Chemical modification of carotid body chemoreception by sulfhydryls

General redox environment and carotid body chemoreceptor function

Carotid body (CB) chemoreceptor cells detect physiological levels of hypoxia and generate a hyperventilation, homeostatic in nature, aimed to minimize the deleterious effects of hypoxia. Intimate mechanisms involved in oxygen sensing in chemoreceptor cells remain largely unknown, but reactive oxygen species (ROS) had been proposed as mediators of this process. We have determined glutathione levels and calculated glutathione redox potential (E(GSH); indicator of the general redox environment of cells) in rat diaphragms incubated in the presence of oxidizing agents of two types: nonpermeating and permeating through cell membranes; in the latter group, unspecific oxidants and inhibitors of ROS-disposing enzymes were used. Selected concentrations of oxidizing agents were tested for their ability to modify the normoxic and hypoxic activity of chemoreceptor cells measured in vitro as their rate of release of neurotransmitters. Results evidence variable relationships between E(GSH) and the activity of chemoreceptor cells. The independence of chemoreceptor cell activity from the E(GSH) would imply that the ability of the CB to play its homeostatic role is largely preserved in any pathological or toxicological contingency causing oxidative stress. Consistent with this suggestion, it was also found that CB-mediated hypoxic hyperventilation was not altered by treatment of intact animals with agents that markedly decreased the E(GSH) in all tissues assayed.

Reflections on the role of the thiol group in biology

This essay is concerned with the role of the thiol or sulfhydrvl group in cellular function and metabolism and with the important investigations over many years that have led us to a better understanding of the importance of this molecular moiety that plays such a vital role in biology. The tools for measuring the SH group and for inhibiting or regenerating it will be discussed as will its essential role in the actions of many enzymes. The importance of the thiol group in glycolysis and in energy production by mitochondria will be emphasized. Of special interest at present is the fact that certain low molecular weight SH-containing substances can mimic some of the actions of insulin and may become of benefit in the treatment of diabetes mellitus. Finally, the toxic effects of oxygen on metabolism and function will be discussed with particular reference to the possibility that oxidation of thiol groups may play a role in the manifestations of oxygen toxicity.

CO reveals dual mechanisms of O2 chemoreception in the cat carotid body

The hypothesis that CO-binding pigments in the carotid body participate in O2 chemoreception was tested. The chemosensory nerve discharges of cat carotid body perfused and superfused in vitro at 36-37 degrees C with cell-free solution containing CO2-HCO3- (pH approximately equal to 7.39) were recorded to monitor O2 chemoreception. Several levels of PCO (60-550 Torr) at two levels of PO2 (50 Torr-140 Torr) were used. With high PCO of 500-550 Torr at any PO2 the discharge rate peaked promptly but the effect was significantly less than that to hypoxia. At any stage of the CO effect, exposure to light promptly attenuated or eliminated the response, as if the stimulatory effect of hypoxia was absent. Lower PCO of 60-70 Torr attenuated the response to hypoxia which was not suppressed by light. PCO of 140 Torr also attenuated the response to hypoxia and made the activity partially photolabile. During high PCO exposure the excitatory response to cyanide but not to nicotine was attenuated, consistent with the idea that the effects of nicotine are downstream from those of CO. Both inhibitory and excitatory effects of CO were promptly reversible. The results indicate that two types of CO-binding chromophores participate in O2 chemoreception in the carotid body.

Dependence of carotid chemosensory responses on metabolic substrates

The dependence of the carotid chemosensory response to hypoxia on metabolic substrate and the hypothesis that lactic acidosis is essential for O2 chemoreception were tested. Effects of 3 types of substrate (glucose, glutamate and a mixture of amino acids) on the response to hypoxia (perfusate flow interruption) were measured (n = 33 carotid bodies). The response to nicotine (n = 25) was used to determine whether these effects were exclusive to the hypoxic response. The cat carotid body was perfused and superfused in vitro with modified Tyrode solution (pO2 > 400 Torr, pCO2 < 1 Torr, pH = 7.4) at 36 degrees C containing a given substrate for at least 15 min prior to flow interruption or nicotine injection. Without substrate, responses to flow interruption (n = 4) and nicotine (n = 2) were irreversibly depressed. With glucose, responses to flow interruption (n = 13) and nicotine (n = 8) increased in a concentration-dependent fashion. Glutamate (42 mM) alone (n = 11) or a mixture of amino acids (4.2 mM) plus 5.5 mM glucose (n = 12) substituted for 11 mM glucose (n = 10). Thus, glutamate (42 mM), or a mixture of amino acids (4.2 mM) or a high concentration of glucose (11 mM) can support chemosensory responses to flow interruption and nicotine. Since glutamate undergoes oxidative deamination to alpha-ketoglutarate without lactic acid production, O2 chemoreception does not depend on lactic acidosis.

Modulation of ionic currents in isolated type I cells of the neonatal rat carotid body by p-chloromercuribenzenesulfonic acid

The sulphydryl reagent, p-chloromercuribenzenesulfonic acid (PCMBS), irreversibly excites the carotid body in vivo. We tested the effects of PCMBS on ionic currents in isolated type I cells, using the whole-cell patch-clamp technique. PCMBS selectively and irreversibly inhibited the Ca(2+)-activated K+ current (IKCa) in a dose-dependent manner (0.01-1 mM). The same concentrations of PCMBS did not affect the Ca(2+)-independent K+ current (IKv), but caused a transient enhancement of the Ca2+ current (ICa). The inhibition of IKCa by PCMBS is similar to the previously reported effects of hypoxia, and suggests a central role for the channels underlying this current in chemotransduction.

Light absorbance changes in the mouse carotid body during hypoxia and cyanide poisoning

The isolated carotid body (CB) of the mouse was bathed with modified Tyrode's solution (pO2, 170 Torr) and transilluminated at different wave-lengths (500-620 nm). Difference spectra (normoxia vs hypoxia--pO2, 10 Torr) showed peaks of increased light transmittance (LT) at 500 and especially at 540 and 580 nm. There were troughs of transmittance decrease at 560 and 600 nm. Similar spectra (normoxia vs NaCN 10 mM) revealed increased LT at 500 and 590 nm, whereas there was decreased LT at 550 nm. Therefore, absorbance changes induced by hypoxia and cyanide were different. As as control for residual hemoglobin (Hb) in the tissues, a segment of carotid artery was filled with blood and the same light scanning was performed. The differential signals evoked by hypoxia were much larger. There was some LT increase at 500 and 540 nm, but this effect was especially marked at 570 and 590 nm. NaCN showed the same absorbance pattern, but peaks were of lower amplitude. Thus, the CB-produced signals were not elicited by residual Hb. A stepwise decrease in the medium pO2 (from 170 to 10 Torr) surrounding the CB produced a progressive increase in LT at 540 and 580 nm starting at 120 Torr. A different pattern was evoked by 550 (cytochrome c) and 600 (cytochrome aa3) nm. With 550 nm, transmittance began to increase at about 70 Torr. LT decreased with 600 nm starting at about 120 Torr.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Oxygen chemoreception by carotid body cells in culture

Chemoreceptors for oxygen reside within the carotid body, but it is not known which cells actually sense hypoxia and by what mechanisms they transduce this information into afferent signals in the carotid sinus nerve. We have developed systems for the growth of glomus cells of the carotid body in dissociated cell culture. Here we demonstrate that, as in vivo, these cells contain the putative neurotransmitters dopamine, serotonin, and norepinephrine. Oxygen tension regulates the rate of dopamine secretion from the glomus cells. Similar to chemically stimulated catecholamine secretion from other adrenergic cells this hypoxia-stimulated release requires extracellular calcium. These results are compatible with the suggestion that the glomus cells of the carotid body are chemoreceptor cells and that they signal hypoxia by regulated secretion of dopamine.