David J. Pallot

Catecholamines in the carotid body of several mammalian species: Effects of surgical and chemical sympathectomy

The catecholamine content of the carotid body of several mammalian species has been assayed using high performance liquid chromatography coupled to electrochemical detection and radioenzymatic assays. Although there were strain differences in the content of catecholamines in the carotid body of the rat, noradrenaline was equal to or exceeded the dopamine level in this species. No apparent differences were found in carotid bodies of animals killed by cervical dislocation or those dissected from anaesthetized animals. Noradrenaline concentrations were found to be substantially higher than those of dopamine in the cat and guinea-pig carotid body, though dopamine was the predominant amine in the rabbit and ferret. Unilateral superior cervical ganglionectomy or chemical sympathectomy with 6-hydroxydopamine substantially depleted noradrenaline without influencing dopamine in the rat carotid body. A marked selective reduction in noradrenaline was also observed in the rabbit and guinea-pig following ganglionectomy, though similar procedures in the cat failed to alter the levels of either catecholamine in the carotid body. The present data highlights the marked species variation in catecholamine content and the contribution to the latter by sympathetic innervation to this organ. This information will be useful in determining the species specificity regarding the relative roles of dopamine and noradrenaline in the modulation of chemoreceptor afferent discharge.

Biochemical studies on the release of catecholamines from the rat carotid body in vitro

The effects of hypoxia and carbachol on the release of newly synthesized catecholamines from superfused rat carotid bodies have been examined. Hypoxic superfusion medium was found to evoke catecholamine release which was dependent on the extracellular calcium concentration and was reduced by nitrendipine and atropine. Superfusion with the muscarinic agonist, carbachol, stimulated catecholamine release independently of the oxygen tension of the medium. The effect of carbachol on catecholamine release was abolished by atropine, suggesting that it was mediated by activation of cholinergic receptors of the muscarinic type. Both hypoxia and carbachol stimulated the release of 45Ca from carotid bodies prelabelled with 45Ca. The release of 45Ca with either stimulus was reduced by atropine and nitrendipine. These results suggest that although extracellular calcium plays an important role in the exocytotic secretory process of the carotid body, the mobilization of intracellular calcium pools may also contribute to the secretory response.

Division of Type I and Endothelial Cells in the Hypoxic Rat Carotid Body

The mammalian carotid body is enlarged under conditions of chronic hypoxaemia. There has been some discussion as to whether this is due to hypertrophy or to hyperplasia. We have subjected rats to 1, 2 or 7 days of 10% oxygen and, 4 h before removing the carotid bodies, injected each animal with vincristine sulphate, an inhibitor of mitosis. The results of this study indicate that numerous mitoses can be found in the carotid bodies of rats exposed to 10% oxygen, but not in control animals maintained in air. These experiments thus provide direct evidence that at least a proportion of the increase in size of the carotid body induced by chronic hypoxaemia is due to a cellular hyperplasia.

Biogenic amine-stimulated cyclic adenosine-3',5'-monophosphate formation in the rat carotid body.

Abstract: The subcutaneous injection of isoprenaline, salbutamol, histamine, and adrenaline to rats, which were subsequently killed by microwave irradiation, resulted in a rapid increase in the cyclic AMP content of the carotid body. On the other hand, noradrenaline, dopamine, adenosine, and 5‐hydroxytryptamine, at doses at least 100 times greater than that of isoprenaline, did not significantly alter the cyclic nucleotide content in vivo. The response to isoprenaline was dose related, with an ED50 of 15 μg · kg−1, and reached a peak level 1–1.5 min after injection. Incubation of intact carotid bodies with isoprenaline (10‐−5M)in vitro also resulted in a 10‐fold increase in cyclic AMP content. The in vivo response to isoprenaline could be blocked stereo‐selectively by propranolol, and ICI 118.551, a β2‐selective antagonist, blocks the isoprenaline‐elicited increase in cyclic AMP completely at a dose of 30 μg · kg−1; whereas betaxolol, a β1‐selective antagonist, was ineffective, even at a dose of 300 μg · kg−1. Hypoxia (5% oxygen in 95% N2) did not result in a significant increase in the cyclic AMP content, nor did it significantly alter the isoprenaline‐stimulated increase in the cyclic AMP content of the rat carotid body. These results suggest that some catecholamines may stimulate cyclic AMP formation by interacting with a β2‐adrenoceptor in the rat carotid body.

Immunohistochemical approach to the study of the cat carotid body

The mammalian carotid body contains a number of different cell types which are not always easy to identify in routine histological sections. We have devised a battery of immunohistochemical tests which overcome this difficulty and offer the possibility of performing routine morphometric analyses of the response of the organ to various pathological processes in paraffin-embedded sections. The type 1 cells can be identified on the basis of their reaction with neuronal specific enolase, whilst type II cells react with antibodies to S-100 protein. Schwann cells do not react with S-100 antibodies but do so with antibodies to glial fibrillary acidic protein; nerve fibres can be identified by their reaction to neurofibrillary protein.

Quantitative studies of rat carotid body type I cells.

The general structure and results of quantitative studies of rat carotid body type I cells are described. In contrast to previous reports, there was no change in mitochondrial V/v% on stimulating the carotid body with 10% oxygen. The volume of cytoplasm occupied by electron-dense cored vesicles was significantly increased, whilst their density per square micrometre of cytoplasm was decreased during hypoxia. Thus, the size of vesicles is increased by hypoxic stimulation. On the basis of vesicle diameter and density we were unable to find evidence of more than one variety of type I cell.

A Case of Dextrocardia with Normal Situs

We report here a case of dextrocardia with normal situs in an 81-year-old man who died from non-pulmonary causes. Removal of the chest wall revealed a large, but otherwise anatomically normal, left lung occupying the entire left hemithorax and extending across the midline to overlap the left border of the heart which was positioned in the right hemithorax. The gross anatomy of the heart was normal save its position and the presence of only 2 pulmonary veins. Dissection of the heart showed all chambers in their classically described position, and there were no valvular defects. The anatomy of the great vessels was also normal. The right lung was hypoplastic and lay posterior to the heart. This lung lacked any lobular structure, but the presence of carbon particles throughout it suggested that it was capable of normal inflation.

Innervation of the Carotid Body

Without doubt the outstanding contribution to the study of the carotid body innervation was made by de Castro (1926, 1928) in his classical studies of methylene blue and silver stain preparations. He showed (1926) that single nerve fibres branched to innervate a number of cells within the carotid body and that the nerve terminals were of variable shapes, such as boutons, calyces and large plate-like structures; in addition, he provided the first experimental demonstration that the carotid body was innervated by the ninth nerve, as most endings had degenerated 6–12 days after section of this nerve at its exit from the jugular foramen. De Castro (1928) went on to show that 5–12 days after intracranial section of the glossopharyngeal nerve there was no degeneration of nerve endings in the carotid body, and hence concluded that the carotid body endings were derived from cell bodies within the petrosal ganglion. As his previous studies of the ganglion indicated that it was composed of typical pseudo-unipolar cells, he concluded that the nerve endings were sensory terminals. The ability of neuronal cell bodies to take up tritiated amino acids and then transport them, or compounds synthesised utilising them, to their peripheral processes has also provided evidence of the sensory nature of the innervation to Type I cells (Smith and Mills 1976, 1977; Fidone et al. 1977 a, b). Both groups of workers showed labelling of Type I cell endings after injection of labelled amino acids into the petrosal ganglion; in the absence of evidence that the petrosal ganglion contains any efferent neurones, the authors concluded that the endings are therefore sensory. The problem with these tracer experiments is that by their very nature they are difficult to quantitate; hence it may be that more than one type of ending can exist on Type I cells (see below).

Identity of the Chemosensor

The question of the identity of the chemosensor has vexed workers for 60 years; judging by the progress in the last 10 years, it will probably do so for many years to come!

Catecholamines and the Carotid Body

The carotid bodies of all species studied to date contain catecholamines, as shown by the presence of formaldehyde-induced fluorescence in the Type I cells (e.g. Palkama 1965). In the last 15 years, such cytochemical observations have been followed up by numerous chemical determinations of carotid body catecholamine levels. Noradrenaline and dopamine were found to be the main catecholamines in several species (Chiocchio et al. 1966; Dearnaley et al. 1968; Hellstrom and Koslow 1975; Hellstrom et al. 1976; Hellstrom 1977a, b; Mills et al. 1978), whilst in the carotid labyrinth of Rana temporaria, adrenaline is the predominant catecholamine with only traces of dopamine and noradrenaline (Bannister et al. 1967).