A. Mokashi

The primary oxygen sensor of the cat carotid body is cytochrome a3 of the mitochondrial respiratory chain

Carbon monoxide was shown to be competitive with O2 in oxygen sensing by perfused carotid bodies isolated from cats, afferent electrical activity increasing with either decreasing O2 or increasing CO. The CO‐induced increase in afferent activity was fully reversed by bright light. At submaximal light intensities the extent of reversal, after correcting to equal light intensity of light quanta at each wavelength, was maximal for light of 432 ± 2 and 590 ± 2 nm, with a ratio (432/590) of approximately 6. This spectrum is characteristic of the CO compound of mitochondrial cytochrome a 3. The photo‐reversible inhibition of oxygen sensing activity by CO accounts for at least 80% of the oxygen chemosensory activity of the carotid body.

Mice lacking in gp91 phox subunit of NAD(P)H oxidase showed glomus cell [Ca2+]i and respiratory responses to hypoxia

The hypothesis that NAD(P)H oxidase may serve as an oxygen sensor was tested using the mice deficient (knock-out) in gp91phox subunit of NAD(P)H oxidase enzyme complex and compared with wild-type (C57BL/6J) strain measuring the ventilatory and glomus cell intracellular calcium ([Ca(2+)](i)) responses of carotid body to hypoxia. The hypoxic ventilatory responses as well as the [Ca(2+)](i) were preserved in the NAD(P)H oxidase knock-out mice. NAD(P)H oxidase, though a major source of oxygen radical production, is not the oxygen sensor in mice carotid body.

Nitric oxide-related inhibition of carotid chemosensory nerve activity in the cat.

The hypothesis that endogenous nitric oxide may play a physiological role in the regulation of carotid chemosensory activity was tested in this study. The nitric oxide synthase (NOS) inhibitors, L-nitro-arginine-methyl ester (L-NAME, 25-200 microM) and NG-monomethyl-L-arginine acetate (L-NMMA, 50 and 100 microM) were used to study its effects on the chemosensory activity of perfused and superfused cat carotid bodies (n = 21) in vitro at 37-37 degrees C. L-NAME elicited slow excitation of the sensory activity as did L-NMMA. The peak-response was dose-dependent, and approached saturation around 200 microM. The excitation by L-NAME showed the following characteristics (mean +/- SEM): latency of response, 2.2 min +/- 0.3 min; time to peak response, 5.5 min +/- 1.0 min and the peak response increased to 407 +/- 42 imp/sec from 88 +/- 13 imp/sec. The peak response was significantly different (P < 0.05) from the baseline activity. L-arginine (50-500 microM) only briefly reversed the stimulation. Hypoxia enhanced the excitation by L-NAME. On the other hand, sodium nitroprusside (SNP, 0.5-10 microM) which supplies NO, terminated the excitatory effect of L-NAME. The results provide evidence in favor of an inhibitory role of endogenous NO in the carotid body, and exogenous application of NO confirms the inhibitory effect.

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.

Peripheral and central dopamine receptors in respiratory control

The role of peripheral and central dopaminergic mechanisms in respiratory control was studied in anesthetized cats. In one series, we simultaneously measured carotid chemoreceptor and ventilatory responses to hypoxia and hypercapnia before and after a saturation dose of intravenous domperidone, a peripheral dopamine (D2) receptor antagonist. Both carotid chemoreceptor and ventilatory responses were augmented by domperidone essentially in proportion, suggesting that they reflected the increase of peripheral chemoreceptor activity. Haloperidol which crosses into the brain from blood, given subsequent to domperidone, did not further affect carotid chemoreceptor responses but attenuated ventilatory responses to hypoxia without significantly altering those to hypercapnia. Thus, the additional ventilatory effect of haloperidol is mediated through central dopaminergic mechanisms involving peripheral chemoreflex pathway alone. In another series, the anesthetized cats were paralyzed and artificially ventilated to study carotid chemoreceptor responses to step increases in the end-tidal PCO2 before and after domperidone. Domperidone significantly augmented the responses to CO2. The results support the hypothesis that both peripheral and central dopaminergic mechanisms play a significant modulatory role in chemoreflex respiratory control.

K+-current modulated by PO2 in type I cells in rat carotid body is not a chemosensor

According to the membrane channel hypothesis of carotid body O2 chemoreception, hypoxia suppresses K+ currents leading to cell depolarization, [Ca2+]i rise, neurosecretion, increased neural discharge from the carotid body. We show here that tetraethylammonium (TEA) plus 4-aminopyridine (4-AP) which suppressed the Ca2+ sensitive and other K+ currents in rat carotid body type I cells, with and without low [Ca2+]o plus high [Mg2+]o, did not essentially influence low PO2 effects on [Ca2+]i and chemosensory discharge. Thus, hypoxia may suppress the K+ currents in glomus cells but K+ current suppression of itself does not lead to chemosensory excitation. Therefore, the hypothesis that K+-O2 current is linked to events in chemoreception is not substantiated. K+-O2 current is an epiphemenon which is not directly linked with O2 chemoreception.

Reciprocal photolabile O2 consumption and chemoreceptor excitation by carbon monoxide in the cat carotid body: evidence for cytochrome a3 as the primary O2 sensor

High carbon monoxide (CO) gas tensions (> 500 Torr) at normoxic PO2 (125-140 Torr) stimulates carotid chemosensory discharge in the perfused carotid body (CB) in the absence but not in the presence of light. According to a metabolic hypothesis of O2 chemoreception, the increased chemosensory discharge should correspond to a photoreversible decrease of O2 consumption, unlike a non-respiratory hypothesis. We tested the respiratory vs. non-respiratory hypotheses of O2 chemoreception in the cat CB by measuring the effect of high CO. Experiments were conducted using CBs perfused and superfused in vitro with high CO in normoxic, normocapnic cell-free CO2-HCO3- buffer solution at 37 degrees C. Simultaneous measurements of the rate of O2 disappearance with recessed PO2 microelectrodes and chemosensory discharge were made after flow interruption with and without CO in the perfusate. The control O2 disappearance rate without CO was -3.66 +/- 0.43 (S.E.) Torr/s (100 measurements in 12 cat CBs). In the dark, high CO reduced the O2 disappearance rate to -2.35 +/- 0.33 Torr/s, or 64.2 +/- 9.0% of control (P < 0.005, 34 measurements). High CO was excitatory in the dark, with an increase in baseline neural discharge from 129.2 +/- 47.0 to 399.3 +/- 49.1 impulses per s (P < 0.0001), and maximum discharge rate of 659 +/- 76 impulses/s (N.S. compared to control) during flow interruption. During perfusion with high CO in the light, there were no significant differences in baseline neural discharge or in the maximum neural discharge after flow interruption, and little effect on O2 metabolism (88.8 +/- 11.5% of control, N.S., 29 measurements).(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between sympathetic and phrenic nerve responses to peripheral chemoreflex in the cat

To test the hypothesis that peripheral chemoreflex effect on the preganglionic cervical sympathetic nerve (PSN) activity is entirely mediated by the central respiratory drive (CRD), as expressed in the phrenic nerve (PN) activity, we studied the relationship between PN and PSN activities under controlled conditions of carotid chemosensory excitation in the anesthetized cats. The cats were vagotomized, paralyzed and artificially ventilated. Tracheal pO2 and pCO2, systemic blood pressure, activities of single or a few PSN and PN fibers and a PN bundle were simultaneously recorded. The PSN preparations, which were responsive to hypoxia and showed PN rhythm, were selected for the study. Carotid chemoreceptor excitation, produced by hypoxia (end-tidal pO2 approximately equal to 50 Torr) or by sodium cyanide injection (50-100 micrograms, i.v.), elicited 3 types of responses: (1) the PSN discharged during the silent period of PN activity, although the PSN peak activity was still coupled to the PN peak activity, (2) PSN discharged only during PN activity, and (3) during the absence of PN discharge induced by hyperventilation hypocapnia, cyanide injection stimulated the PSN without PN activity. These observations suggest a model of chemoreflex control of sympathetic nerve activity which consists of two parts: one is dependent on PN activity and the other is not. Accordingly, all PSN chemoreflex responses may not be integrated with all inspiratory chemoreflexes.

Nitric Oxide Synthase Occurs in Neurons and Nerve Fibers of the Carotid Body

Nitric oxide (NO), a free radical, can be produced by vascular endothelial cells and certain neurons of the central and peripheral nervous systems upon activation of its biosynthetic enzyme, nitric oxide synthase (NOS). NO may function as a non-conventional neurotransmitter in the nervous system, and either NO or an NO-producing compound is believed to be the endothelium-derived relaxing factor of the vascular system. An antiserum recognizing the molecular form of NOS present in brain has been used to localize the enzyme in brain and peripheral tissues (Bredt et al., 1990). NOS containing neurons also display intense NADPH-diaphorase activity (Dawson et al., 1991; Hope et al., 1991), and this histochemical reaction, though less specific than immunohistochemistry, provides a label for NOS activity in both central and peripheral neurons.

Cat carotid body chemosensory discharge (in vitro) is insensitive to charybdotoxin

Charybdotoxin (ChTX), a venom protein, suppresses Ca2+-activated K+ (K+(Ca)) currents in the glomus cell of neonatal rat carotid body. If it works similarly for cat carotid body chemoreceptors, charybdotoxin is expected to stimulate the chemosensory discharge during normoxia, and particularly hypoxia and hypercapnia. We studied the effects of charybdotoxin (20-40 nM) in vitro (perfused/superfused) on the cat carotid chemosensory discharge, and simultaneously tissue PO2 (PtiO2), as a measure of positive control. ChTX (20 nM) only increased PtiO2 and decreased carotid chemosensory discharge during hypoxia, indicating vasodilation. We conclude that K+(Ca) channels do not appear to play a significant role in chemotransduction in the cat carotid body.