Cardiorespiratory kinetics in exercise physiology: estimates and predictions using randomized changes in work rate.

PURPOSE: Kinetics of cardiorespiratory parameters (CRP) in response to work rate (WR) changes are evaluated by pseudo-random binary sequences (PRBS testing). In this study, two algorithms were applied to convert responses from PRBS testing into appropriate impulse responses to predict steady states values and responses to incremental increases in exercise intensity. METHODS: 13 individuals (age: 41 ± 9 years, BMI: 23.8 ± 3.7 kg m-2), completing an exercise test protocol, comprising a section of randomized changes of 30 W and 80 W (PRBS), two phases of constant WR at 30 W and 80 W and incremental WR until subjective fatigue, were included in the analysis. Ventilation ([Formula: see text]), O2 uptake ([Formula: see text]), CO2 output ([Formula: see text]) and heart rate (HR) were monitored. Impulse responses were calculated in the time domain and in the frequency domain from the cross-correlations of WR and the respective CRP. RESULTS: The algorithm in the time domain allows better prediction for [Formula: see text] and [Formula: see text], whereas for [Formula: see text] and HR the results were similar for both algorithms. Best predictions were found for [Formula: see text] and HR with higher (3-4%) 30 W steady states and lower (1-4%) values for 80 W. Tendencies were found in the residuals between predicted and measured data. CONCLUSION: The CRP kinetics, resulting from PRBS testing, are qualified to assess steady states within the applied WR range. Below the ventilatory threshold, [Formula: see text] and HR responses to incrementally increasing exercise intensities can be sufficiently predicted.

The GTPase Arl8B Plays a Principle Role in the Positioning of Interstitial Axon Branches by Spatially Controlling Autophagosome and Lysosome Location.

Interstitial axon branching is an essential step during the establishment of neuronal connectivity. However, the exact mechanisms on how the number and position of branches are determined are still not fully understood. Here, we investigated the role of Arl8B, an adaptor molecule between lysosomes and kinesins. In chick retinal ganglion cells (RGCs), downregulation of Arl8B reduces axon branch density and shifts their location more proximally, while Arl8B overexpression leads to increased density and more distal positions of branches. These alterations correlate with changes in the location and density of lysosomes and autophagosomes along the axon shaft. Diminishing autophagy directly by knock-down of atg7, a key autophagy gene, reduces branch density, while induction of autophagy by rapamycin increases axon branching, indicating that autophagy plays a prominent role in axon branch formation. In vivo, local inactivation of autophagy in the retina using a mouse conditional knock-out approach disturbs retino-collicular map formation which is dependent on the formation of interstitial axon branches. These data suggest that Arl8B plays a principal role in the positioning of axon branches by spatially controlling autophagy, thus directly controlling formation of neural connectivity in the brain.SIGNIFICANCE STATEMENT The formation of interstitial axonal branches plays a prominent role in numerous places of the developing brain during neural circuit establishment. We show here that the GTPase Arl8B controls density and location of interstitial axon branches, and at the same time controls also density and location of the autophagy machinery. Upregulation or downregulation of autophagy in vitro promotes or inhibits axon branching. Local disruption of autophagy in vivo disturbs retino-collicular mapping. Our data suggest that Arl8B controls axon branching by controlling locally autophagy. This work is one of the first reports showing a role of autophagy during early neural circuit development and suggests that autophagy in general plays a much more prominent role during brain development than previously anticipated.

Confinement, partial sleep deprivation and defined physical activity-influence on cardiorespiratory regulation and capacity.

INTRODUCTION: Adequate cardiorespiratory fitness is of utmost importance during spaceflight and should be assessable via moderate work rate intensities, e.g., using kinetics parameters. The combination of restricted sleep, and defined physical exercise during a 45-day simulated space mission is expected to slow heart rate (HR) kinetics without changes in oxygen uptake ([Formula: see text]) kinetics. METHODS: Overall, 14 crew members (9 males, 5 females, 37 ± 7 yrs, 23.4 ± 3.5 kg m-2) simulated a 45-d-mission to an asteroid. During the mission, the sleep schedule included 5 nights of 5 h and 2 nights of 8 h sleep. The crew members were tested on a cycle ergometer, using pseudo-random binary sequences, changing between 30 and 80 W on day 8 before (MD-8), day 22 (MD22) and 42 (MD42) after the beginning and day 4 (MD + 4) following the end of the mission. Kinetics information was assessed using the maxima of cross-correlation functions (CCFmax). Higher CCFmax indicates faster responses. RESULTS: CCFmax(HR) was significantly (p = 0.008) slower at MD-8 (0.30 ± 0.06) compared with MD22 (0.36 ± 0.06), MD42 (0.38 ± 0.06) and MD + 4 (0.35 ± 0.06). Mean HR values during the different work rate steps were higher at MD-8 and MD + 4 compared to MD22 and MD42 (p < 0.001). DISCUSSION: The physical training during the mission accelerated HR kinetics, but had no impact on mean HR values post mission. Thus, HR kinetics seem to be sensitive to changes in cardiorespiratory fitness and may be a valuable parameter to monitor fitness. Kinetics and capacities adapt independently in response to confinement in combination with defined physical activity and sleep.

Cardiorespiratory kinetics: comparisons between athletes with different training habits.

PURPOSE: Fast muscular oxygen uptake ([Formula: see text]) kinetics are limiting factors for high exercise capacities. It is hypothesized that [Formula: see text] and heart rate (HR) kinetics would be faster in individuals, performing long-distance endurance training (CONT) compared with athletes performing predominantly interval-based sports (INT). METHODS: 17 subjects (INT: n = 7, 24 ± 5 years, 183 ± 7 cm, 85 ± 10 kg, 6 ± 3 h of training per week, CONT: n = 10, 37 ± 7 years, 175 ± 9 cm, 69 ± 10 kg, 6 ± 3 h of training per week) completed a treadmill work rate (WR) protocol with pseudo-randomized WR changes with velocities of 6.5 and 9.5 km h-1. [Formula: see text]O2musc and the respective kinetics were estimated from the measured pulmonary oxygen uptake and HR combined with a circulatory model. Kinetics information were calculated using time series analysis. Higher maxima of the cross-correlation function (CCF) of WR and the respective parameter ([Formula: see text], HR) indicate faster kinetics responses. RESULTS: The kinetics of HR (INT: 0.23 ± 0.04 vs. CONT: 0.42 ± 0.18; P = 0.001), [Formula: see text]O2pulm (0.30 ± 0.05 vs. 0.53 ± 0.20; P = 0.005) and [Formula: see text]O2musc (0.31 ± 0.06 vs. 0.53 ± 0.16; P = 0.005) were significantly slower in INT compared with the CONT athletes. CONCLUSIONS: It seems that at least in the long-term CONT exercise, training without the need of changing intensities is favorable for fast [Formula: see text]O2 and HR kinetics compared with INT exercise including frequently changing intensities.

Effect of acute ambient temperature exposure on cardio-pulmonary and respiratory kinetics in men.

AIM: The goal of the study was to compare the kinetic responses of heart rate (HR) and pulmonary (V̇O2pulm) and muscular (V̇O2musc) oxygen uptake during dynamic leg exercise across different acute ambient temperature conditions in a climatic chamber. METHODS: Thirteen physically healthy, active, male volunteers demonstrated pseudorandom binary sequence (PRBS) work rate (WR) changes between 30 and 80 W at 15 °C, 25 °C and 35 °C, respectively. HR was measured beat-to-beat using an echocardiogram and V̇O2pulm by breath-by-breath gas exchange; V̇O2musc estimations were assessed by applying a circulatory model and cross-correlation functions. RESULTS: No significant differences were observed across the various temperature conditions in each case for HR, V̇O2pulm or V̇O2musc kinetics (p > 0.05). Baroreflex regulation based on HR kinetics does not seem to be influenced between ambient temperatures of 15 °C and 35 °C during dynamic exercise. CONCLUSIONS: The results imply that ambient temperatures of 15 °C, 25 °C and 35 °C have no effect on HR, V̇O2pulm or V̇O2musc kinetics during dynamic moderate exercise. The applied approach may be of interest for assessments of the cardio-pulmonary and respiratory health statuses of individuals working or performing sports in extreme temperature environments. Furthermore, differentiation between systemic (e.g. cardio-dynamic: HR) and specific (e.g. exercising tissues: V̇O2musc) determinants of the relevant physiological systems may improve the evaluation of an individual's health status.

Non-invasive estimation of muscle oxygen uptake kinetics with pseudorandom binary sequence and step exercise responses.

PURPOSE: The aim of the study was to test for significant differences in non-invasively estimated muscle oxygen uptake ([Formula: see text]) kinetics, assessed by a square-wave exercise protocol (STEP) as well as by a time series approach with pseudorandom binary sequence (PRBS) work rate (WR) changes. METHODS: Seventeen healthy and active individuals (10 women, 7 men; 23 ± 2 years old; height 175 ± 11 cm; body mass 73 ± 14 kg [mean ± SD]) completed five repetitions of WR transitions from 30 to 80 W for the STEP approach and two sequences of pseudorandom binary WR changes between 30 and 80 W for the PRBS approach. Pulmonary oxygen uptake ([Formula: see text]) was measured breath by breath. [Formula: see text] kinetics were estimated during phase II [Formula: see text] in the STEP approach and during the pseudorandom binary sequence WR changes in the PRBS approach. RESULTS: No significant differences were observed between different models of the STEP and the PRBS approach for estimation of [Formula: see text] kinetics (p > 0.05). In addition, a very high variability between the models was determined for [Formula: see text] kinetics [mean time constants (τ) difference: - 2.5 ± 11.4 s]. A significant correlation for τ of [Formula: see text] between the STEP approach with experimentally determined phase I [Formula: see text] lengths and the PRBS approach was noticed (r = 0.536; p < 0.05). CONCLUSIONS: Both approaches (STEP and PRBS) are not significantly different for estimating the [Formula: see text] kinetics, but the very high variability impairs the predictability between the models. However, the determination of the length of phase I [Formula: see text] should be as appropriate as possible because predefined duration lengths can result in overestimations in [Formula: see text] kinetics.

BDNF promotes axon branching of retinal ganglion cells via miRNA-132 and p250GAP.

A crucial step in the development of the vertebrate visual system is the branching of retinal ganglion cell (RGC) axons within their target, the superior colliculus/tectum. A major player in this process is the neurotrophin brain-derived neurotrophic factor (BDNF). However, the molecular basis for the signaling pathways mediating BDNF action is less well understood. As BDNF exerts some of its functions by controlling the expression of microRNAs (miRNAs), we investigated whether miRNAs are also involved in BDNF-mediated retinal axon branching. Here, we demonstrate that the expression pattern of miRNA-132 in the retina is consistent with its involvement in this process, and that BDNF induces the upregulation of miRNA-132 in retinal cultures. Furthermore, in vitro gain-of-function and loss-of-function approaches in retinal cultures reveal that miRNA-132 mediates axon branching downstream of BDNF. A known target of miRNA-132 is the Rho family GTPase-activating protein, p250GAP. We find that p250GAP is expressed in RGC axons and mediates the effects of miRNA-132 in BDNF-induced branching. BDNF treatment or overexpression of miRNA-132 leads to a reduction in p250GAP protein levels in retinal cultures, whereas the overexpression of p250GAP abolishes BDNF-induced branching. Finally, we used a loss-of-function approach to show that miRNA-132 affects the maturation of RGC termination zones in the mouse superior colliculus in vivo, while their topographic targeting remains intact. Together, our data indicate that BDNF promotes RGC axon branching during retinocollicular/tectal map formation via upregulation of miRNA-132, which in turn downregulates p250GAP.

Protein tyrosine phosphatase receptor type O inhibits trigeminal axon growth and branching by repressing TrkB and Ret signaling.

Axonal branches of the trigeminal ganglion (TG) display characteristic growth and arborization patterns during development. Subsets of TG neurons express different receptors for growth factors, but these are unlikely to explain the unique patterns of axonal arborizations. Intrinsic modulators may restrict or enhance cellular responses to specific ligands and thereby contribute to the development of axon growth patterns. Protein tyrosine phosphatase receptor type O (PTPRO), which is required for Eph receptor-dependent retinotectal development in chick and for development of subsets of trunk sensory neurons in mouse, may be such an intrinsic modulator of TG neuron development. PTPRO is expressed mainly in TrkB-expressing (TrkB(+)) and Ret(+) mechanoreceptors within the TG during embryogenesis. In PTPRO mutant mice, subsets of TG neurons grow longer and more elaborate axonal branches. Cultured PTPRO(-/-) TG neurons display enhanced axonal outgrowth and branching in response to BDNF and GDNF compared with control neurons, indicating that PTPRO negatively controls the activity of BDNF/TrkB and GDNF/Ret signaling. Mouse PTPRO fails to regulate Eph signaling in retinocollicular development and in hindlimb motor axon guidance, suggesting that chick and mouse PTPRO have different substrate specificities. PTPRO has evolved to fine tune growth factor signaling in a cell-type-specific manner and to thereby increase the diversity of signaling output of a limited number of receptor tyrosine kinases to control the branch morphology of developing sensory neurons. The regulation of Eph receptor-mediated developmental processes by protein tyrosine phosphatases has diverged between chick and mouse.

Axonal ephrinA/EphA interactions, and the emergence of order in topographic projections.

In the classical view of axon guidance, neurons send out axons which are endowed with guidance receptors enabling them to find their (distant) target areas by an interaction with their ligands expressed in specific spatio-temporal patterns along their pathways and in their target area. However, this view has recently been confounded by more detailed analyses of, for example, the expression patterns of EphAs and ephrinAs in the retinotectal projection. Here ephrinA 'ligands' are expressed not only in the target area but also on the projecting RGC axons, and EphA 'receptors' not only on retinal ganglion cell (RGC) axons but also in the target area itself. This review describes the on-going functional characterisation of the surprising co-expression of ephrinAs and EphAs on retinal ganglion cell (RGC) axons and other cell types. It also investigates the function of ephrinAs as receptors and describes their interaction with co-receptors involved in mediating this function.

Alternative splicing controls teneurin-3 compact dimer formation for neuronal recognition.

Neuronal network formation is facilitated by recognition between synaptic cell adhesion molecules at the cell surface. Alternative splicing of cell adhesion molecules provides additional specificity in forming neuronal connections. For the teneurin family of cell adhesion molecules, alternative splicing of the EGF-repeats and NHL domain controls synaptic protein-protein interactions. Here we present cryo-EM structures of the compact dimeric ectodomain of two teneurin-3 isoforms that harbour the splice insert in the EGF-repeats. This dimer is stabilised by an EGF8-ABD contact between subunits. Cryo-EM reconstructions of all four splice variants, together with SAXS and negative stain EM, reveal compacted dimers for each, with variant-specific dimeric arrangements. This results in specific trans-cellular interactions, as tested in cell clustering and stripe assays. The compact conformations provide a structural basis for teneurin homo- and heterophilic interactions. Altogether, our findings demonstrate how alternative splicing results in rearrangements of the dimeric subunits, influencing neuronal recognition and likely circuit wiring.

Sexual dimorphism in the social behaviour of Cntnap2-null mice correlates with disrupted synaptic connectivity and increased microglial activity in the anterior cingulate cortex.

A biological understanding of the apparent sex bias in autism is lacking. Here we have identified Cntnap2 KO mice as a model system to help better understand this dimorphism. Using this model, we observed social deficits in juvenile male KO mice only. These male-specific social deficits correlated with reduced spine densities of Layer 2/3 and Layer 5 pyramidal neurons in the Anterior Cingulate Cortex, a forebrain region prominently associated with the control of social behaviour. Furthermore, in male KO mice, microglia showed an increased activated morphology and phagocytosis of synaptic structures compared to WT mice, whereas no differences were seen in female KO and WT mice. Our data suggest that sexually dimorphic microglial activity may be involved in the aetiology of ASD, disrupting the development of neural circuits that control social behaviour by overpruning synapses at a developmentally critical period.

EphrinA6 on chick retinal axons is a key component for p75(NTR)-dependent axon repulsion and TrkB-dependent axon branching.

A characteristic of the ephrin/Eph family is their capacity for bi-directional signalling. This means that an ephrin, for example, can function either as a ligand for an Eph 'receptor', or as a receptor for an Eph 'ligand'. A system in which this phenomenon is well studied is the retinotectal projection in which the guidance of retinal ganglion cell (RGC) axons to their target area in the tectum is controlled by both Ephs and ephrins expressed in gradients in both the retina and tectum. Here we have analysed the receptor function of ephrinAs on RGC axons in further detail by focussing on ephrinA6, which is the most strongly expressed ephrinA in the chick retina. EphrinAs are GPI-anchored proteins and therefore require the interaction with transmembrane proteins to exert this receptor function. Previous work has shown that ephrinAs interact on RGC axons in cis with the neurotrophin receptors p75(NTR) and TrkB. P75(NTR) then was shown to be necessary for the repulsion of ephrinA-expressing RGC axons from an EphA substrate and for the downregulation of axon branching. In turn, an interaction of ephrinAs with TrkB as well as an increase in axonal ephrinA expression augments the axon branch-promoting activity of TrkB. We now show that ephrinA6 is the necessary ephrinA component of the repulsive ephrinA/p75(NTR) receptor complex on chick RGC axons as axons lacking ephrinA6 no longer avoid an EphA matrix in stripe assay experiments. We also demonstrate that the branch-promoting activity of TrkB is dependent on ephrinA6 as a knockdown of ephrinA6 renders RGC axons insensitive to BDNF, the high affinity ligand for TrkB. In sum our data further strengthen the hypothesis that a fine-tuned interplay of ephrinAs with p75(NTR) and TrkB is important for the guidance and branching of RGC axons.

Silencing of EphA3 through a cis interaction with ephrinA5.

EphAs and ephrinAs are expressed in multiple areas of the developing brain in overlapping countergradients, notably in the retina and tectum. Here they are involved in targeting retinal axons to their correct topographic position in the tectum. We have used truncated versions of EphA3, single-amino acid point mutants of ephrinA5 and fluorescence resonance energy transfer technology to uncover a cis interaction between EphA3 and ephrinA5 that is independent of the established ligand-binding domain of EphA3. This cis interaction abolishes the induction of tyrosine phosphorylation of EphA3 and results in a loss of sensitivity of retinal axons to ephrinAs in trans. Our data suggest that formation of this complex transforms the uniform expression of EphAs in the nasal part of the retina into a gradient of functional EphAs and has a key role in controlling retinotectal mapping.

Opposing gradients of ephrin-As and EphA7 in the superior colliculus are essential for topographic mapping in the mammalian visual system.

During development of the retinocollicular projection in mouse, retinal axons initially overshoot their future termination zones (TZs) in the superior colliculus (SC). The formation of TZs is initiated by interstitial branching at topographically appropriate positions. Ephrin-As are expressed in a decreasing posterior-to-anterior gradient in the SC, and they suppress branching posterior to future TZs. Here we investigate the role of an EphA7 gradient in the SC, which has the reverse orientation to the ephrin-A gradient. We find that in EphA7 mutant mice the retinocollicular map is disrupted, with nasal and temporal axons forming additional or extended TZs, respectively. In vitro, retinal axons are repelled from growing on EphA7-containing stripes. Our data support the idea that EphA7 is involved in suppressing branching anterior to future TZs. These findings suggest that opposing ephrin-A and EphA gradients are required for the proper development of the retinocollicular projection.

A TrkB/EphrinA interaction controls retinal axon branching and synaptogenesis.

Toward understanding topographically specific branching of retinal axons in their target area, we have studied the interaction between neurotrophin receptors and members of the Eph family. TrkB and its ligand BDNF are uniformly expressed in the retina and tectum, respectively, and exert a branch-promoting activity, whereas EphAs and ephrinAs are expressed in gradients in retina and tectum and can mediate a suppression of axonal branching. We have identified a novel cis interaction between ephrinA5 and TrkB on retinal ganglion cell axons. TrkB interacts with ephrinA5 via its second cysteine-rich domain (CC2), which is necessary and sufficient for binding to ephrinA5. Their functional interaction is twofold: ephrinA5 augments BDNF-promoted retinal axon branching in the absence of its activator EphA7-Fc, whereas EphA7-Fc application abolishes branching in a local and concentration-dependent manner. The importance of TrkB in this process is shown by the fact that overexpression of an isolated TrkB-CC2 domain interfering with the ephrinA/TrkB interaction abolishes this regulatory interplay, whereas knockdown of TrkB via RNA interference diminishes the ephrinA5-evoked increase in branching. The ephrinA/Trk interaction is neurotrophin induced and specifically augments the PI-3 kinase/Akt pathway generally known to be involved in the promotion of branching. In addition, ephrinAs/TrkB modulate axon branching and also synapse formation of hippocampal neurons. Our findings uncover molecular mechanisms of how spatially restricted axon branching can be achieved by linking globally expressed branch-promoting with differentially expressed branch-suppressing activities. In addition, our data suggest that growth factors and the EphA-ephrinA system interact in a way that affects axon branching and synapse development.

Robos are required for the correct targeting of retinal ganglion cell axons in the visual pathway of the brain.

Axonal projections from the retina to the brain are regulated by molecules including the Slit family of ligands [Thompson, H., Barker, D., Camand, O., Erskine, L., 2006a. Slits contribute to the guidance of retinal ganglion cell axons in the mammalian optic tract. Dev. Biol. 296, 476-484, Thompson, H., Camand, O., Barker, D., Erskine, L., 2006b. Slit proteins regulate distinct aspects of retinal ganglion cell axon guidance within dorsal and ventral retina. J. Neurosci. 26, 8082-8091]. However, the roles of Slit receptors in mammals, (termed Robos), have not been investigated in visual system development. Here we examined Robo1 and 2 mutant mice and found that Robos regulate the correct targeting of retinal ganglion cell (RGC) axons along the entire visual projection. We noted aberrant projections of RGC axons into the cerebral cortex, an area not normally targeted by RGC axons. The optic chiasm was expanded along the rostro-caudal axis (similar to Slit mutant mice, Plump, A.S., Erskine, L., Sabatier, C., Brose, K., Epstein, C.J., Goodman, C.S., Mason, C.A., Tessier-Lavigne, M., 2002. Slit1 and Slit2 cooperate to prevent premature midline crossing of retinal axons in the mouse visual system. Neuron 33, 219-232), with ectopic crossing points, and some axons projecting caudally toward the corticospinal tract. Further, we found that axons exuberantly projected into the diencephalon. These defects were more pronounced in Robo2 than Robo1 knockout animals, implicating Robo2 as the predominant Robo receptor in visual system development.

Adaptation of Systemic and Pulmonary Circulation to Acute Changes in Gravity and Body Position.

INTRODUCTION: Changes in gravity or body position provoke changes in hydrostatic pressure in the arterial system and in venous return. Potential asymmetries between left (QLV) and right ventricular (QRV) cardiac output during transient gravity changes were investigated. It was hypothesized that blood volume is temporarily stored in the pulmonary vessels, with amount and duration depending on the level and directions of gravity.METHODS: Eight healthy, male subjects (32 ± 3 yr, 182 ± 7 cm, 82 ± 6 kg) were tested on a tilt seat (TS), in a long arm human centrifuge (laHC), and during parabolic flights (PF). The gravitational changes during PF were reconstructed by changing gravity in a laHC and different body positions on a TS. All participants were tested in the seated, resting position. Heart rate and blood pressure were recorded continuously and QLV was calculated, applying the Modelflow Algorithm. Gas exchange was measured breath-by-breath. QRV was calculated from these data according to the Fick Principle. Four sequences were superimposed and analyzed by ANOVA with the factors Time, Ventricle (QRV, QLV), and Mode (TS, PF, laHC).RESULTS: After reductions in gravity QRV and QLV were transiently desynchronized. ANOVA showed no main effect for Mode, but significant changes were found for Time and Ventricle and all interactions.DISCUSSION: Phases of reduced gravity seem to lead to transiently increased storage of blood volume inside the pulmonary vascular system. A more detailed understanding of these mechanisms might help to describe the compliance of the pulmonary vascular system in diseases of the pulmonary circulation.Hoffmann U, Koschate J, Appell Coriolano H-J, Drescher U, Thieschäfer L, Dumitrescu D, Werner A. Adaptation of systemic and pulmonary circulation to acute changes in gravity and body position. Aerosp Med Hum Perform. 2019; 90(8):688-695.

Eph family functions from an evolutionary perspective.

Eph receptor tyrosine kinases and ephrins have been identified in organisms ranging from sponges to flies and worms to chick, mice and humans, thus allowing their function to be approached also from an evolutionary perspective. The structural analysis of Eph/ephrin crystals is providing hints for the existence of Eph and ephrin folds in plants and suggests a mechanism for triggering bi-directional signalling.

Impact of venous return on pulmonary oxygen uptake kinetics during dynamic exercise: in silico time series analyses from muscles to lungs.

The aim of the present study was to investigate whether a single-compartment (SCM) and a multi-compartment (MCM) venous return model will produce significantly different time-delaying and distortive effects on pulmonary oxygen uptake (V̇o2pulm) responses with equal cardiac outputs (Q̇) and muscle oxygen uptake (V̇o2musc) inputs. For each model, 64 data sets were simulated with alternating Q̇ and V̇o2musc kinetics-time constants (τ) ranging from 10 to 80 s-as responses to pseudorandom binary sequence work rate (WR) changes. Kinetic analyses were performed by using cross-correlation functions (CCFs) between WR with V̇o2pulm and V̇o2musc. Higher maxima of the CCF courses indicate faster system responses-equal to smaller τ values of the variables of interest (e.g., τV̇o2musc). The models demonstrated a highly significant relationship for the resulting V̇o2pulm responses ( r = 0.976, P < 0.001, n = 64). Both models showed significant differences between V̇o2pulm and V̇o2musc kinetics for τV̇o2musc ranging from 10 to 30 s ( P < 0.05 each). In addition, a significant difference in V̇o2pulm kinetics ( P < 0.05) between the models was observed for very fast V̇o2musc kinetics (τ = 10 s). The combinations of fast Q̇ dynamics and slow V̇o2musc kinetics yield distinct deviations in the resultant V̇o2pulm responses compared with V̇o2musc kinetics. Therefore, the venous return models should be used with care and caution if the aim is to infer V̇o2musc by means of V̇o2pulm kinetics. Finally, the resultant V̇o2pulm responses seem to be complex and most likely unpredictable if no cardiodynamic measurements are available in vivo. NEW & NOTEWORTHY A single-compartment and a multi-compartment venous return model were tested to see whether they result in different pulmonary oxygen uptake (V̇o2pulm) kinetics from equal cardiac output and muscle oxygen uptake (V̇o2musc) kinetics. To infer V̇o2musc kinetics by means of V̇o2pulm kinetics, both models should only be used for V̇o2musc time constants ranging from 40 to 80 s. The resultant V̇o2pulm responses seem to be complex and most likely unpredictable if no cardiodynamic measurements are available.