The influence of the sympathetic outflow on aortic chemoreceptors of the cat during hypoxia and hypercapnia

Systemic Administration of Tempol, a Superoxide Dismutase Mimetic, Augments Upper Airway Muscle Activity in Obese Zucker Rats

Obstructive sleep apnea (OSA) is characterized by repetitive partial/complete collapse of the pharynx during sleep, which results in apnea/hypopnea leading to arterial oxygen desaturations and arousals. Repetitive apnea/hypopnea-arousal episodes cause hypoxia/reoxygenation cycles, which increase free radical generation and oxidative stress that cause motor/sensory nerve impairments and muscle damage. We hypothesize that antioxidants may protect and/or reverse from oxidative stress-induced damage in OSA patients. To understand the acute protective effects of antioxidants on respiratory muscles, we studied the systemic effects of a membrane permeable superoxide dismutase mimetic, Tempol, on genioglossus (EMG_GG) and diaphragmatic (EMG_DIA) electro-myographic activities, hypoglossal motoneuron (HMN) nerve activity and cardiorespiratory parameters (mean arterial blood pressure, heart rate) in adult isoflurane-anesthetized obese Zucker rats (OZR) and age-matched lean Zucker rats (LZR). Tempol dose-dependently (1-100 mg/kg) increased EMG_GG without changing EMG_DIA in OZR and LZR. Tempol increased respiratory rate and tidal volume in OZR and LZR. Tempol (1-25 mg/kg) dose-dependently increased HMN nerve activity in healthy Sprague Dawley rats. Tempol (100 mg/kg) increased EMG_GG output by 189% in OZR and 163% in LZR. With respect to mechanisms of effect, Tempol (100 mg/kg) did not augment EMG_GG after bilateral HMN transection in Sprague Dawley rats. Although future studies are warranted, available data suggest that in addition to its antioxidant and antihypertensive properties, Tempol can selectively augment EMG_GG through modulating HMN and this effect may prevent collapsibility and/or improve stability of the upper airway pharyngeal dilator muscles during episodes of partial and/or complete collapse of the upper airway in OSA human subjects.

The discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input

In order to determine whether serotonergic cells in the medullary raphe magnus (RM) receive baroreceptor input, cells were tested for their responses to descending aortic occlusion, aortic nerve stimulation, or systemic phenylephrine administration in the lightly anesthetized rat. Serotonergic cells were identified physiologically by a quantitative analysis of their slow and steady discharge. Greater than 40% of the serotonergic RM cells tested responded to brief occlusion of the descending aorta at the level of the coeliac arteries, a stimulus that elevated blood pressure by about 30 mmHg. Similarly, about 40% of the serotonergic RM cells responded to stimulation of the aortic nerve, a nerve that contains primarily baroreceptor afferents from the aortic arch. Greater than 70% of RM serotonergic cells responded to phenylephrine administration which elevated blood pressure by an average of 50 mmHg. Serotonergic cell responses to all methods of baroreceptor activation were small in magnitude and were largely restricted in time to the stimulus duration. The results indicate that a subset of serotonergic cells in RM are influenced by baroreceptor activity.

The present status of the mechanical hypothesis for chemoreceptor stimulation

Reasons are given to show why the transmitter based hypothesis for the stimulation of chemoreceptors needs to be reviewed. On the other hand evidence is presented to show that chemoreceptors can be stimulated by various mechanical stimuli and how the local PO2 can be sensed by the type II cell which by getting mechanically deformed causes this cell to shrink. This shrinkage is transmitted to the generator region of the nerve terminal thereby leading to the production of propagated impulses at the regenerative region thus making the whole process of generation of information about the local PO2, similar to the generation of sensory information by other sensory receptors.

Role of aortic chemoreceptors in the hypertensive response to cigarette smoke

To determine the role of aortic chemoreceptors in the rise of systemic blood pressure (BP) seen with smoking cigarettes, their responses to injecting various doses of nicotine (Nic) into the left atrium and to a puff of cigarette smoke were studied in 31 cats. The activity of 71% (n = 36) of the chemoreceptor fibres was stimulated significantly by a puff of cigarette smoke (delivering approximately 10.0 micrograms kg-1 Nic). The mean threshold dose to stimulation by nicotine of these fibres was 8.0 micrograms kg-1. The activity of the remaining 29% (n = 15) was not stimulated; their mean threshold dose to stimulation by Nic was 24 micrograms kg-1. Stimulation of fibres lasted for about 14 +/- 0.2 sec and the rise in BP was seen for 20-60 sec. By using hexamethonium it was established that the stimulation was produced exclusively by Nic contained in cigarette smoke. It was concluded that aortic chemoreceptors must contribute to the reflex rise in BP produced by smoking cigarettes.

Reflex stimulation of aortic chemoreceptors through the stellate ganglion during hypoxia and hypotension in cats

1. In cats anaesthetized with sodium pentobarbitone (35 mg kg-1), an investigation was made of the mechanisms underlying the enhancement of aortic chemoreceptor activity through the excitatory effect mediated by the sympathetic outflow to them during hypoxia and hypotension. 2. Removal of the right stellate ganglion significantly (P < 0.01) reduced the responses of the chemoreceptors to hypoxia and hypotension. Injection of a bolus of sodium pentobarbitone reduced the responses before, but not after, stellectomy. This indicated that the excitatory sympathetic influence on the chemoreceptors (attributed to glomeral vasoconstriction leading to reduction in blood flow) is exerted entirely through the right stellate ganglion. This excitatory effect was abolished after cutting the vagi and both carotid sinus nerves. 3. It is concluded that, in addition to the direct excitatory effect of hypoxia on aortic chemoreceptors, a reflex chemoreceptor excitatory component is mediated through the vagi, carotid sinus nerves and right stellate ganglion.

Effect of halothane and isoflurane on in situ diameter responses of small mesenteric veins to acute graded hypercapnia

The purpose of the present study was to quantify the inhibitory effect of inhaled halothane and isoflurane on acute hypercapnia-induced responses of capacitance-regulating veins and related cardiovascular variables in response to sequential 40-s periods of 5%, 10%, 15%, and 20% inspired CO2 (FICO2). Measurements were made in normoxic alpha-chloralose-anesthetized rabbits before, during, and after either 0.75 minimum alveolar anesthetic concentration inhaled halothane or isoflurane. The graded hypercapnia caused graded venoconstriction and bradycardia but minimal pressor responses. Hypercapnia-induced venoconstriction was blocked by prior local superfusion of the exposed veins with 3 x 10(-6) M tetrodotoxin. Both the hypercapnia-induced venoconstriction and bradycardia responses were significantly attenuated by halothane or isoflurane and did not fully recover after removal of the anesthetics from the circulation. Both anesthetics produced a significant baseline (i.e., prehypercapnia) hypotension and a tendency toward a resultant tachycardia. The baseline hypotension did not recover completely after elimination of the anesthetic. Neither anesthetic altered baseline vein diameter. These results agree with previous studies demonstrating that hypercapnic acidosis produces mesenteric venoconstriction by elevating excitatory sympathetic efferent neural input via activation of peripheral and central chemoreceptors and that bradycardia results from activation of compensatory baroreflexes. The neural components of these reflexes are possible primary sites for attenuation of these cardiovascular responses by halothane and isoflurane.

Ganglioglomerular nerves influence responsiveness of cat carotid body chemoreceptors to almitrine

A bolus injection of almitrine bismesylate (0.5 mg.kg-1 i.v.) in anaesthetised artificially ventilated cats caused a significantly greater increase in carotid chemosensory discharge in animals with sectioned ipsilateral ganglioglomerular sympathetic nerves in comparison with a group in which these nerves were intact. Plasma levels of almitrine were similar in both groups. Responses to hypoxia and hypercapnia post-almitrine were also bigger if the ganglioglomerular nerves were cut. Domperidone (10-50 micrograms.kg-1 i.a), a dopamine D2 receptor antagonist, greatly increaed the responsiveness of chemoreceptors to almitrine in ganglioglomerular nerve-intact preparations. Almitrine-induced chemosensory activity was unaffected by illuminating the carotid bifurcation with light from a fibre optic lamp, regardless of whether or not the ganglioglomerular nerves were cut. It is concluded that almitrine may directly or indirectly activate an efferent pathway in the ganglioglomerular nerves to cause depression of chemoreceptor activity, possibly by releasing dopamine to act at D2 dopamine receptors in the carotid body.