Reflex increases in breathing in response to acute hypoxia are dependent on activation of the carotid body (CB)—A specialised peripheral chemoreceptor. Central to CB O₂-sensing is their unique mitochondria but the link between mitochondrial inhibition and cellular stimulation is unresolved. The objective of this study was to evaluate if ex vivo intact CB nerve activity and in vivo whole body ventilatory responses to hypoxia were modified by alterations in succinate metabolism and mitochondrial ROS (mitoROS) generation in the rat. Application of diethyl succinate (DESucc) caused concentration-dependent increases in chemoafferent frequency measuring approximately 10–30% of that induced by severe hypoxia. Inhibition of mitochondrial succinate metabolism by dimethyl malonate (DMM) evoked basal excitation and attenuated the rise in chemoafferent activity in hypoxia. However, approximately 50% of the response to hypoxia was preserved. MitoTEMPO (MitoT) and 10-(6′-plastoquinonyl) decyltriphenylphosphonium (SKQ1) (mitochondrial antioxidants) decreased chemoafferent activity in hypoxia by approximately 20–50%. In awake animals, MitoT and SKQ1 attenuated the rise in respiratory frequency during hypoxia, and SKQ1 also significantly blunted the overall hypoxic ventilatory response (HVR) by approximately 20%. Thus, whilst the data support a role for succinate and mitoROS in CB and whole body O₂-sensing in the rat, they are not the sole mediators. Treatment of the CB with mitochondrial selective antioxidants may offer a new approach for treating CB-related cardiovascular–respiratory disorders.

The carotid body (CB) is the key peripheral oxygen sensor. CB mitochondria are hypothesised to be uniquely adapted with unusually low intrinsic oxygen affinity which, in association with nitric oxide (NO) and reactive oxygen species signalling, enables acute responsiveness to hypoxia. However, CB mitochondrial physiology or intrinsic oxygen affinity have never been measured directly. We sought to address this key gap by isolating sheep CB mitochondria and comprehensively characterising their phenotype and contrasting them to a non-oxygen sensing tissue, left ventricular myocardium (LV). High resolution respirometry, liquid chromatography mass spectrometry, enzymatic assays and in silico modelling were used to characterise mitochondrial content, aerobic capacity, oxygen affinity, complex subunit abundance and activity, H2O2 production and NO sensitivity in ovine CB and LV. Mitochondrial oxygen affinity (P50 = 0.089 mmHg) was lower in the CB than the LV (P50 = 0.058 mmHg; p = 0.005). Whilst mitochondrial content was lower in the CB, CB mitochondria had higher respiratory rates and enzymatic activity than LV. H2O2 production and NO sensitivity were similar in the two tissues. While intrinsic mitochondrial oxygen affinity is slightly lower in the oxygen sensing CB than in the non-oxygen sensing LV, this difference is small. Hence, any role of mitochondria in CB oxygen sensing is not due to an intrinsic difference in the O2 affinity of cytochrome oxidase due to differential expression of its subunits. Instead, this work suggests that differences in O2 affinity in vivo are secondary to other factors, perhaps including NO, that alter mitochondrial O2 affinity.

We establish the first protocol for high-fidelity echocardiography in conscious sheep, overcoming the long-standing barrier of apical view acquisition. This enables advanced indices, including volumetric ejection fraction and tissue Doppler, and provides reproducible reference values. Applied to a tachypacing model, it revealed progressive biventricular and atrial remodeling and an early therapeutic window, offering a powerful and clinically relevant platform for translational heart failure research.