Various heme-containing proteins have been proposed as primary molecular O2 sensors for hypoxia-sensitive type I cells in the mammalian carotid body. One set of data in particular supports the involvement of a cytochrome b NADPH oxidase that is commonly found in neutrophils. Subunits of this enzyme have been immunocytochemically localized in type I cells, and diphenyleneiodonium, an inhibitor of the oxidase, increases carotid body chemoreceptor activity. The present study evaluated immunocytochemical and functional properties of carotid bodies from normal mice and from mice with a disrupted gp91 phagocytic oxidase (gp91 phox ) DNA sequence gene knockout (KO), a gene that codes for a subunit of the neutrophilic form of NADPH oxidase. Immunostaining for tyrosine hydroxylase, a signature marker antigen for type I cells, was found in groups or lobules of cells displaying morphological features typical of the O2-sensitive cells in other species, and the incidence of tyrosine hydroxylase-immunopositive cells was similar in carotid bodies from both strains of mice. Studies of whole cell K+currents also revealed identical current-voltage relationships and current depression by hypoxia in type I cells dissociated from normal vs. KO animals. Likewise, hypoxia-evoked increases in intracellular Ca2+ concentration were not significantly different for normal and KO type I cells. The whole organ response to hypoxia was evaluated in recordings of carotid sinus nerve activity in vitro. In these experiments, responses elicited by hypoxia and by the classic chemoreceptor stimulant nicotine were also indistinguishable in normal vs. KO preparations. Our data demonstrate that carotid body function remains intact after sequence disruption of the gp91 phox gene. These findings are not in accord with the hypothesis that the phagocytic form of NADPH oxidase acts as a primary O2 sensor in arterial chemoreception.
Immunohistochemical Characteristics of the Human Carotid Body in the Antenatal and Postnatal Periods of Development
