Cervical spinal hemisection effects on spinal tissue oxygenation and long-term facilitation of phrenic, renal and splanchnic sympathetic nerve activity

HYPOTHESES

Moderate acute intermittent hypoxia (mAIH) elicits plasticity in both respiratory (phrenic long-term facilitation; pLTF) and sympathetic nerve activity (sympLTF) in rats. Although mAIH produces pLTF in normal rats, inconsistent results are reported after cervical spinal cord injury (cSCI), possibly due to greater spinal tissue hypoxia below the injury site. There are no reports concerning cSCI effects on sympLTF. Since mAIH is being explored as a therapeutic modality to restore respiratory and non-respiratory movements in humans with chronic SCI, both effects are important. To understand cSCI effects on mAIH-induced pLTF and sympLTF, partial or complete C2 spinal hemisections (C2Hx) were performed and, 2 weeks later, we assessed: 1) ipsilateral cervical spinal tissue oxygen tension; 2) ipsilateral & contralateral pLTF; and 3) ipsilateral sympLTF in splanchnic and renal sympathetic nerves.

METHODS

Male Sprague-Dawley rats were studied intact, or after partial (single slice) or complete C2Hx (slice with ∼1 mm aspiration). Two weeks post-C2Hx, rats were anesthetized and prepared for recordings of bilateral phrenic nerve activity and spinal tissue oxygen pressure (PtO2). Splanchnic and renal sympathetic nerve activity was recorded in intact and complete C2Hx rats.

RESULTS

Spinal PtO2 near phrenic motor neurons was decreased after C2Hx, an effect most prominent with complete vs. partial injuries; baseline PtO2 was positively correlated with mean arterial pressure. Complete C2Hx impaired ipsilateral but not contralateral pLTF; with partial C2Hx, ipsilateral pLTF was unaffected. In intact rats, mAIH elicited splanchnic and renal sympLTF. Complete C2Hx had minimal impact on baseline ipsilateral splanchnic or renal sympathetic nerve activity and renal, but not splanchnic, sympLTF remained intact.

CONCLUSION

Greater tissue hypoxia likely impairs pLTF and splanchnic sympLTF post-C2Hx, although renal sympLTF remains intact. Increased sympathetic nerve activity post-mAIH may have therapeutic benefits in individuals living with chronic SCI since anticipated elevations in systemic blood pressure may mitigate hypotension characteristic of people living with SCI.

Magnitude and Mechanism of Phrenic Long-term Facilitation Shift Between Daily Rest Versus Active Phase

Plasticity is a fundamental property of the neural system controlling breathing. One key example of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a persistent increase in phrenic nerve activity elicited by acute intermittent hypoxia (AIH). pLTF can arise from distinct cell signaling cascades initiated by serotonin versus adenosine receptor activation, respectively, and interact via powerful cross-talk inhibition. Here, we demonstrate that the daily rest/active phase and the duration of hypoxic episodes within an AIH protocol have profound impact on the magnitude and mechanism of pLTF due to shifts in serotonin/adenosine balance. Using the historical "standard" AIH protocol (3, 5-min moderate hypoxic episodes), we demonstrate that pLTF magnitude is unaffected by exposure in the midactive versus midrest phase, yet the mechanism driving pLTF shifts from serotonin-dominant (midrest) to adenosine-dominant (midactive). This mechanistic "flip" results from combined influences of hypoxia-evoked adenosine release and daily fluctuations in basal spinal adenosine. Since AIH evokes less adenosine with shorter (15, 1-min) hypoxic episodes, midrest pLTF is amplified due to diminished adenosine constraint on serotonin-driven plasticity; in contrast, elevated background adenosine during the midactive phase suppresses serotonin-dominant pLTF. These findings demonstrate the importance of the serotonin/adenosine balance in regulating the amplitude and mechanism of AIH-induced pLTF. Since AIH is emerging as a promising therapeutic modality to restore respiratory and nonrespiratory movements in people with spinal cord injury or ALS, knowledge of how time-of-day and hypoxic episode duration impact the serotonin/adenosine balance and the magnitude and mechanism of pLTF has profound biological, experimental, and translational implications.

Effect of acute intermittent hypoxia on cortico-diaphragmatic conduction in healthy humans

Acute intermittent hypoxia (AIH) is a strategy to improve motor output in humans with neuromotor impairment. A single AIH session increases the amplitude of motor evoked potentials (MEP) in a finger muscle (first dorsal interosseous), demonstrating enhanced corticospinal neurotransmission. Since AIH elicits phrenic/diaphragm long-term facilitation (LTF) in rodent models, we tested the hypothesis that AIH augments diaphragm MEPs in humans. Eleven healthy adults (7 males, age = 29 ± 6 years) were tested. Transcranial and cervical magnetic stimulation were used to induce diaphragm MEPs and compound muscle action potentials (CMAP) recorded by surface EMG, respectively. Stimulus-response curves were generated prior to and 30-60 min after AIH. Diaphragm LTF was assessed by measurement of integrated EMG burst amplitude and frequency during eupnoeic breathing before and after AIH. Following baseline measurements, AIH was delivered from an oxygen generator connected to a facemask under poikilocapnic conditions (15 one minute episodes of 9% inspired oxygen with one minute room air intervals). There were no detectable changes in MEP (-1.5 ± 12.1%, p = 0.96) or CMAP (+0.1 ± 7.8%, p = 0.97) amplitudes across the stimulus-response curve. At stimulation intensities approximating 50% of the difference between minimum and maximum baseline amplitudes, MEP and CMAP amplitudes were also unchanged (p > 0.05). Further, no AIH effect was observed on diaphragm EMG activity during eupnoea post-AIH (p > 0.05). We conclude that unlike hand muscles, poikilocapnic AIH does not enhance diaphragm MEPs or produce diaphragm LTF in healthy humans.

Cervical spinal injury compromises caudal spinal tissue oxygenation and undermines acute intermittent hypoxia-induced phrenic long-term facilitation

An important model of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a persistent increase in phrenic burst amplitude following acute intermittent hypoxia (AIH). Moderate AIH elicits pLTF by a serotonin-dependent mechanism known as the Q pathway to phrenic motor facilitation. In contrast, severe AIH (greater hypoxemia) increases spinal adenosine accumulation and activates phrenic motor neuron adenosine 2A receptors, thereby initiating a distinct mechanism of plasticity known as the S pathway. Since the Q and S pathways interact via mutual cross-talk inhibition, the balance between spinal serotonin release and adenosine accumulation is an important pLTF regulator. Spinal injury decreases spinal tissue oxygen pressure (PtO₂) caudal to injury. Since AIH is being explored as a neurotherapeutic to restore breathing ability after cervical spinal injury, we tested the hypothesis that decreased PtO₂ in the phrenic motor nucleus after C2 spinal hemisection (C2Hx) undermines moderate AIH-induced pLTF, likely due to shifts in the adenosine/serotonin balance. We recorded C3/4 ventral cervical PtO₂ with an optode, and bilateral phrenic nerve activity in anesthetized, paralyzed and ventilated rats, with and without C2Hx. In intact rats, PtO₂ was lower during severe versus moderate AIH as expected. In chronic C2Hx rats (> 8 weeks post-injury), PtO₂ was lower during baseline and moderate hypoxic episodes, approaching severe AIH levels in intact rats. After C2Hx, pLTF was blunted ipsilateral, but observed contralateral to injury. We conclude that C2Hx compromises PtO₂ near the phrenic motor nucleus and undermines pLTF, presumably due to a shift in the serotonin versus adenosine balance during hypoxic episodes. These findings have important implications for optimizing AIH protocols in our efforts to restore breathing ability with therapeutic AIH in people with chronic cervical spinal injury.

Baseline Arterial CO2 Pressure Regulates Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation in Rats

Moderate acute intermittent hypoxia (mAIH) elicits a progressive increase in phrenic motor output lasting hours post-mAIH, a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH-induced pLTF is initiated by activation of spinally-projecting raphe serotonergic neurons during hypoxia and subsequent serotonin release near phrenic motor neurons. Since raphe serotonergic neurons are also sensitive to pH and CO₂, the prevailing arterial CO₂ pressure (PaCO₂) may modulate their activity (and serotonin release) during hypoxic episodes. Thus, we hypothesized that changes in background PaCO₂ directly influence the magnitude of mAIH-induced pLTF. mAIH-induced pLTF was evaluated in anesthetized, vagotomized, paralyzed and ventilated rats, with end-tidal CO₂ (i.e., a PaCO₂ surrogate) maintained at: (1) ≤39 mmHg (hypocapnia); (2) ∼41 mmHg (normocapnia); or (3) ≥48 mmHg (hypercapnia) throughout experimental protocols. Although baseline phrenic nerve activity tended to be lower in hypocapnia, short-term hypoxic phrenic response, i.e., burst amplitude (Δ = 5.1 ± 1.1 μV) and frequency responses (Δ = 21 ± 4 bpm), was greater than in normocapnic (Δ = 3.6 ± 0.6 μV and 8 ± 4, respectively) or hypercapnic rats (Δ = 2.0 ± 0.6 μV and −2 ± 2, respectively), followed by a progressive increase in phrenic burst amplitude (i.e., pLTF) for at least 60 min post mAIH. pLTF in the hypocapnic group (Δ = 4.9 ± 0.6 μV) was significantly greater than in normocapnic (Δ = 2.8 ± 0.7 μV) or hypercapnic rats (Δ = 1.7 ± 0.4 μV). In contrast, although hypercapnic rats also exhibited significant pLTF, it was attenuated versus hypocapnic rats. When pLTF was expressed as percent change from maximal chemoreflex stimulation, all pairwise comparisons were found to be statistically significant (p < 0.05). We conclude that elevated PaCO₂ undermines mAIH-induced pLTF in anesthetized rats. These findings contrast with well-documented effects of PaCO₂ on ventilatory LTF in awake humans.

Systemic inflammation suppresses spinal respiratory motor plasticity via mechanisms that require serine/threonine protein phosphatase activity

Background

Inflammation undermines multiple forms of neuroplasticity. Although inflammation and its influence on plasticity in multiple neural systems has been extensively studied, its effects on plasticity of neural networks controlling vital life functions, such as breathing, are less understood. In this study, we investigated the signaling mechanisms whereby lipopolysaccharide (LPS)-induced systemic inflammation impairs plasticity within the phrenic motor system—a major spinal respiratory motor pool that drives contractions of the diaphragm muscle. Here, we tested the hypotheses that lipopolysaccharide-induced systemic inflammation (1) blocks phrenic motor plasticity by a mechanism that requires cervical spinal okadaic acid-sensitive serine/threonine protein phosphatase (PP) 1/2A activity and (2) prevents phosphorylation/activation of extracellular signal-regulated kinase 1/2 mitogen activated protein kinase (ERK1/2 MAPK)—a key enzyme necessary for the expression of phrenic motor plasticity.

Methods

To study phrenic motor plasticity, we utilized a well-characterized model for spinal respiratory plasticity called phrenic long-term facilitation (pLTF). pLTF is characterized by a long-lasting, progressive enhancement of inspiratory phrenic nerve motor drive following exposures to moderate acute intermittent hypoxia (mAIH). In anesthetized, vagotomized and mechanically ventilated adult Sprague Dawley rats, we examined the effect of inhibiting cervical spinal serine/threonine PP 1/2A activity on pLTF expression in sham-vehicle and LPS-treated rats. Using immunofluorescence optical density analysis, we compared mAIH-induced phosphorylation/activation of ERK 1/2 MAPK with and without LPS-induced inflammation in identified phrenic motor neurons.

Results

We confirmed that mAIH-induced pLTF is abolished 24 h following low-dose systemic LPS (100 μg/kg, i.p.). Cervical spinal delivery of the PP 1/2A inhibitor, okadaic acid, restored pLTF in LPS-treated rats. LPS also prevented mAIH-induced enhancement in phrenic motor neuron ERK1/2 MAPK phosphorylation. Thus, a likely target for the relevant okadaic acid-sensitive protein phosphatases is ERK1/2 MAPK or its upstream activators.

Conclusions

This study increases our understanding of fundamental mechanisms whereby inflammation disrupts neuroplasticity in a critical population of motor neurons necessary for breathing, and highlights key roles for serine/threonine protein phosphatases and ERK1/2 MAPK kinase in the plasticity of mammalian spinal respiratory motor circuits.

Synergy between Acute Intermittent Hypoxia and Task-Specific Training

Acute intermittent hypoxia (AIH) and task-specific training (TST) synergistically improve motor function after spinal cord injury; however, mechanisms underlying this synergistic relation are unknown. We propose a hypothetical working model of neural network and cellular elements to explain AIH-TST synergy. Our goal is to forecast experiments necessary to advance our understanding and optimize the neurotherapeutic potential of AIH-TST.

Hypoxia-induced hypotension elicits adenosine-dependent phrenic long-term facilitation after carotid denervation

Moderate acute intermittent hypoxia (AIH) elicits a persistent, serotonin-dependent increase in phrenic amplitude, known as phrenic long-term facilitation (pLTF). Although pLTF was originally demonstrated by carotid sinus nerve stimulation, AIH still elicits residual pLTF in carotid denervated (CBX) rats via a distinct, but unknown mechanism. We hypothesized that exaggerated hypoxia-induced hypotension after carotid denervation leads to greater spinal tissue hypoxia and extracellular adenosine accumulation, thereby triggering adenosine 2A receptor (A_2A)-dependent pLTF. Phrenic activity, arterial pressure and spinal tissue oxygen pressure were measured in anesthetized CBX rats. Exaggerated hypoxia-induced hypotension after CBX was prevented via intravenous phenylephrine; without the hypotension, spinal tissue hypoxia during AIH was normalized, and residual pLTF was no longer observed. Spinal A_2A (MSX-3), but not serotonin 2 receptor (5-HT₂) inhibition (ketanserin), abolished residual pLTF in CBX rats. Thus, pLTF regulation may be altered in conditions impairing sympathetic activity and arterial pressure regulation, such as spinal cord injury.

Circulatory control of phrenic motor plasticity

Acute intermittent hypoxia (AIH) elicits distinct mechanisms of phrenic motor plasticity initiated by brainstem neural network activation versus local (spinal) tissue hypoxia. With moderate AIH (mAIH), hypoxemia activates the carotid body chemoreceptors and (subsequently) brainstem neural networks associated with the peripheral chemoreflex, including medullary raphe serotonergic neurons. Serotonin release and receptor activation in the phrenic motor nucleus then elicits phrenic long-term facilitation (pLTF). This mechanism is independent of tissue hypoxia, since electrical carotid sinus nerve stimulation elicits similar serotonin-dependent pLTF. In striking contrast, severe AIH (sAIH) evokes a spinal adenosine-dependent, serotonin-independent mechanism of pLTF. Spinal tissue hypoxia per se is the likely cause of sAIH-induced pLTF, since local tissue hypoxia elicits extracellular adenosine accumulation. Thus, any physiological condition exacerbating spinal tissue hypoxia is expected to shift the balance towards adenosinergic pLTF. However, since these mechanisms compete for dominance due to mutual cross-talk inhibition, the transition from serotonin to adenosine dominant pLTF is rather abrupt. Any factor that compromises spinal cord circulation will limit oxygen availability in spinal cord tissue, favoring a shift in the balance towards adenosinergic mechanisms. Such shifts may arise experimentally from treatments such as carotid denervation, or spontaneous hypotension or anemia. Many neurological disorders, such as spinal cord injury or stroke compromise local circulatory control, potentially modulating tissue oxygen, adenosine levels and, thus, phrenic motor plasticity. In this brief review, we discuss the concept that local (spinal) circulatory control and/or oxygen delivery regulates the relative contributions of distinct pathways to phrenic motor plasticity.

Protein kinase Cδ constrains the S-pathway to phrenic motor facilitation elicited by spinal 5-HT7 receptors or severe acute intermittent hypoxia

KEY POINTS

Concurrent 5-HT_2A (Q pathway) and 5-HT₇ (S pathway) serotonin receptor activation cancels phrenic motor facilitation due to mutual cross-talk inhibition. Spinal protein kinase Cδ (PKCδ) or protein kinase A inhibition restores phrenic motor facilitation with concurrent Q and S pathway activation, demonstrating a key role for these kinases in cross-talk inhibition. Spinal PKCδ inhibition enhances adenosine-dependent severe acute intermittent hypoxia-induced phrenic long-term facilitation (S pathway), consistent with relief of cross-talk inhibition.

ABSTRACT

Intermittent spinal serotonin receptor activation elicits long-lasting phrenic motor facilitation (pMF), a form of respiratory motor plasticity. When activated alone, spinal Gq protein-coupled serotonin 2A receptors (5-HT_2A ) initiate pMF by a mechanism that requires ERK-MAP kinase signalling and new BDNF protein synthesis (Q pathway). Spinal Gs protein-coupled serotonin 7 (5-HT₇ ) and adenosine 2A (A_2A ) receptor activation also elicits pMF, but via distinct mechanisms (S pathway) that require Akt signalling and new TrkB protein synthesis. Although studies have shown inhibitory cross-talk interactions between these competing pathways, the underlying cellular mechanisms are unknown. We propose the following hypotheses: (1) concurrent 5-HT_2A and 5-HT₇ activation undermines pMF; (2) protein kinase A (PKA) and (3) NADPH oxidase mediate inhibitory interactions between Q (5-HT_2A ) and S (5-HT₇ ) pathways. Selective 5-HT_2A (DOI hydrochloride) and 5HT₇ (AS-19) agonists were administered intrathecally at C4 (three injections, 5-min intervals) in anaesthetized, vagotomized and ventilated male rats. With either spinal 5-HT_2A or 5-HT₇ activation alone, phrenic amplitude progressively increased (pMF). In contrast, concurrent 5-HT_2A and 5-HT₇ activation failed to elicit pMF. The 5-HT_2A -induced Q pathway was restored by inhibiting PKA activity (Rp-8-Br-cAMPS). NADPH oxidase inhibition did not prevent cross-talk inhibition. Therefore, we investigated alternative mechanisms to explain Q to S pathway inhibition. Spinal protein kinase C (PKC) inhibition with Gö6983 or PKCδ peptide inhibitor restored the 5-HT₇ -induced S pathway to pMF, revealing PKCδ as the relevant isoform. Spinal PKCδ inhibition enhanced the S pathway-dependent form of pMF elicited by severe acute intermittent hypoxia. We suggest that powerful constraints between 5-HT_2A and 5-HT₇ or A_2A receptor-induced pMF are mediated by PKCδ and PKA, respectively.

Phrenic motor neuron adenosine 2A receptors elicit phrenic motor facilitation

KEY POINTS

Although adenosine 2A (A2A ) receptor activation triggers specific cell signalling cascades, the ensuing physiological outcomes depend on the specific cell type expressing these receptors. Cervical spinal adenosine 2A (A2A ) receptor activation elicits a prolonged facilitation in phrenic nerve activity, which was nearly abolished following intrapleural A2A receptor siRNA injections. A2A receptor siRNA injections selectively knocked down A2A receptors in cholera toxin B-subunit-identified phrenic motor neurons, sparing cervical non-phrenic motor neurons. Collectively, our results support the hypothesis that phrenic motor neurons express the A2A receptors relevant to A2A receptor-induced phrenic motor facilitation. Upregulation of A2A receptor expression in the phrenic motor neurons per se may potentially be a useful approach to increase phrenic motor neuron excitability in conditions such as spinal cord injury.

ABSTRACT

Cervical spinal adenosine 2A (A2A ) receptor activation elicits a prolonged increase in phrenic nerve activity, an effect known as phrenic motor facilitation (pMF). The specific cervical spinal cells expressing the relevant A2A receptors for pMF are unknown. This is an important question since the physiological outcome of A2A receptor activation is highly cell type specific. Thus, we tested the hypothesis that the relevant A2A receptors for pMF are expressed in phrenic motor neurons per se versus non-phrenic neurons of the cervical spinal cord. A2A receptor immunostaining significantly colocalized with NeuN-positive neurons (89 ± 2%). Intrapleural siRNA injections were used to selectively knock down A2A receptors in cholera toxin B-subunit-labelled phrenic motor neurons. A2A receptor knock-down was verified by a ∼45% decrease in A2A receptor immunoreactivity within phrenic motor neurons versus non-targeting siRNAs (siNT; P < 0.05). There was no evidence for knock-down in cervical non-phrenic motor neurons. In rats that were anaesthetized, subjected to neuromuscular blockade and ventilated, pMF induced by cervical (C3-4) intrathecal injections of the A2A receptor agonist CGS21680 was greatly attenuated in siA2A (21%) versus siNT treated rats (147%; P < 0.01). There were no significant effects of siA2A on phrenic burst frequency. Collectively, our results support the hypothesis that phrenic motor neurons express the A2A receptors relevant to A2A receptor-induced pMF.

Cross-talk inhibition between 5-HT2B and 5-HT7 receptors in phrenic motor facilitation via NADPH oxidase and PKA

Intermittent spinal serotonin receptor activation elicits phrenic motor facilitation (pMF), a form of spinal respiratory motor plasticity. Episodic activation of either serotonin type 2 (5-HT₂) or type 7 (5-HT₇) receptors elicits pMF, although they do so via distinct cellular mechanisms known as the Q (5-HT₂) and S (5-HT₇) pathways to pMF. When coactivated, these pathways interact via mutual cross-talk inhibition. Although we have a rudimentary understanding of mechanisms mediating cross-talk interactions between spinal 5-HT₂ subtype A (5-HT_2A) and 5-HT₇ receptor activation, we do not know if similar interactions exist between 5-HT₂ subtype B (5-HT_2B) and 5-HT₇ receptors. We confirmed that either spinal 5-HT_2B or 5-HT₇ receptor activation alone elicits pMF and tested the hypotheses that 1) concurrent activation of both receptors suppresses pMF due to cross-talk inhibition; 2) 5-HT₇ receptor inhibition of 5-HT_2B receptor-induced pMF requires protein kinase A (PKA) activity; and 3) 5-HT_2B receptor inhibition of 5-HT₇ receptor-induced pMF requires NADPH oxidase (NOX) activity. Selective 5-HT_2B and 5-HT₇ receptor agonists were administered intrathecally at C4 (3 injections, 5-min intervals) to anesthetized, paralyzed, and ventilated rats. Whereas integrated phrenic nerve burst amplitude increased after selective spinal 5-HT_2B or 5-HT₇ receptor activation alone (i.e., pMF), pMF was no longer observed with concurrent 5-HT_2B and 5-HT₇ receptor agonist administration. With concurrent receptor activation, pMF was rescued by inhibiting either NOX or PKA activity, demonstrating their roles in cross-talk inhibition between these pathways to pMF. This report demonstrates cross-talk inhibition between 5-HT_2B- and 5-HT₇ receptor-induced pMF and that NOX and PKA activity are necessary for that cross-talk inhibition.

Cardiovascular and respiratory outcome of preconditioned rats submitted to chronic intermittent hypoxia

NEW FINDINGS

What is the central question of this study? What are the effects of hypoxic preconditioning upon the cardiovascular and respiratory responses to subsequent episodes of chronic intermittent hypoxia? What is the main finding and its importance? The cardiovascular and respiratory responses to a chronic intermittent hypoxia protocol were not altered by previous exposure to intermittent or sustained hypoxia. These findings show that preconditioning to hypoxia produced neither facilitation nor protection from the cardiovascular and respiratory dysfunctions in response to subsequent episodes of chronic intermittent hypoxia in juvenile rats. Rats exposed to chronic intermittent hypoxia (CIH) develop hypertension, which is associated with changes in the coupling of sympathetic and respiratory activities. In this study, we hypothesized that previous preconditioning to intermittent or sustained hypoxia would affect cardiovascular and respiratory changes produced by subsequent protocols of CIH. To test this hypothesis, male Wistar rats were preconditioned to either 10 days of CIH or 24 h of sustained hypoxia (SH). After the initial exposure to hypoxia, rats were maintained in normoxic conditions for 15 days before a new protocol of CIH during 10 days. Cardiovascular and respiratory variables obtained from groups of preconditioned rats were compared with a group of rats exposed to CIH for the first time and also to a group of rats maintained in normoxic conditions throughout the period of time of the respective preconditioning protocol. The data show that CIH produced a similar increase in arterial pressure and heart rate in both CIH and SH preconditioning protocols. Respiratory parameters during basal conditions were also not affected by preconditioning to either CIH or SH. We conclude that previous exposure to CIH or SH preconditioning does not facilitate or prevent the cardiovascular changes produced by CIH.

Stability of relative oxygen pulse curve during repeated maximal cardiopulmonary testing in professional soccer players

During cardiopulmonary exercise testing (CPET), stroke volume can be indirectly assessed by O(2) pulse profile. However, for a valid interpretation, the stability of this variable over time should be known. The objective was to analyze the stability of the O(2) pulse curve relative to body mass in elite athletes. VO(2), heart rate (HR), and relative O(2) pulse were compared at every 10% of the running time in two maximal CPETs, from 2005 to 2010, of 49 soccer players. Maximal values of VO(2) (63.4 ± 0.9 vs 63.5 ± 0.9 mL O(2)•kg(-1)•min(-1)), HR (190 ± 1 vs188 ± 1 bpm) and relative O(2) pulse (32.9 ± 0.6 vs 32.6 ± 0.6 mL O(2)•beat(-1)•kg(-1)) were similar for the two CPETs (P > 0.05), while the final treadmill velocity increased from 18.5 ± 0.9 to 18.9 ± 1.0 km/h (P < 0.01). Relative O(2) pulse increased linearly and similarly in both evaluations (r(2) = 0.64 and 0.63) up to 90% of the running time. Between 90 and 100% of the running time, the values were less stable, with up to 50% of the players showing a tendency to a plateau in the relative O(2) pulse. In young healthy men in good to excellent aerobic condition, the morphology of the relative O(2) pulse curve is consistent up to close to the peak effort for a CPET repeated within a 1-year period. No increase in relative O(2)pulse at peak effort could represent a physiologic stroke volume limitation in these athletes.