The impact of PCO2 and H+ on the release of acetylcholine from the cat carotid body

The carotid body (CB) is a sensor of oxygen, carbon dioxide, hydrogen ion, and glucose in the arterial blood. Many studies of the CB's responses to low oxygen (hypoxia) have been reported. Recently attention has been increasingly focused on its responses to elevated CO2 (hypercapnia). An increase in ventilation or carotid body neural output (CBNO) can result from stimulating the CB with blood or perfusion fluids having an elevated CO2 or H+. The increase in ventilation seen with a hypoxic stimulus is accompanied with an increase in CBNO and an increased release of both acetylcholine (ACh) and ATP from the CB. The present in vitro study using both CBs harvested from six cats was undertaken to determine if hypercapnia also provoked an increased release of ACh from the incubated CBs. The anesthetizing, handling, and euthanizing of the animals were according to the guidelines of the Johns Hopkins Animal Care and Use Committee which are totally consonant with those of the NIH. CBs, once harvested and prepared for the experimental protocol, were subjected to the following steps each lasting 10 min: (1) control; (2) stress; (3) recovery. The stresses were respiratory acidosis (RAC; acidic hypercapnia), compensated respiratory acidosis (CRAC; isohydric hypercapnia), and metabolic acidosis (MtAC). The first and last forms of acidosis generated small but significant increases in the release of ACh from the CBs; the second generated a very small and insignificant increase in ACh release. Since it is generally accepted that ACh is a key excitatory neurotransmitter in the CB along with ATP, these data are consistent with other studies measuring the increase in ventilation in response to a small increase in CO2 and those studies recording CBNO in response to hypercapnia. In five of the six animals the responses to RAC and MtAC were compared to the responses to hypoxia. The latter were statistically indistinguishable from the former two.

Genetic background affects cardiovascular responses to obstructive and simulated apnea

We have recently demonstrated that genetic background significantly impacts the blood pressure and heart rate response to hypoxia (Campen MJ, Tagaito Y, Li J, Balbir A, Tankersley CG, Smith P, Schwartz A, and O'Donnell CP. Physiol Genomics 20: 15-20, 2005). Because hypoxia is considered a mediator of the acute and chronic cardiovascular complications of obstructive sleep apnea, we investigated whether genetic factors also influence the cardiovascular response to experimentally induced obstructive apnea (OA) and simulated apnea (SA). In three strains of inbred mice (C57BL/6J, DBA/2J, and FVB/J) anesthetized with urethane (1.2 g/kg), apnea was induced at end-expiration for 5- and 10-s periods in spontaneously breathing (OA) and mechanically ventilated (SA; pancuronium, 0.2 mg/kg bolus + 0.003 mg.kg(-1).min(-1)) animals before and after administration of an autonomic ganglionic blocker (hexamethonium, 20 mg/kg). In contrast to our previous findings with hypoxia, OA produced a marked hypertensive response in all three strains. However, strain impacted on the degree of bradycardia during OA, which was large in C57BL/6J and FVB/J mice and effectively absent in DBA/2J mice. In C57BL/6J but not FVB/J mice, the bradycardia was abolished with SA under mechanical ventilation. Cardiovascular responses to SA in all strains were eliminated by autonomic blockade. These data show that 1) DBA/2J mice, in contrast to the previous demonstration of marked bradycardia during hypoxia, unexpectedly do not produce bradycardia during apnea; 2) C57BL/6J mice exhibit a bradycardia that is dependent on input from thoracic afferents; and 3) FVB/J mice exhibit a bradycardia despite the loss of thoracic afferent input, consistent with a potent pressure response eliciting a baroreceptor-mediated bradycardia. Thus genetic background can affect both the pattern and magnitude of the cardiovascular response to apnea.

The effect of a nitric oxide donor, sodium nitroprusside, on the release of acetylcholine from the in vitro cat carotid body

The purpose of the present study was to determine the impact of a nitric oxide (NO) donor, sodium nitroprusside (SNP), on the release of acetylcholine (ACh), an essential excitatory neurotransmitter, from the in vitro cat carotid body (CB). Bilateral CBs were harvested from five deeply anesthetized cats according to the regulations contained in the policies of the Johns Hopkins Animal Care and Use Committee. After recovering from the surgical procedures for extraction and cleaning, the CBs were taken through a 15-step protocol in which they were exposed to a hyperoxic gas mixture (40% O2/5% CO2; 20 min), then a hypoxic gas mixture (6% O2/5% CO2; 20 min), and a final 10 min hyperoxic mixture. This sequence was applied twice, followed by the same sequence in the presence, first, of 5 microM SNP, and secondly in the presence of 10 microM SNP. After washing and a recovery period the CBs were again exposed to the gases as in the first two non-SNP trials. The SNP exposures significantly reduced the overall release of ACh by about 20% (P=0.039). Further, SNP significantly reduced the hypoxia-induced increase in ACh release (without SNP: 82.4+/-19.1 fmol/20 microL versus with SNP: 49.7+/-15.0 fmol/20 microL; mean+/-S.E.M.; P=0.032). Trials #1 and #2 which preceded the application of SNP and Trial #3 which followed SNP were statistically indistinguishable. The CBs had recovered their original status. The data support the hypothesis that the frequently reported NO-induced reduction in CB neural output during hypoxia is at least in part due to the reduction in ACh release. The results are consistent with a previous report in which l-arginine, an NO precursor, had the same reducing effect. Possible mechanisms are discussed.

l-arginine's effect on the hypoxia-induced release of acetylcholine from the in vitro cat carotid body

NO is known to reduce the hypoxia-induced increase in carotid body neural activity (CBNA). Acetylcholine (ACh), a known excitatory transmitter in the cat carotid body (CB), is released during hypoxia. This study addressed the impact of an NO precursor on ACh release during hypoxia. Both CBs from nine cats were prepared for incubation, then inserted into a medium and bubbled with three consecutive gas mixtures, hyperoxic, hypoxic, and a final hyperoxic mixture. This series of exposures was performed in the absence of L-arginine, followed by the three exposures in a 1mM L-arginine medium, and followed, thirdly, in a 10mM L-arginine medium. L-Arginine significantly attenuated the hypoxia-induced release of ACh. Two post-arginine procedures suggested strongly that the reduction in the ACh release was not due to a gradual exhaustion of carotid body ACh stores over the course of the experiment. The data are consistent with those reports showing that NO donors and precursors reduce the hypoxia-induced increase in CBNA, and further support a role for ACh in the hypoxia-induced increase in CBNA.

The impact of adenosine on the release of acetylcholine, dopamine, and norepinephrine from the cat carotid body

Exogenously administered adenosine provokes an increase in respiration in both animal models and in man. Administered near the carotid body adenosine increases neural output from the carotid body in rats and cats. Hypoxia has the same effect. Hypoxia also provokes a release of acetylcholine (ACh), dopamine (DA), and norepinephrine (NE) from the carotid body. The present study aimed to determine the effect of exogenous adenosine on the release of ACh, DA, and NE from the carotid bodies of cats. After a recovery period (from surgery) carotid bodies were first incubated for 10 (DA, NE) or 15 (ACh) min in Eppendorf tubes containing 85 microL of a physiological salt solution equilibrated with 40% O2/5% CO2 at 37 degrees C (hyperoxia). At the end of the incubation period the medium was drawn off, and measured for ACh, DA, and NE using HPLC-ECD methods. Next 85 microL of the medium and the tubes were equilibrated with a hypoxic gas mixture (4% O2/5% CO2) and the carotid bodies were incubated for 10 (DA, NE) or 15 (ACh) min, at the end of which the medium was drawn off and measured for ACh, DA, and NE. In the ACh studies there followed a post-hypoxic hyperoxic exposure (40% O2/5% CO2). ACh tubes were then made 100 microM with respect to adenosine, and the hyperoxic, hypoxic, and post-hypoxic hyperoxic challenges were repeated. One of the two DA, NE tubes had the 100 microM adenosine from the start. Adenosine significantly increased the release of ACh, but significantly decreased the hypoxia-induced release of DA. Potential mechanisms for these changes are reviewed.

Patch clamp study of mouse glomus cells using a whole carotid body

Some electrophysiological characteristics of mouse glomus cells (DBA/2J strain) were investigated using an undissociated carotid body. The carotid body with major carotid arteries was placed in a recording chamber, and glomus cells were visualized with a water immersion lens combined with an infrared differential interference video camera. Patch clamp experiments revealed that voltage-gated outward current, but not inward current, was easily observed in glomus cells. Pharmacological experiments and the kinetics of the current suggest that outward current is via delayed rectifier, A type, and large conductance calcium-activated K channels. Furthermore, K current was reversibly attenuated by mild hypoxia. The results suggest electrophysiological similarities of glomus cells among the cat, the rat, and the DBA/2J mouse. The method appears useful for physiological experiments.

Identification of M1 and M2 muscarinic acetylcholine receptors in the cat carotid body chemosensory system

The carotid body is a major arterial chemoreceptor that senses low O2 tension, high CO2 tension and low pH in the arterial blood. It is generally believed that neurotransmitters, including acetylcholine (ACh), participate in the genesis of afferent neural output from the carotid body and modulate the function of chemoreceptor cells (glomus cells). Previous pharmacological studies suggest that M1 and M2 muscarinic ACh receptors (mAChRs) are involved in these processes. This study was designed to demonstrate the presence and localization of M1 and M2 mAChRs in the carotid body and in the petrosal ganglion of the cat. Since DNA sequences of the cat M1 and M2 mAChRs were not known, we first determined partial DNA sequences. These sequences and deduced amino acid sequences highly resembled those of human and the rat. Subsequent reverse transcription-polymerase chain reaction (RT-PCR)analysis has demonstrated that mRNAs for M1 and M2 mAChRs are present in the carotid body and the petrosal ganglion of the cat. Immunohistochemistry has indicated that the localization of these receptors appears different. Immunoreactivity for M1 mAChR was strong in nerves in the carotid body. Nerve endings positively stained for M1 mAChR appear to innervate glomus cells. Weak staining for M1 mAChRs was seen in glomus cells. On the other hand, M2 receptor protein seems to be present in glomus cells but not on nerve endings. One third of the neurons in the petrosal ganglion showed immunoreactivity for M1 mAChR. Many neurons and nerve fibers in the petrosal ganglion expressed M2 mAChR immunoreactivity. The results were consistent with previous pharmacological studies. Thus, activation of M1 mAChRs on afferent nerve endings may be linked to the increase in neural output during hypoxia. Further, M1 and M2 mAChRs on glomus cells modulate the release of neurotransmitters.

Differences in Sleep-induced Hypoxia between A/J and DBA/2J Mouse Strains

In obstructive sleep apnea, hypoxic ventilatory sensitivity may affect the degree of hypoxic stress and sleep disruption that occurs in response to upper airway obstruction. We induced (1) sleep-induced hypoxia (SIH) or (2) sleep fragmentation (SF) without hypoxia for 5 days (12-hour light/dark cycle) in two inbred mouse strains with low (A/J) and high (DBA/2J) hypoxic ventilatory sensitivities. During SIH, the time to arousal (26.4 +/- 1.1 vs. 21.3 +/- 1.5 seconds, p<0.025) and the severity of hypoxic exposure (nadir FIO2: 11.5 +/- 0.4 vs. 13.6 +/- 0.1%, p<0.002) was greater in A/J than DBA/2J mice. Furthermore, A/J mice had a greater frequency of hypoxic events (640 +/- 29 vs. 368 +/- 33 events per 24 hours, p<0.001) and total sleep time (47.5 +/- 2.8% vs. 26.5 +/- 2.4% per 24 hours, p<0.0001) during SIH than DBA/2J mice. In contrast, the event characteristics and total sleep time during SF were the same in both strains. Furthermore, in the light phase, both strains showed a longer (p<0.01) time to arousal during SIH and SF compared with the dark phase. We conclude that genetic background can influence respiratory events and sleep architecture during SIH and that the arousal threshold is subject to circadian variation. Our data imply that individuals with low hypoxic sensitivity may be at a greater risk for hypoxia-related complications of obstructive sleep apnea.

Characterization of nicotinic acetylcholine receptors in cultured arterial chemoreceptor cells of the cat

Neurotransmitters appear to be involved in chemotransmission of the carotid body, a major arterial chemoreceptor. Substantial data indicate that acetylcholine (ACh) is an excitatory neurotransmitter in the carotid body, regulating the excitability of afferent nerve endings and glomus cells (putative chemoreceptor cells). In this study we characterized properties of nicotinic ACh receptors (nAChRs) in cultured cat glomus cells using immunocytochemistry and whole cell patch clamp techniques. Cultured glomus cells expressed immunoreactivity for alpha3, alpha4, and beta2 subunits of nAChRs. An application of ACh elicited inward current. Nicotinic AChRs of glomus cells showed high affinity for ACh. The current-voltage relationship showed strong inward rectification at positive membrane potential. alpha-Conotoxin MII (20 nM), dihydro-beta-erythroidine (DHbetaE; 1 nM), and hexamethonium (300 microM) significantly inhibited ACh-induced current. These results indicate that cultured cat glomus cells possess functional nAChRs, and that their characteristics are consistent with those of alpha3, alpha4 and beta2 containing nAChRs.

Structural and functional differences of the carotid body between DBA/2J and A/J strains of mice

In a previous study, DBA/2J and A/J inbred mice showed extremely different hypoxic ventilatory responses, suggesting variations in their carotid bodies. We have assessed the morphological and functional differences of the carotid bodies in these mice. Histological examination revealed a clearly delineated carotid body only in the DBA/2J mice. Many typical glomus cells and glomeruli appeared in the DBA/2J but not in the A/J mice. The size of the carotid body in the DBA/2J and A/J mice was 6.3 +/- 0.5 x 10(6) and 1.5 +/- 0.3 x 10(6) micro m(3), respectively. The area immunostained for tyrosine hydroxylase, an estimation of the glomus cell quantity, was four times larger in the DBA/2J mice than in the A/J mice. The individual data points in the DBA/2J mice segregated from those in the A/J mice. ACh increased intracellular Ca(2+) in most clusters (81%) of cultured carotid body cells from the DBA/2J mice, but only in 18% of clusters in the A/J mice. These data suggest that genetic determinants account for the strain differences in the structure and function of the carotid body.