Mitochondrial Succinate Metabolism and Reactive Oxygen Species Are Important but Not Essential for Eliciting Carotid Body and Ventilatory Responses to Hypoxia in the Rat

Abstract

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.

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

“…In our own experiments, exogenous application of the cell permeable form of succinate (diethyl succinate, DESucc) did lead to chemoafferent stimulation in the isolated rat CB, but this measured only approximately 10–30% of that induced by a subsequent severe hypoxic stimulus ( Swiderska et al, 2021 ). Addition of a high concentration of dimethyl malonate (DMM; a competitive inhibitor of SDH) also produced a significant rise in basal chemoafferent frequency and caused partial (20%–50%) reduction of the activity in hypoxia.…”

“…To probe the importance of mitochondrial ROS further, we have recently measured rat CB chemoafferent activity and ventilation in the presence of the mitochondrial antioxidants, MitoTEMPO and SKQ1 ( Swiderska et al, 2021 ). Both agents caused a moderate decrease (20%–50%) of the chemoafferent response to hypoxia, although a significant component remained intact ( Figure 2 ).…”

“…Lower panel shows the inhibitory effect of these pharmacological agents on the rises in carotid body chemoafferent discharge frequency, respiratory frequency and minute ventilation during hypoxia. IMS, intermembrane space; cyt c, cytochrome c. Adapted from ( López-Barneo and Ortega-Sáenz, 2022 ), ( Swiderska et al, 2021 ) and ( Chouchani et al, 2014 ).…”

“…This has been an important research focus over recent years and a better understanding of these specific mechanisms could offer new strategies to manipulate CB function in health and disease ( Chang, 2017 ). A number of mitochondrial/metabolic signalling pathways have been proposed including an elevation in mitochondrial reactive oxygen species (mitoROS) generation ( Fernandez-Aguera et al, 2015 ; Arias-Mayenco et al, 2018 ; Swiderska et al, 2021 ), an alteration in nicotinamide adenine dinucleotide (NAD + ) and flavoprotein (Fp) redox status ( Bernardini et al, 2020 ), a fall in MgATP ( Varas et al, 2007 ) and an increase in lactate production ( Chang et al, 2015 ) ( Figure 1 ). Although there is compelling evidence for each of these processes, they are all not without question.…”

Hypothesis: Why Different Types of SDH Gene Variants Cause Divergent Tumor Phenotypes

“…ROS-mediated toxicity might also play a role in a broader range of tissues, perhaps explaining the tissue selectivity of SDH variants. We predict that sympathetic paraganglia have weaker anti-oxidative defenses and/or weaker protection against ROS-induced apoptosis compared to oxygen-sensing parasympathetic tissues, which likely require tight redox control due to the direct involvement of ROS in oxygen-sensing [ 39 , 40 ]. Both missense and truncating SDH variants initiate tumorigenesis in parasympathetic tissues, whereas missense-driven toxicity and cell death largely confines tumorigenesis in sympathetic tissues to low-ROS truncating (null) variants.…”