Adenosine and nitric oxide (NO) are important local mediators of vasodilatation. The aim of this study was to elucidate the mechanisms underlying adenosine receptor‐mediated NO release from the endothelium. In studies on freshly excised rat aorta, second‐messenger systems were pharmacologically modulated by appropriate antagonists while a NO‐sensitive electrode was used to measure adenosine‐evoked NO release from the endothelium. We showed that A₁‐mediated NO release requires extracellular Ca²⁺, phospholipase A₂(PLA₂) and ATP‐sensitive K⁺(K_ATP) channel activation whereas A_2A‐mediated NO release requires extracellular Ca²⁺and Ca²⁺‐activated K⁺(K_Ca) channels. Since our previous study showed that A1‐ and A_2A‐receptor‐mediated NO release requires activation of adenylate cyclase (AC), we propose the following novel pathways. The K⁺efflux resulting from A₁‐receptor‐coupled K_ATP‐channel activation facilitates Ca²⁺influx which may cause some stimulation of endothelial NO synthase (eNOS). However, the increase in [Ca²⁺]_ialso stimulates PLA₂to liberate arachidonic acid and stimulate cyclooxygenase to generate prostacyclin (PGI₂). PGI₂acts on its endothelial receptors to increase cAMP, so activating protein kinase A (PKA) to phosphorylate and activate eNOS resulting in NO release. By contrast, the K⁺efflux resulting from A_2A‐coupled K_Cachannels facilitates Ca²⁺influx, thereby activating eNOS and NO release. This process may be facilitated by phosphorylation of eNOS by PKA via the action of A_2A‐receptor‐mediated stimulation of AC increasing cAMP. These pathways may be important in mediating vasodilatation during exercise and systemic hypoxia when adenosine acting in an endothelium‐ and NO‐dependent manner has been shown to be important.

It is accepted that NO plays a role in hypoxic vasodilatation in several tissues. For rat hindlimb muscle there is evidence that during systemic hypoxia endogenously released adenosine acts on endothelial A₁ receptors to evoke dilatation in a NO-dependent fashion, implying requirement for, or mediation by, NO. We tested in vivo whether systemic hypoxia and adenosine release NO from muscle. In anaesthetized rats, arterial blood pressure (ABP) and femoral blood flow (FBF) were recorded allowing computation of femoral vascular conductance (FVC). Blood samples taken from femoral artery and vein allowed electrochemical measurement of plasma [NO] after reduction of NO₃⁻ and NO₂⁻. Systemic hypoxia and adenosine infusion for 5 min each, evoked an increase in FVC that was attenuated by the NO synthase (NOS) inhibitor l-NAME (Group 1, n= 8) and adenosine A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, Group 2, n= 6). Concomitant systemic hypoxia and adenosine infusion evoked increases in venous–arterial [NO] difference ([NO]_v-a) from −1.4 ± 0.85 to 6.6 ± 1.6 and 2.3 ± 0.78 to 8.4 ± 1.8 nmol l⁻¹, respectively (mean ±s.e.m), which were abolished by l-NAME (−0.72 ± 0.90 to −0.87 ± 0.74 and 0.72 ± 0.85 to −0.97 ± 1.1 nmol l⁻¹, respectively). DPCPX also abolished the hypoxia-evoked increase in [NO]_v-a (control −4.2 ± 1.8 to 12.5 ± 3.7 nmol l⁻¹, with DPCPX −0.63 ± 2.6 to 3.3 ± 2.9 nmol l⁻¹) and decreased the adenosine-evoked increase in [NO]_v-a (control 1.1 ± 1.5 to 24 ± 14, with DPCPX −0.43 ± 2.9 to 12 ± 5.9 nmol l⁻¹). These results allow the novel conclusion that the muscle vasodilatation of systemic hypoxia is partly mediated by adenosine acting at endothelial A₁ receptors to stimulate synthesis and release of NO, which then induces dilatation.

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

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.