Discontinuous gas exchange in Madagascan hissing cockroaches is not a consequence of hysteresis around a fixed PCO2 threshold

It has been hypothesised that insects display discontinuous gas-exchange cycles (DGCs) due to hysteresis in their ventilatory control, where CO2-sensitive respiratory chemoreceptors respond to changes in hemolymph PCO2 only after some delay. If correct, DGCs would be a manifestation of an unstable feedback loop between chemoreceptors and ventilation causing PCO2 to oscillate around some fixed threshold value: PCO2 above this ventilatory threshold would stimulate excessive hyperventilation, driving PCO2 below the threshold and causing a subsequent apnoea. This hypothesis was tested by implanting micro-optodes into the hemocoel of Madagascar hissing cockroaches and measuring hemolymph PO2 and PCO2 simultaneously during continuous and discontinuous gas exchange. The mean hemolymph PCO2 of 1.9 kPa measured during continuous gas exchange was assumed to represent the threshold level stimulating ventilation, and this was compared with PCO2 levels recorded during DGCs elicited by decapitation. Cockroaches were also exposed to hypoxic (PO2 10 kPa) and hypercapnic (PCO2 2 kPa) gas mixtures to manipulate hemolymph PO2 and PCO2. Decapitated cockroaches maintained DGCs even when their hemolymph PCO2 was forced above or below the putative ∼2 kPa ventilation threshold, demonstrating that the characteristic oscillation between apnoea and gas exchange is not driven by a lag between changing hemolymph PCO2 and a PCO2 chemoreceptor with a fixed ventilatory threshold. However, it was observed that the gas exchange periods within the DGC were altered to enhance O2 uptake and CO2 release during hypoxia and hypercapnia exposure. This indicates that while respiratory chemoreceptors do modulate ventilatory activity in response to hemolymph gas levels, their role in initiating or terminating the gas exchange periods within the DGC remains unclear.

Uncoupling effect of lipid peroxidation in spinach thylakoids exposed to peroxyl radicals generated by 2,2′-azobis (2-amidinopropane) dihydrochloride

In this study, we characterized how lipid peroxidation alters the functionality of spinach thylakoids exposed to peroxyl radicals generated by the azo compound 2,2-azobis(2-amidinopropane) dihydrochloride (AAPH). Incubation of thylakoids in presence of different concentrations (0 to 200 mM) of AAPH inhibited the formation of ΔpH (IC50 ≈ 1.5 mM) estimated by the quenching of 9-aminoacridine fluorescence (Q9-AA). The Q9-AA inhibition was correlated (R2=0.98) to the extent of lipid peroxidation determined by the accumulation of thiobarbituric acid reactive substances (TBARS). Much higher AAPH concentrations were required to inhibit the maximum (Fv/Fm) and effective (ΔF/Fm’) photochemical efficiencies of photosystem II (IC50 ≈ 120 mM and 50 mM respectively), indicating that moderate lipid peroxidation caused the uncoupling of spinach thylakoids. This was confirmed by the 62 % stimulation of the O2 uptake rates measured without the artificial uncoupler NH4Cl when the AAPH concentrations increased from 0 to at 20 mM, reaching similar values to the rates measured in presence of NH4Cl. Above 20 mM AAPH, the O2 uptake rates measured with and without NH4Cl declined similarly to the decrease of ΔF/Fm’. These results suggest that the increased H+-leakiness of thylakoid membranes could be one of the primary effects of oxidative stress.

Carbon and Oxygen Gas Exchange in Woody Debris: The Process and Climate-Related Drivers

The carbon-to-oxygen relationship and gas exchange balance, organic carbon to CO2 conversion intensity and efficiency, and their relevance to climate parameters and wood decay fungi were investigated for birch woody debris (WD) in the Mid-Urals mixed pine and birch forests. It was shown that, within the range of temperatures from 10 to 40 °C and relative moisture (RM) of wood of 40% and 70%, aerobic gas exchange was observed in the WD, encompassing the physiologically entwined processes of CO2 emission and O2 uptake. Their volumetric ratio (0.9) confirmed that (1) the WD represents a globally significant CO2 source and appropriate O2 consumer and (2) the oxidative conversion of organic carbon is highly efficient in the WD, with an average ratio of CO2 released to O2 consumed equal to 90%. The balance of carbon-to-oxygen gas exchange and oxidizing conversion efficiency in the WD were not affected by either fungal species tested or by moisture or temperature. However, the intensity of gas exchange was unique for each wood decay fungi, and it could be treated as a climate-reliant parameter driven by temperature (Q10 = 2.0–2.1) and moisture (the latter induced a corresponding trend and value changes in CO2 emission and O2 uptake). Depending on the direction and degree of the change in temperature and moisture, their combined effect on the intensity of gas exchange led to its strengthening or weakening; otherwise, it was stabilized. Aerobic respiration of wood decay Basidiomycetes is an essential prerequisite and the major biotic factor in the WD gas exchange, while moisture and temperature are its climatic controllers only.

Influence of the fitting window on the O2 uptake kinetics at the onset of moderate intensity exercise

The O2 uptake (V'O2) data at the onset of an exercise are usually fitted with a mono-exponential function, after removal of the data pertaining to a conventional initial time period (ΔTr) lasting ~20s. We performed a thorough quantitative analysis on the effects of removing data pertaining to different ΔTr, aiming at identifying an objective method to establish the appropriate ΔTr. Breath-by-breath O2 uptake responses, acquired on 25 healthy adults performing a step moderate-intensity exercise, and 104 simulated bi-exponential responses, were analyzed. For all the responses, the kinetic parameters of a mono-exponential function, and the corresponding Asymptotic Standard Errors (ASE), were estimated by non-linear regression, removing the data pertaining to progressively longer initial periods (1s each) up to 60s. Four methods to establish objectively ΔTr were compared. The minimum estimated tau was obtained for ΔTr≅35s in both the V'O2 and simulated data, that was about 30% lower compared to that obtained for ΔTr≅0s. The average ASE values remained quite constant up to ΔTr≅35s, thereafter they increased remarkably. The tau used to generate the simulated response fell within the confidence intervals of the estimated tau in ~85% of cases for ΔTr=20s ("20s-w" method); this percentage increased to ~92% of cases when ΔTr was established according to both the minimum tau and its narrowest confidence interval ("Mixed" method). In conclusion, the effects of removing the V'O2 data pertaining to different ΔTr are remarkable. The "Mixed" method provided estimated parameters close to those used to generate the simulated responses and is thus endorsed.

Is hypoxia vulnerability in fishes a by-product of maximum metabolic rate?

ABSTRACT The metabolic index concept combines metabolic data and known thermal sensitivities to estimate the factorial aerobic scope of animals in different habitats, which is valuable for understanding the metabolic demands that constrain species' geographical distributions. An important assumption of this concept is that the O2 supply capacity (which is equivalent to the rate of oxygen consumption divided by the environmental partial pressure of oxygen: ) is constant at O2 tensions above the critical O2 threshold (i.e. the where O2 uptake can no longer meet metabolic demand). This has led to the notion that hypoxia vulnerability is not a selected trait, but a by-product of selection on maximum metabolic rate. In this Commentary, we explore whether this fundamental assumption is supported among fishes. We provide evidence that O2 supply capacity is not constant in all fishes, with some species exhibiting an elevated O2 supply capacity in hypoxic environments. We further discuss the divergent selective pressures on hypoxia- and exercise-based cardiorespiratory adaptations in fishes, while also considering the implications of a hypoxia-optimized O2 supply capacity for the metabolic index concept.

Closed loop stimulation in heart failure patients with severe chronotropic incompetence: responder versus non-responders

Funding Acknowledgements Type of funding sources: Private company. Main funding source(s): Biotronik SE & Co. KG Woermannkehre 1 12359 Berlin Background The prevalence of chronotropic incompetence (CI) in heart failure (HF) population is high and negatively impacts prognosis. Rate-adaptive pacing (RAP) is an important treatment option for CI. However, only a proportion of HF patients treated with cardiac resynchronisation therapy (CRT) devices benefit from accelerometer-based RAP in terms of exercise tolerance, functional capacity, and quality of life (QoL). Further research is needed to identify patient characteristics predicting positive response to RAP, and to evaluate performance of alternative sensors such as closed loop stimulation (CLS) driven by cardiac impedance measurements. An optimal outcome measure is represented by ventilatory efficiency (VE) slope during cardio-pulmonary exercise test (CPX) because of superior prognostic value. Purpose The purpose of the BIO|Create pilot study was to assess the benefit of CLS in CRT patients with CI. In this predefined subanalysis, we identify predictors of positive response to CLS (reduction of VE slope by ≥5%) and compare study outcomes in responders vs non-responders. Methods The study enrolled CRT patients with NYHA class II or III and severe CI (<75% of age-predicted maximum heart rate [HR] or <50% of HR reserve utilised at end-exercise). Patients were randomised to DDD-CLS mode or to DDD pacing at 40 beats/min for 1 month, followed by crossover for another month. At 1- and 2-month follow-ups, exercise tolerance was assessed by treadmill CPX, functional capacity by 6-min walk test, and QoL by the EQ-5D-5L and Minnesota Living with HF (MLHFQ) questionnaires. Results Among 17 patients with full follow-up datasets, 8 (47%) were responders to CLS. Compared to non-responders, responders had larger left ventricular (LV) ejection fraction at baseline (46 ± 3 vs 36 ± 9 %; p = 0.0070), smaller end-diastolic (121 ± 34 vs 181 ± 41 ml; p = 0.0085) and end-systolic (65 ± 23 vs 114 ± 39 ml; p = 0.0076) LV volumes, and were predominantly in NYHA class II (p = 0.0498). For study outcomes, the mean difference between DDD-CLS and DDD-40 modes in responders vs non-responders was - 6.1 (-16.4%) vs +2.7 (+6.8%) for VE slope (both p < 0.05), +0.5 vs -0.2 ml/min (O2 uptake efficiency slope), +1.3 vs -0.3 ml/kg/min (peak O2 uptake), +1.4 vs -0.75 mmHg (end-exercise end-tidal CO2), 16 vs 7 m (6-min walk distance), 0.08 vs 0.06 (EQ-5D-5L index), 1.9 vs 0 (EQ-5D-5L scale), and -2.5 vs +1.75 (MLHFQ). Conclusions For the first time, predictors for positive outcome of RAP in CRT patients have been identified. Patients with less advanced HF were responders to RAP driven by CLS principle. In addition, a consistent increase in exercise and functional capacity and QoL in these patients could be achieved. In contrast, patients with advanced HF experienced worse exercise capacity and QoL during RAP, suggesting caution if RAP is desirable due to CI. Further clinical research is needed to evaluate if positive response to RAP can improve hard clinical outcomes.

Sex differences in cardiorespiratory fitness are explained by blood volume and oxygen carrying capacity

Aims Intrinsic sex differences in fundamental blood attributes have long been hypothesized to contribute to the gap in cardiorespiratory fitness between men and women. This study experimentally assessed the role of blood volume and oxygen (O2) carrying capacity on sex differences in cardiac function and aerobic power. Methods and results Healthy women and men (n = 60) throughout the mature adult lifespan (42–88 yr) were matched by age and physical activity levels. Transthoracic echocardiography, central blood pressure, and O2 uptake were assessed throughout incremental exercise (cycle ergometry). Main outcomes such as left ventricular end-diastolic volume (LVEDV), stroke volume (SV), cardiac output (Q), and peak O2 uptake (VO2peak), as well as blood volume (BV) were determined with established methods. Measurements were repeated in men following blood withdrawal and O2 carrying capacity reduction matching women’s levels. Prior to blood normalization, BV and O2 carrying capacity were markedly reduced in women compared with men (P < 0.001). Blood normalization resulted in a precise match of BV (82.36 ± 9.83 vs. 82.34 ± 7.70 ml·kg−1, P = 0.993) and O2 carrying capacity (12.0 ± 0.6 vs. 12.0 ± 0.7 g·dl−1, P = 0.562) between women and men. Body size-adjusted cardiac filling and output (LVEDV, SV, Q) during exercise as well as VO2peak (30.8 ± 7.5 vs. 35.6 ± 8.7 ml·min−1·kg−1, P < 0.001) were lower in women compared with men prior to blood normalization. VO2peak did not differ between women and men after blood normalization (30.8 ± 7.5 vs. 29.7 ± 7.4 ml·min−1·kg−1, P = 0.551). Conclusions Sex differences in cardiorespiratory fitness are abolished when blood attributes determining O2 delivery are experimentally matched between adult women and men.

NaCl-altered oxygen flux profiles and H+-ATPase activity in roots of two contrasting poplar species

Maintaining mitochondrial respiration is crucial for proving ATP for H+ pumps to continuously exclude Na+ under salt stress. NaCl-altered O2 uptake, mitochondrial respiration, and the relevance to H+-ATPase activity were investigated in two contrasting poplar species, Populus euphratica (salt-tolerant) and P. popularis 35–44 (salt-sensitive). Compared with P. popularis, P. euphratica roots exhibited a greater capacity to extrude Na+ under NaCl stress (150 mM). The cytochemical analysis with Pb(NO3)2 staining revealed that P. euphratica root cells retained higher H+ hydrolysis activity than the salt-sensitive poplar during a long-term (LT) of increasing salt stress (50 to 200 mM NaCl, 4 weeks). Long-sustained activation of proton pumps require long-lasting supply of energy (ATP), delivered by aerobic respiration. Taking advantage of the vibrating-electrodes technology combined with the use of membrane-tipped, polarographic oxygen microelectrodes, the species, spatial, and temporal differences in root O2 uptake were characterized under conditions of salt stress. Oxygen uptake upon NaCl shock (150 mM) was less declined in P. euphratica than in P. popularis, although the salt-induced transient kinetics were distinct from the drastic drop of O2 caused by hyperosmotic shock (255 mM mannitol). Short-term (ST) treatment (150 mM NaCl, 24 h) stimulated O2 influx in P. euphratica roots, and LT-treated P. euphratica displayed an increased O2 influx along root axis, whereas O2 influx declined with increasing salinity in P. popularis roots. The spatial localization of O2 influxes revealed that the apical zone was more susceptible than elongation region upon high NaCl (150, 200 mM) during ST and LT stress. Pharmacological experiments showed that the Na+ extrusion and H+-ATPase activity in salinized roots were correspondingly suppressed when O2 uptake was inhibited by a mitochondrial respiration inhibitor, NaN3. Therefore, we conclude that the stable mitochondrial respiration energized H+-ATPase of P. euphratica root cells for maintaining Na+ homeostasis under salt environments.