Elsevier

Brain Research

Volume 789, Issue 2, 13 April 1998, Pages 229-238
Brain Research

Research report
Electrophysiological and immunocytological demonstration of cell-type specific responses to hypoxia in the adult cat carotid body

https://doi.org/10.1016/S0006-8993(97)01472-8Get rights and content

Abstract

We have recently shown two types of cat carotid body cells based on the oxygen sensitivity of voltage-gated potassium channels. In the present study, we attempted to determine the correlation between cell types (glomus cells, sheath cells, and subtypes of glomus cells) and oxygen sensitivity of potassium channels. Further, changes in membrane potentials in response to hypoxia were also examined. Carotid body cells harvested from adult cats were cultured, and a whole cell patch clamp method was applied to determine the oxygen sensitivity of outward current. The tested cells were identified by Lucifer Yellow in the patch pipette. Glomus cells and sheath cells were immunocytochemically identified using tyrosine hydroxylase (TH) and glial fibrillary acidic protein (GFAP) as markers. The cells whose outward current was inhibited by hypoxia showed TH-immunoreactivity but not GFAP-immunoreactivity. The cells whose outward current was not sensitive to hypoxia were GFAP-positive or TH-negative. One TH-positive cell had oxygen-insensitive outward current. The resting membrane potentials of the cells having oxygen-sensitive outward current were significantly higher (−55±3 mV) than those of the cells having oxygen-insensitive outward current (−35±2 mV). The former type of cells was depolarized during hypoxia, but not the latter type of cells. These results suggest that most glomus cells of the adult cat carotid body possess oxygen-sensitive potassium channels and are depolarized in response to hypoxia. On the other hand, sheath cells and possibly a small fraction of glomus cells possess oxygen-insensitive potassium channels and their membrane potential is not affected by hypoxia.

Introduction

The carotid body is a major arterial chemoreceptor, whose sensitivity to oxygen tension plays an essential role in systemic responses to hypoxia 9, 15. A current hypothesis of carotid body chemoreception implies that inhibition of oxygen-sensitive potassium channels by hypoxia depolarizes glomus cells. Depolarization activates voltage-gated calcium channels and triggers neurotransmitter release 17, 24, 34. The presence of voltage-gated oxygen-sensitive potassium channels in carotid body cells has been shown in the rabbit, rat and cat (8, 10, 20, 25, 26, 33, 38; for reviews, see Refs. 24, 34). In agreement with the hypothesis described above depolarization of carotid body cells during hypoxia was observed in the fetal rabbit [10]. In the adult rabbit glomus cells generated action potentials and the frequencies of action potentials increased during hypoxia [25]. However, there were some inconsistent observations. Biscoe and Duchen [3]reported that carotid body cells of the adult rabbit were hyperpolarized and that outward potassium current was increased, instead of decreased, during hypoxia. Other studies using microelectrodes did not show any consistent changes in membrane potentials during hypoxia 14, 31. The inconsistency may be due to differences in species, age, or experimental procedures. It is also possible that subtypes of glomus cells respond differently. Earlier morphological studies distinguished several types of glomus cells in rats, cats, and human 19, 28. Recent electrophysiological and fluorometric studies have indicated heterogeneous responses of glomus cells to hypoxia and cyanide 4, 11, 35.

Our previous study has shown that one type of cultured cat carotid body cell has oxygen-sensitive potassium channels and the other has oxygen-insensitive potassium channels [8]. The electrophysiological properties of these two types of potassium channels were very similar, and we could not detect any clear morphological differences between the cells which differentially expressed these channels. Questions are whether these cells consist of subtypes of glomus cells, or whether glomus cells and sheath cells differentially express different potassium channels. This study was designed to clarify these questions. We further examined the correlation between the oxygen sensitivity of potassium channels and changes in the membrane potential during hypoxia. A preliminary study was reported previously [7].

Section snippets

Cell culture

Carotid body cells were cultured as previously reported with a slight modification [36]. In short, carotid bodies were harvested from adult cats that were deeply anesthetized with pentobarbital (30–40 mg/kg, i.p., then 50 mg/kg, i.v.) and decapitated. Both carotid bodies were cleaned, and dissociated with collagenase (0.1–0.2%, type XI, Sigma) and gentle trituration. The cells were then centrifuged (100–200×g, 5 min), the pellet was resuspended in a culture medium, and the cell suspension was

Experimental set 1

Fig. 1 represents electrophysiological and immunohistological results from a cell which had oxygen-sensitive outward current. The voltage clamp experiment showed that the outward current was reversibly inhibited by hypoxia (Fig. 1A). Lucifer Yellow contained in the patch pipette spread out in the cytoplasm and processes and, even after the patch pipette was removed, the patched cell was readily identified under a fluorescent microscope with a fluorescein filter (Fig. 1B). This concentration of

Discussion

The present study has demonstrated that the cat carotid body cells whose outward current was inhibited by hypoxia were TH-positive or GFAP-negative. The cells whose outward currents were insensitive to hypoxia were GFAP-positive or TH-negative except one case. The resting membrane potentials of cells having oxygen-sensitive outward current were much higher than those of cells having oxygen-insensitive outward current. The former type of cell was depolarized during hypoxia, but not the latter

Acknowledgements

The authors are grateful to Drs. Thomas Croxton and Bradly Undem for their helpful suggestions. This study was supported by HL47044 and HL50712.

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