Lactate sensing mechanisms in arterial chemoreceptor cells.

Classically considered a by-product of anaerobic metabolism, lactate is now viewed as a fundamental fuel for oxidative phosphorylation in mitochondria, and preferred over glucose by many tissues. Lactate is also a signaling molecule of increasing medical relevance. Lactate levels in the blood can increase in both normal and pathophysiological conditions (e.g., hypoxia, physical exercise, or sepsis), however the manner by which these changes are sensed and induce adaptive responses is unknown. Here we show that the carotid body (CB) is essential for lactate homeostasis and that CB glomus cells, the main oxygen sensing arterial chemoreceptors, are also lactate sensors. Lactate is transported into glomus cells, leading to a rapid increase in the cytosolic NADH/NAD+ ratio. This in turn activates membrane cation channels, leading to cell depolarization, action potential firing, and Ca2+ influx. Lactate also decreases intracellular pH and increases mitochondrial reactive oxygen species production, which further activates glomus cells. Lactate and hypoxia, although sensed by separate mechanisms, share the same final signaling pathway and jointly activate glomus cells to potentiate compensatory cardiorespiratory reflexes.

Acute O2 Sensing: Role of Coenzyme QH2/Q Ratio and Mitochondrial ROS Compartmentalization.

Acute O2 sensing by peripheral chemoreceptors is essential for mammalian homeostasis. Carotid body glomus cells contain O2-sensitive ion channels, which trigger fast adaptive cardiorespiratory reflexes in response to hypoxia. O2-sensitive cells have unique metabolic characteristics that favor the hypoxic generation of mitochondrial complex I (MCI) signaling molecules, NADH and reactive oxygen species (ROS), which modulate membrane ion channels. We show that responsiveness to hypoxia progressively disappears after inducible deletion of the Ndufs2 gene, which encodes the 49 kDa subunit forming the coenzyme Q binding site in MCI, even in the presence of MCII substrates and chemical NAD+ regeneration. We also show contrasting effects of physiological hypoxia on mitochondrial ROS production (increased in the intermembrane space and decreased in the matrix) and a marked effect of succinate dehydrogenase activity on acute O2 sensing. Our results suggest that acute responsiveness to hypoxia depends on coenzyme QH2/Q ratio-controlled ROS production in MCI.

HIF-2α is essential for carotid body development and function.

Mammalian adaptation to oxygen flux occurs at many levels, from shifts in cellular metabolism to physiological adaptations facilitated by the sympathetic nervous system and carotid body (CB). Interactions between differing forms of adaptive response to hypoxia, including transcriptional responses orchestrated by the Hypoxia Inducible transcription Factors (HIFs), are complex and clearly synergistic. We show here that there is an absolute developmental requirement for HIF-2α, one of the HIF isoforms, for growth and survival of oxygen sensitive glomus cells of the carotid body. The loss of these cells renders mice incapable of ventilatory responses to hypoxia, and this has striking effects on processes as diverse as arterial pressure regulation, exercise performance, and glucose homeostasis. We show that the expansion of the glomus cells is correlated with mTORC1 activation, and is functionally inhibited by rapamycin treatment. These findings demonstrate the central role played by HIF-2α in carotid body development, growth and function.

Monitoring Functional Responses to Hypoxia in Single Carotid Body Cells.

The carotid body is the main arterial chemoreceptor in mammals that mediates the cardiorespiratory reflexes activated by acute hypoxia. Here we describe the protocols followed in our laboratory to study responsiveness to hypoxia of single, enzymatically dispersed, glomus cells monitored by microfluorimetry and the patch-clamp technique.