Carbamylated erythropoietin enhances mice ventilatory responses to changes in O2 but not CO2 levels.

Erythropoietin (EPO) has beneficial tissue-protective effects in several diseases but erythrocytosis may cause deleterious effects in EPO-treated patients. Thus carbamylated-EPO (C-EPO) and other derivatives retaining tissue-protective but lacking bone marrow-stimulating actions have been developed. Although EPO modulates ventilatory responses, the effects of C-EPO on ventilation have not been investigated. Here, basal breathing and respiratory chemoreflexes were measured by plethysmography after acute and chronic treatments with recombinant human C-EPO (rhC-EPO; 15,000 IU/kg during 5days) or saline (control group). Hematocrit, plasma and brainstem rhC-EPO levels were also quantified. Chronic rhC-EPO significantly elevated tissue rhC-EPO levels but not hematocrit. None of the drug regimen altered basal ventilation (normoxia). Chronic but not acute rhC-EPO enhanced hyperoxic ventilatory depression, and sustained the hypoxic ventilatory response mainly via a reduction of the roll-off phase. By contrast, rhC-EPO did not blunt the ventilatory response to hypercapnia. Thus, chronic C-EPO may be a promising therapy to improve breathing during hypoxia while minimizing adverse effects on cardiovascular function.

Polycythemia and high levels of erythropoietin in blood and brain blunt the hypercapnic ventilatory response in adult mice.

Changes in arterial Po2, Pco2, and pH are the strongest stimuli sensed by peripheral and central chemoreceptors to adjust ventilation to the metabolic demand. Erythropoietin (Epo), the main regulator of red blood cell production, increases the hypoxic ventilatory response, an effect attributed to the presence of Epo receptors in both carotid bodies and key brainstem structures involved in integration of peripheral inputs and control of breathing. However, it is not known whether Epo also has an effect on the hypercapnic chemoreflex. In a first attempt to answer this question, we tested the hypothesis that Epo alters the ventilatory response to increased CO2 levels. Basal ventilation and hypercapnic ventilatory response (HCVR) were recorded from control mice and from two transgenic mouse lines constitutively expressing high levels of human Epo in brain only (Tg21) or in brain and plasma (Tg6), the latter leading to polycythemia. To tease apart the potential effects of polycythemia and levels of plasma Epo in the HCVR, control animals were injected with an Epo analog (Aranesp), and Tg6 mice were treated with the hemolytic agent phenylhydrazine after splenectomy. Ventilatory parameters measured by plethysmography in conscious mice were consistent with data from electrophysiological recordings in anesthetized animals and revealed a blunted HCVR in Tg6 mice. Polycythemia alone and increased levels of plasma Epo blunt the HCVR. In addition, Tg21 mice with an augmented level of cerebral Epo also had a decreased HCVR. We discuss the potential implications of these findings in several physiopathological conditions.

Erythropoietin and the sex-dimorphic chemoreflex pathway.

During hypoxic or hypoxemic conditions, tissue oxygenation and arterial O(2) carrying capacity are upregulated by two complementary systems, namely the neural respiratory network (central and peripheral) that leads to increased minute ventilation thereby increasing tissue oxygenation, and erythropoietin (Epo) release by the kidney that activates erythropoiesis in bone marrow to augment arterial blood O(2) carrying capacity. Despite the fact that both neural respiratory control and Epo-mediated elevation of red blood cells are responsible for keeping arterial O(2) content optimal, no interaction between these systems has been described so far. Here we review data obtained in our laboratory demonstrating that ventilatory and erythropoietic systems are tightly connected. We found Epo is the key factor mediating this relationship through modulation of the chemoreflex pathway. Moreover, we showed that this interaction occurs in a sex-dependent manner.

Erythropoietin and its antagonist regulate hypoxic fictive breathing in newborn mice.

Clinical use of erythropoietin in adult and newborn patients has revealed its involvement in neuroprotection, neurogenesis, and angiogenesis. More recently, we showed in adult mouse, that brain erythropoietin interacts with the major brainstem centers associated with respiration to enhance the ventilatory response to acute and chronic conditions of physiological hypoxia (e.g., as occurring at high altitude). However, whether brain erythropoietin is involved in breathing regulation in newborns remains unknown. In this study, en bloc brainstem-spinal cord preparations were obtained from mice at postnatal day 4. After various periods (30, 60, or 90 min) of incubation with 0, 25, or 250 U of erythropoietin, preparations were superfused with artificial cerebrospinal fluid bubbled with normoxic or hypoxic gas mixtures. The electrophysiological fictive breathing produced by axons at the C4 ventral root was next recorded. Our results show that erythropoietin attenuates the hypoxia-mediated decrease of the central respiratory activity and improves post-hypoxic recovery. Additional analysis revealed that the soluble erythropoietin receptor (the endogenous erythropoietin antagonist) dramatically decreases neural hypoxic respiratory activity, confirming the specific erythropoietin effect on respiratory drive. These results imply that erythropoietin exerts main modulation and maintenance of respiratory motor output during hypoxic and post-hypoxic challenges in 4-days old mice.

Differential respiratory control of the upper airway and diaphragm muscles induced by 5-HT1A receptor ligands.

BACKGROUND: Serotonin (5-HT) has a role in respiratory function and dysfunction. Although 5-HT affects respiratory drive to both phrenic and cranial motoneurons, relatively little is known about the role of 5-HT receptor subtypes in the control of upper airway muscle (UAM) respiratory activity. MATERIALS AND METHODS: Here, we performed central injections of 5-HT1A agonist (8-OHDPAT) or antagonist (WAY100635) in anesthetized rats and analyzed changes in the electromyographic activity of several UAM and other cardiorespiratory parameters. We also compared the pattern of Fos expression induced after central injection of a control solution or 8-OHDPAT. RESULTS: Results showed that 8-OHDPAT induced a robust increase in UAM activity, associated with either tachypnea under volatile anesthesia or bradypnea under liquid anesthesia. Injection of WAY100635 switched off UAM respiratory activity and led to bradypnea, suggesting a tonic excitatory role of endogenous 5-HT1A receptor activation. Co-injection of the agonist and the antagonist blocked the effects produced by each drug alone. Besides drug-induced changes in respiratory frequency, only slight increases in surface of diaphragm bursts were observed. Significant increases in Fos expression after 5-HT1A receptor activation were seen in the nucleus tractus solitarius, nucleus raphe pallidus, parapyramidal region, retrotrapezoid nucleus, lateral parabrachial, and Kölliker-Fuse nuclei. This restricted pattern of Fos expression likely identified the neural substrate responsible for the enhancement of UAM respiratory activity observed after 8-OHDPAT injection. CONCLUSIONS: These findings suggest an important role for the 5-HT1A receptors in the neural control of upper airway patency and may be relevant to counteract pharyngeal atonia during obstructive sleep apneas.