Effect of hypoxia on the lactate dehydrogenase isoenzyme composition in the rat carotid body

The inevitability of ATP as a transmitter in the carotid body

Atmospheric oxygen concentrations rose markedly at several points in evolutionary history. Each of these increases was followed by an evolutionary leap in organismal complexity, and thus the cellular adaptions we see today have been shaped by the levels of oxygen within our atmosphere. In eukaryotic cells, oxygen is essential for the production of adenosine 5'-triphosphate (ATP) which is the 'Universal Energy Currency' of life. Aerobic organisms survived by evolving precise mechanisms for converting oxygen within the environment into energy. Higher mammals developed specialised organs for detecting and responding to changes in oxygen content to maintain gaseous homeostasis for survival. Hypoxia is sensed by the carotid bodies, the primary chemoreceptor organs which utilise multiple neurotransmitters one of which is ATP to evoke compensatory reflexes. Yet, a paradox is presented in oxygen sensing cells of the carotid body when during periods of low oxygen, ATP is seemingly released in abundance to transmit this signal although the synthesis of ATP is theoretically halted because of its dependence on oxygen. We propose potential mechanisms to maintain ATP production in hypoxia and summarise recent data revealing elevated sensitivity of purinergic signalling within the carotid body during conditions of sympathetic overactivity and hypertension. We propose the carotid body is hypoxic in numerous chronic cardiovascular and respiratory diseases and highlight the therapeutic potential for modulating purinergic transmission.

Carotid body type I cells engage flavoprotein and Pin1 for oxygen sensing

Carotid body (CB) type I cells sense the blood Po₂ and generate a nervous signal for stimulating ventilation and circulation when blood oxygen levels decline. Three oxygen-sensing enzyme complexes may be used for this purpose: 1) mitochondrial electron transport chain metabolism, 2) heme oxygenase 2 (HO-2)-generating CO, and/or 3) an NAD(P)H oxidase (NOX). We hypothesize that intracellular redox changes are the link between the sensor and nervous signals. To test this hypothesis type I cell autofluorescence of flavoproteins (Fp) and NAD(P)H within the mouse CB ex vivo was recorded as Fp/(Fp+NAD(P)H) redox ratio. CB type I cell redox ratio transiently declined with the onset of hypoxia. Upon reoxygenation, CB type I cells showed a significantly increased redox ratio. As a control organ, the non-oxygen-sensing sympathetic superior cervical ganglion (SCG) showed a continuously reduced redox ratio upon hypoxia. CN^-, diphenyleneiodonium, or reactive oxygen species influenced chemoreceptor discharge (CND) with subsequent loss of O₂ sensitivity and inhibited hypoxic Fp reduction only in the CB but not in SCG Fp, indicating a specific role of Fp in the oxygen-sensing process. Hypoxia-induced changes in CB type I cell redox ratio affected peptidyl prolyl isomerase Pin1, which is believed to colocalize with the NADPH oxidase subunit p47_phox in the cell membrane to trigger the opening of potassium channels. We postulate that hypoxia-induced changes in the Fp-mediated redox ratio of the CB regulate the Pin1/p47_phox tandem to alter type I cell potassium channels and therewith CND.

Extracellular pH changes in the superfused cat carotid body during hypoxia and hypercapnia

Extracellular pH changes were measured in the superfused cat carotid body with double barreled pH glass microelectrodes, under constant pH (7.45 +/- 0.02), temperature (35 degrees C) and flow (3.6 ml/min) of the superfusion medium. Changes of pO2 in the medium from about 188 Torr (30% O2) to 35 or 12 Torr (5% and 2% respectively) called hypoxia, induced a change of the pH signal of about 0.1 units indicating acidification of the tissue. Medium pH monitored with a pH macroelectrode did not change during hypoxic stimulation. An increase of pCO2 in the medium from about 20 Torr (3% CO2, pH 7.45 +/- 0.02) to 70 Torr (12% CO2, pH 6.98 +/- 0.01) called hypercapnia, under constant pO2 (188 +/- 2 Torr), temperature (35 degrees C) and flow (3.6 ml/min) resulted in acidification of the tissue of about 0.3 pH units. Extracellular pH changes during hypoxia did not occur when the superfusion medium had no glucose; however, pH changes during hypercapnia persisted under these conditions. The hypoxic and hypercapnic chemosensory response of the sinus nerve were decreased or abolished during glucose deprivation in a time-dependent manner. Replacement of glucose with 2-deoxyglucose in the medium led to a similar pattern, i.e. inhibition of the hypoxic and hypercapnic chemosensory nerve response and of the extracellular hypoxic pH changes. These results indicate that glycolysis takes place and contributes to O2 and CO2-chemoreception in the carotid body.

Redox changes in the mouse carotid body during hypoxia

The carotid body of the mouse is ideally suited for in vitro photometric and fluorometric studies using transillumination. It is small, thin and translucent. Photometric recordings were made by using a dual wavelength method. Wavelengths of 550 nm (maximal absorbance, MA) and 540 nm (isosbestic point, IP) were employed to study cytochrome c, 600 nm (MA) and 620 nm (IP) for a pigment presumed to be cytochrome aa3, 580 nm (MA) and 570 or 542 nm (IP) to control for possible effects of hemoglobin (Hb). Exposure of the organ to NaCN (10-50 nM) reduced cytochromes c and putative aa3. Hypoxia (produced by superfusing with a medium equilibrated with 100% N2) reduced cytochrome c but tended to oxidize the presumed cytochrome aa3. These effects were apparent at a medium pO2 of 100 torr or less. Fluorometry revealed reduction of NADH under both cyanide exposure and hypoxia. There was little or no interference by Hb when the preparations were carefully washed of remaining red cells. The precise site of the recorded mitochondria is unknown: they could have been located in the parenchymal cells (type I and/or II), nerve terminals, smooth muscle fibers, etc. Resolution of this point will need using dissociated carotid body cells. Further, the possible presence of a special respiratory pigment responding at a wavelength similar to that exciting cytochrome aa3 has not been discarded. Its study would require isolating carotid body mitochondria.