Differential Expression of Large-Conductance Ca-Activated K Channels in the Carotid Body between DBA/2J and A/J Strains of Mice

The carotid body (CB) is a primary chemosensory organ for arterial hypoxia. Inhibition of K channels in chemosensory glomus cells (GCs) are considered to be responsible for hypoxic chemoreception and/or chemotransduction of the CB. Hypoxic sensitivity of large-conductance calcium-activated K (BK) channels has been established in the rat CB. Our previous work has shown the BK channel β2 subunits are more expressed in the CB of the DBA/2J mouse than that of the A/J mouse. Because the DBA/2J mouse is more sensitive to hypoxia than the A/J mouse, our general hypothesis is that BK channels play a role in the sensitivity of the mouse CB to mild hypoxia. We performed vigorous analysis of the gene expression of α, β2, and β4 subunits of BK channels in the CB. We found that α and β2 subunits were expressed more in the CB of the DBA/2J mice than that of the A/J mice. No differences were found in the β4 subunit expression. These differences were not seen in the neighboring tissues, the superior cervical ganglion and the carotid artery, suggesting that the differences are CB specific. Further, the sensitivity of BK channels in GCs to mild hypoxia was examined in patch clamp experiments using undissociated CBs. Iberiotoxin significantly inhibited K current of GCs in the DBA/2J mice, but not in the A/J mice. When reducing PO₂ to ∼70 mmHg, K current reversibly decreased in GCs of the DBA/2J, but not of the A/J mice. In the presence of iberiotoxin, mild hypoxia did not inhibit K current in either strains. Thus, the data suggest that BK channels in GCs of the DBA/2J mice are sensitive to mild hypoxia. Differential expression of BK channel β subunits in the CBs may, at least in part, explain the different hypoxic sensitivity in these mouse strains.

The impact of hydrogen sulfide (H₂S) on neurotransmitter release from the cat carotid body

Do cat carotid bodies (CBs) increase their release of acetylcholine and ATP in response to H(2)S? Two CBs, incubated in a Krebs Ringer bicarbonate solution at 37 ° C, exhibited a normal response to hypoxia-increased release of acetylcholine (ACh) and ATP. They were challenged with several concentrations of Na(2)S, an H(2)S donor. H(2)S, a new gasotransmitter, is reported to open K(ATP) channels. Under normoxic conditions the CBs reduced their release of ACh and ATP below control values. They responded identically to pinacidil, a well-known K(ATP) channel opener. CB glomus cells exhibited a positive immunohistochemical signal for cystathione-β-synthetase, a H(2)S synthesizing enzyme, and for a subunit of the K(ATP) channel. The data suggest that Na(2)S may have opened the glomus cells' K(ATP) channels, hyperpolarizing the cells, thus reducing their tonic release of ACh and ATP. Since during hypoxia H(2)S levels rise, the glomus cells responding very actively to hypoxia may be protected from over-exertion by the H(2)S opening of the K(ATP) channels.

Benzodiazepines and GABA-GABAA receptor system in the cat carotid body

Benzodiazepines (BZs) suppress ventilation possibly by augmenting the GABA(A) receptor activity in the respiratory control system, but precise sites of action are not well understood. The goals of this study were: (1) to identify GABA(A) receptor subunits in the carotid body (CB) and petrosal ganglion (PG); (2) to test if BZs exert their effects through the GABA(A) receptor in the CB chemosensory unit. Tissues were taken from euthanized adult cats. RNA was extracted from the brain, and cDNA sequences of several GABA(A) receptor subunits were determined. Subsequent RT-PCR analysis demonstrated the gene expression of alpha2, alpha3, beta3, and gamma2 subunits in the CB and the PG. Immunoreactivity for GABA and for GABA(A) receptor beta3 and gamma2 subunits was detected in chemosensory glomus cells (GCs) in the CB and neurons in the PG. The functional aspects of the GABA-GABA(A) receptor system in the CB was studied by measuring CB neural output using in vitro perfusion setup. Two BZs, midazolam and diazepam, decreased the CB neural response to hypoxia. With continuous application of bicuculline, a GABA(A) receptor antagonist, the effects of BZs were abolished. In conclusion, the GABA-GABA(A) receptor system is functioning in the CB chemosensory system. BZs inhibit CB neural response to hypoxia by enhancing GABA(A) receptor activity.

The impact of hypoxia and low glucose on the release of acetylcholine and ATP from the incubated cat carotid body

The carotid body (CB) is a polymodal sensor which increases its neural output to the nucleus tractus solitarii with a subsequent activation of several reflex cardiopulmonary responses. Current reports identify acetylcholine (ACh) and adenosine triphosphate (ATP) as two essential excitatory neurotransmitters in the cat and rat CBs. This study explored the impact of hypoxia, low glucose, and the two together on the release of both ACh and ATP from two incubated cat CBs. The CBs were prepared with standard procedures in accordance with the policies and regulations of the Institutional Animal Care and Use Committee. When normalized to their controls, a significant increase of ACh in the incubation medium was measured in response to hypoxia, low glucose, and the combined stimuli. When normalized to their controls, a significant increase in ATP in the incubation medium was measured in response to hypoxia and to the combined stimuli. Low glucose generated an increase in ATP which was not statistically significant (P>0.05). Second, normalizing the initial 3-4 or 2-3 min Time Segment of the challenge Stage to the final 3-4 or 2-3 min Time Segment of the control Stage for both ACh and ATP generated significant increases in response to hypoxia, low glucose (ACh only), and the combined stimuli. The data suggested the possibility that in the cat the increased CB neural output in response to low glucose might be due primarily to ACh.

Behavioral and respiratory characteristics during sleep in neonatal DBA/2J and A/J mice

The ventilatory response to hypoxia depends on the carotid body function and sleep-wake states. Therefore, the response must be measured in a consistent sleep-wake state. In mice, EMG with behavioral indices (coordinated movements, CMs; myoclonic twitches, MTs) has been used to assess sleep-wake states. However, in neonatal mice EMG instrumentation could induce stress, altering their behavior and ventilation. Accordingly, we examined: (1) if EMG can be eliminated for assessing sleep-wake states; and (2) behavioral characteristics and carotid body-mediated respiratory control during sleep with EMG (EMG+) or without EMG (EMG-). Seven-day-old DBA/2J and A/J mice were divided into EMG+ and EMG- groups. In both strains, CMs occurred when EMG was high; MTs were present during silent/low EMG activity. The durations of high EMG activity and of CMs were statistically indifferent. Thus, CMs can be used to indicate wake state without EMG. The stress caused by EMG instrumentation may be distinctively manifested based on genetic background. Prolonged agitation was observed in some EMG+ DBA/2J (5 of 13), but not in A/J mice. The sleep time and MT counts were indifferent between the groups in DBA/2J mice. The EMG+ A/J group showed longer sleep time and less MT counts than the EMG- A/J group. Mean respiratory variables (baseline, hyperoxic/hypoxic responses) were not severely influenced by EMG+ in either strain. Individual values were more variable in EMG+ mice. Carotid body-mediated respiratory responses (decreased ventilation upon hyperoxia and increased ventilation upon mild hypoxia) during sleep were clearly observed in these neonatal mice with or without EMG instrumentation.

Oxygen sensing in the carotid body and its relation to heart failure

This brief review first touches on the origins of the earth's oxygen. It then identifies and locates the principal oxygen sensor in vertebrates, the carotid body (CB). The CB is unique in that in human subjects, it is the only sensor of lower than normal levels in the partial pressure of oxygen (hypoxia, HH). Another oxygen sensor, the aortic bodies, are mostly vestigial in higher vertebrates. At least they play a much smaller role than the CB. In such an important role, the many reflexes in response to CB stimulation by HH are presented. After briefly reviewing what CB stimulation does, the next topic is to describe how the CB chemotransduces HH into neural signals to the brain. Several mechanisms are known, but critical steps in the mechanisms of chemosensation and chemotransduction are still under investigation. Finally, a brief glance at the operation of the CB in chronic heart failure patients is presented. Specifically, the role of nitric oxide, NO, is discussed.

A search for genes that may confer divergent morphology and function in the carotid body between two strains of mice

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

Role of acetylcholine in neurotransmission of the carotid body

Acetylcholine (ACh) has been considered an important excitatory neurotransmitter in the carotid body (CB). Its physiological and pharmacological effects, metabolism, release, and receptors have been well documented in several species. Various nicotinic and muscarinic ACh receptors are present in both afferent nerve endings and glomus cells. Therefore, ACh can depolarize or hyperpolarize the cell membrane depending on the available receptor type in the vicinity. Binding of ACh to its receptor can create a wide variety of cellular responses including opening cation channels (nicotinic ACh receptor activation), releasing Ca(2+) from intracellular storage sites (via muscarinic ACh receptors), and modulating activities of K(+) and Ca(2+) channels. Interactions between ACh and other neurotransmitters (dopamine, adenosine, nitric oxide) have been known, and they may induce complicated responses. Cholinergic biology in the CB differs among species and even within the same species due to different genetic composition. Development and environment influence cholinergic biology. We discuss these issues in light of current knowledge of neuroscience.