea. This effect, however, has not previously been studied during normobaric hypoxia whilst breathing. After 20 min of horizontal rest and normoxic breathing, five subjects underwent 20-min of normobaric hypoxic breathing (12.8% oxygen) followed by 10 min of normoxic breathing. Ultrasound measurements of spleen volume and samples for venous hemoglobin concentration (Hb) and hematocrit (Hct) were taken simultaneously at short intervals from 20 min before until 10 min after the hypoxic period. Heart rate, arterial oxygen saturation (SaO(2)) and respiration rate were recorded continuously. During hypoxia, a reduction in SaO(2) by 34% (P < 0.01) was accompanied by an 18% reduction in spleen volume and a 2.1% increase in both Hb and Hct (P < 0.05). Heart rate increased 28% above baseline (P < 0.05). Within 3 min after hypoxia SaO(2) had returned to pre-hypoxic levels, and spleen volume, Hb and Hct had all returned to pre-hypoxic levels within 10 min. Respiratory rate remained stable throughout the protocol. This study of short-term exposure to eupneic normobaric hypoxia suggests that hypoxia plays a key role in triggering spleen contraction and subsequent release of stored erythrocytes in humans. This response could be beneficial during early altitude acclimatization.

The Serotonin 5-Hydroxytryptaphan1A Receptor Agonist, (+)8-Hydroxy-2-(di-n-propylamino)-tetralin, Stimulates Sympathetic-Dependent Increases in Venous Tone during Hypovolemic Shock

Adjuvant treatment of hypovolemic shock with vasoconstrictors is controversial due to their propensity to raise arterial resistance and exacerbate ischemia. A more advantageous therapeutic approach would use agents that also promote venoconstriction to augment perfusion pressure through increased venous return. Recent studies indicate that 5-hydroxytryptophan (5-HT)(1A) receptor agonists increase blood pressure by stimulating sympathetic drive when administered after acute hypotensive hemorrhage. Given that venous tone is highly dependent upon sympathetic activation of alpha(2)-adrenergic receptors, we hypothesized that the 5-HT(1A) receptor agonist, (+)8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), would increase venous tone in rats subject to hypovolemic shock through sympathetic activation of alpha(2)-adrenergic receptors. Systemic administration of 8-OH-DPAT produced a sustained rise in blood pressure (+44 +/- 3 mm Hg 35 min after injection, P < 0.01 versus saline) and mean circulatory filling pressure (+4.2 +/- 0.7 mm Hg, P < 0.01 versus saline) in conscious rats subjected to hypovolemic shock. An equipressor infusion of epinephrine failed to influence mean circulatory filling pressure (MCFP). Ganglionic blockade, alpha(1)-, or peripheral alpha(2)-adrenergic receptor blockade prevented the rise in MCFP observed with 8-OH-DPAT, but only alpha(1)-adrenergic receptor blockade diminished the pressor effect of the drug (P < 0.01). 8-OH-DPAT raises blood pressure in rats in hypovolemic shock through both direct vascular activation and sympathetic activation of alpha(1)-adrenergic receptors. The sympathoexcitatory effect of 8-OH-DPAT contributes to elevated venous tone through concurrent activation of both alpha(1)- and alpha(2)-adrenergic receptors. The data suggest that 5-HT(1A) receptor agonists may provide an advantageous alternative to currently therapeutic interventions used to raise perfusion pressure in hypovolemic shock.

The early effects of chronic hypoxia on the cardiovascular system in the rat: role of nitric oxide

Experiments were performed under Saffan anaesthesia on normoxic (N) rats and on chronically hypoxic rats exposed to 12% O2 for 1, 3 or 7 days (1, 3 or 7CH rats): N rats routinely breathed 21% O2 and CH rats 12% O2. The 1, 3 and 7CH rats showed resting hyperventilation relative to N rats, but baseline heart rate (HR) was unchanged and arterial blood pressure (ABP) was lowered. Femoral vascular conductance (FVC) was increased in 1 and 3CH rats, but not 7CH rats. When 1-7CH rats were acutely switched to breathing 21% O2 for 5 min, ABP increased and FVC decreased, consistent with removal of a hypoxic dilator stimulus that is waning in 7CH rats. We propose that this is because the increase in haematocrit and vascular remodelling in skeletal muscle help restore the O2 supply. The increases in FVC evoked by acute hypoxia (8% O2 for 5 min) and by infusion for 5 min of alpha-calcitonin gene-related peptide (alpha-CGRP), which are NO-dependent, were particularly accentuated in 1CH, relative to N rats. The NO synthesis inhibitor L-NAME increased ABP, decreased HR and greatly reduced FVC, and attenuated increases in FVC evoked by acute hypoxia and alpha-CGRP, such that baselines and responses were similar in N and 1-7CH rats. We propose that in the first few days of chronic hypoxia there is tonic NO-dependent vasodilatation in skeletal muscle that is associated with accentuated dilator responsiveness to acute hypoxia and dilator substances that are NO -dependent.

Haemoconcentration via diuresis in short-term hypoxia: a possible role for cardiac natriuretic peptide in rainbow trout

Rainbow trout, exposed to acute hypoxia (decrease of oxygen level from full to 30% air saturation for 1 h, stable 30% air saturation for 2 h), showed more than twofold increase in urine flow rate. Hypoxic diuresis was associated with a sustained increase in dorsal aortic cardiac peptide (sCP) level, and the diuresis could be completely inhibited by a bolus injection of sCP antiserum. These results suggest that hypoxic haemoconcentration, which is partially achieved via increased urine flow rate in vertebrates, is caused by cardiac peptides. The results further suggest that cardiac peptide receptors in hypoxic fish gills modulate the postbranchial systemic level of sCP.

The human diving response, its function, and its control

The purpose of this review is to outline the physiological responses associated with the diving response, its functional significance, and its cardiorespiratory control. This review is separated into four major sections. Section one outlines the diving response and its physiology. Section two provides support for the hypothesis that the primary role of the diving response is the conservation of oxygen. The third section describes how the diving response is controlled and provides a model that illustrates the cardiorespiratory interaction. Finally, the fourth section illustrates potential adaptations that result after regular exposure to an asphyxic environment. The cardiovascular and endocrine responses associated with the diving response and apnea are bradycardia, vasoconstriction, and an increase in secretion of suprarenal catecholamines. These responses require the integration of both the cardiovascular system and the respiratory system. The primary role of the diving response is likely to conserve oxygen for sensitive brain and heart tissue and to lengthen the time before the onset of serious hypoxic damage. We suggest that future research should be focused towards understanding the role of altered ventilatory responses in human breath-hold athletes as well as in patients suffering from sleep-disordered breathing.

The human spleen during physiological stress

Many mammals have the ability to autotransfuse a large quantity of red blood cells from the spleen into the active circulation during times of stress. This enhancement of the oxygen transport system has benefited the athletic mammal, that is, the thoroughbred horse, fox and greyhound in an improved aerobic performance. The role of the spleen in sequestering 50% of the total red cell volume in seals and horses, during times of inactivity, dramatically reduces the viscosity of the blood and therefore the work of the heart. In comparison, the human spleen contains only a small percentage of red blood cells, and has been primarily thought of as a lymphoid organ. The aim of this review is to emphasise the similarities between the human spleen and that of several athletic mammalian species during acute physiological stress. In the athletic mammalian model the expulsion of blood from the spleen is facilitated via the sympathetic nervous system resulting in contraction of smooth muscle within the splenic capsule. In comparison, the lack of smooth muscle contained within the human splenic capsule has meant that active contraction of the spleen has historically been viewed as unlikely, although evidence of contractile proteins within the red pulp have suggested otherwise. Exercise results in haemoconcentration, which has been attributed solely to a reduction in plasma volume. Indirect calculation of plasma volume changes utilise haemoglobin and haematocrit and assume that the circulating red cell volume remains constant. However, several studies have suggested that the human spleen could account for 30% of the increase in haematocrit. This would result in a substantial overestimation of the reduction in plasma volume, indicating that the expulsion of red blood cells from the spleen must not be overlooked when utilising these equations.

Interval hypoxic training

Interval hypoxic training (IHT) is a technique developed in the former Soviet Union, that consists of repeated exposures to 5-7 minutes of steady or progressive hypoxia, interrupted by equal periods of recovery. It has been proposed for training in sports, to acclimatize to high altitude, and to treat a variety of clinical conditions, spanning from coronary heart disease to Cesarean delivery. Some of these results may originate by the different effects of continuous vs. intermittent hypoxia (IH), which can be obtained by manipulating the repetition rate, the duration and the intensity of the hypoxic stimulus. The present article will attempt to examine some of the effects of IH, and, whenever possible, compare them to those of typical IHT. IH can modify oxygen transport and energy utilization, alter respiratory and blood pressure control mechanisms, induce permanent modifications in the cardiovascular system. IHT increases the hypoxic ventilatory response, increase red blood cell count and increase aerobic capacity. Some of these effects might be potentially beneficial in specific physiologic or pathologic conditions. At this stage, this technique appears interesting for its possible applications, but still largely to be explored for its mechanisms, potentials and limitations.

Selected Contribution: Pulmonary hypertension in mice following intermittent hypoxia

Sleep apnea (intermittent periods of hypoxia with or without hypercapnia) is associated with systemic hypertension and increased mortality from cardiovascular disease, but the relationship to pulmonary hypertension is uncertain. Previous studies on intermittent hypoxia (IH) in rats that demonstrated pulmonary hypertension utilized relatively long periods of hypoxia. Recent studies that utilized brief periods of hypoxia have conflicting reports of right ventricular (RV) hypertrophy. In addition, many studies have not measured pulmonary hemodynamics to asses the severity of pulmonary hypertension in vivo. Given the increasing availability of genetically engineered mice and the need to establish a rodent model of IH-induced pulmonary hypertension, we studied the effect of IH (2-min cycles of 10% and 21% O2, 8 h/day, 4 wk) on wild-type mice, correlating in vivo measurements of pulmonary hypertension with RV mass and pulmonary vascular remodeling. RV systolic pressure was increased after IH (36 +/- 0.9 mmHg) compared with normoxia (29.5 +/- 0.6) but was lower than continuous hypoxia (44.2 +/- 3.4). RV mass [RV-to-(left ventricle plus septum) ratio] correlated with pressure measurements (IH = 0.27 +/- 0.02, normoxia = 0.22 +/- 0.01, and continuous hypoxia = 0.34 +/- 0.01). Hematocrits were also elevated after IH and continuous hypoxia (56 +/- 1.6 and 54 +/- 1.1 vs. 44.3 +/- 0.5%). Evidence of neomuscularization of the distal pulmonary circulation was found after IH and continuous hypoxia. We conclude that mice develop pulmonary hypertension following IH, representing a possible animal model of pulmonary hypertension in response to the repetitive hypoxia-reoxygenation of sleep apnea.

alpha2-Adrenergic-receptor response in reversible increase in hemoglobin concentration in intermittent hypoxia

We have previously shown that intermittent hypoxia (IHx, 10% O(2), 60 min/day) leads to an increase in the splenic alpha2-adrenoceptor response and results in a splenic contraction-induced reversible increase in hemoglobin concentration ([Hb]). In the present study, we determined whether IHx of shorter duration (15 min/day (15-min) and 30 min/day (30-min)), produced this phenomenon in rats. A significant increase in [Hb] during hypoxia was observed in both the groups, but its magnitude was larger in the 30-min IHx rats. Even when the cumulative exposure time (time/dayxdays) was shorter, the [Hb] increase was larger in the rats with longer daily hypoxic exposure. The alpha2-adrenoceptor antagonist yohimbine abolished the [Hb] increase of 15- and 30-min IHx. The increase in [Hb] following administration of the alpha2-adrenoceptor agonist oxymetazoline was also higher in 30-min IHx; indicating that the higher [Hb] produced by longer daily hypoxic exposure times is the result of increases in alpha2-adrenergic-receptor response of greater magnitude. In conclusion, IHx for periods as short as 15 and 30 min/day increases the splenic alpha2-adrenoceptor response and its magnitude reaches the maximum value depending on the daily hypoxic exposure time. A reversible increase in [Hb] constitutes a useful mechanism that protects organ oxygen supply during hypoxic episodes of variable duration and intensity.