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

Sympathetic baroreceptor regulation during hypoxic hypotension in humans: new insights

BACKGROUND

Baroreceptor activation by a continuous infusion of phenylephrine selectively abolishes the muscle sympathetic nerve activity (MSNA) response to hypoxia in humans. Baroreceptor deactivation enhances the MSNA rise during hypoxia in animals. Whether this is true in humans is unknown and was tested in the present study.

METHODS

We assessed MSNA responses elicited by isocapnic hypoxia (10% O2 in N2) during baroreflex loading and unloading with phenylephrine and nitroprusside, respectively, in 19 healthy volunteers. The study was randomized and placebo-controlled.

RESULTS

Phenylephrine and nitroprusside increased and decreased, respectively, blood pressure during normoxia and hypoxia, whereas the reverse occurred for heart rate and MSNA (all P < 0.001 vs. placebo). As compared with normoxia, cardiac barosensitivity decreased during the infusion of placebo and nitroprusside in the presence of hypoxia, as well as sympathetic barosensitivity during the infusion of nitroprusside (all P < 0.05). Three patients even disclosed a reduction in arterial pressure, which became apparent at the third minute of hypoxia and worsened steadily thereafter (SBP: 91 ± 7 mmHg; DBP 47 ± 9 mmHg), in spite of a gradual rise in heart rate of 20 ± 4 bpm. Changes in baroreceptor loading conditions did not affect ventilation during normoxia and hypoxia.

CONCLUSION

Cardiac and sympathetic baroreceptor sensitivity decrease during baroreceptor unloading in the presence of peripheral chemoreceptor activation. Normal humans have limited reflex capabilities to sustain simultaneous reductions in oxygen and pressure, and may experience hemodynamic instability episodes in such condition.

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.

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.

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.

Nitric oxide induces tau hyperphosphorylation via glycogen synthase kinase-3beta activation

Nitric oxide is associated with neurofibrillary tangle, which is composed mainly of hyperphosphorylated tau in the brain of Alzheimer's disease (AD). However, the role of nitric oxide in tau hyperphosphorylation is unclear. Here we show that nitric oxide produced by sodium nitroprusside (SNP), a recognized donor of nitric oxide, induces tau hyperphosphorylation at Ser396/404 and Ser262 in HEK293/tau441 cells with a simultaneous activation of glycogen synthase kinase-3beta (GSK-3beta). Pretreatment of the cells with 10 mM lithium chloride (LiCl), an inhibitor of GSK-3, 1 h before SNP administration inhibits GSK-3beta activation and prevents tau from hyperphosphorylation. This is the first direct evidence demonstrating that nitric oxide induces AD-like tau hyperphosphorylation in vitro, and GSK-3beta activation is partially responsible for the nitric oxide-induced tau hyperphosphorylation. It is suggested that nitric oxide may be an upstream element of tau abnormal hyperphosphorylation in AD.