Oxygen sensing, plasticity and catecholaminergic functions in cultured chromaffin cells of rat carotid body and adrenal medulla: Modulation by chronic hypoxia and acetylcholine receptors

Abstract

<p>The mammalian carotid body is a chemosensory organ, located at the bifurcation of the common carotid artery. It senses blood levels of oxygen, carbon dioxide, and acidity and maintains homeostasis via the control of breathing. Type I or glomus cells of the carotid body are the putative chemoreceptors which transduce blood-borne chemical stimuli into electrical signals carried by the carotid sinus nerve, which projects to the respiratory control center in the brainstem. It is likely that oxygen chemoreception in the carotid body involves the concerted actions of multiple neurotransmitters or neuromodulators, e.g. dopamine, acetylcholine and substance P, released from glomus cells onto apposed sensory terminals of the carotid sinus nerve. Further, carotid body catecholamines have been implicated in the resetting of chemoreceptor sensitivity after birth, ventilatory acclimatization to chronic hypoxia, and "blunting" of the ventilatory response in high-altitude dwellers. Catecholamine release is also critical for the animal's ability to survive the hypoxic stress associated with delivery and the transition to extrauterine life, though in this case, the origin is from a different source, the adrenomedullary chromaffin cells. The primary goal of this thesis was to elucidate the cellular and molecular mechanisms underlying oxygen chemoreception and the adaptive responses of chemoreceptors to chronic hypoxia. The use of dispersed cell cultures of the rat carotid body and adrenal medulla permitted the direct exposure of putative oxygen chemoreceptors to low oxygen. In addition, high performance liquid chromatography, immunocytochemistry, and pharmacological tools were used to delineate the cellular and molecular mechanisms which underlie oxygen sensing and adaptation of the isolated chemoreceptors to chronic hypoxia in vitro. In normoxic carotid body cultures, acute hypoxia stimulated dopamine release in a dose- and Ca²⁻dependent manner, possibly via closure of Ca²⁻-dependent K⁻ channels. Exposure of glomus cells to chronic hypoxia in vitro triggered a wide array of adaptive responses with the potential to modify the level of released neurotransmitter. These "plastic" responses include: (1) an apparent down-regulation of functional oxygen-sensitive, Ca²⁻-dependent K⁻ channels; (2) up-regulation of GAP-43 immunoreactivity; and (3) enhanced basal extracellular dopamine, which appears to be set by positive and negative feedback regulation via nicotinic and muscarinic acetylcholine receptors, respectively, and inhibition of dopamine transporters. In chronic hypoxia, acetylcholine appears to be an important autocrine/paracrine modulator of dopaminergic function in carotid body cultures. These cellular adaptations may relate to changes in carotid body chemosensitivity during chronic hypoxia in vivo. Similar to carotid body glomus cells, neonatal adrenomedullary chromaffin cells express oxygen-chemoreceptive properties. Exposure of neonatal chromaffin cultures to acute hypoxia or a specific blocker of Ca²⁻-dependent K⁻ channels stimulated catecholamine (predominantly epinephrine) release in vitro. These findings in glomus and adrenomedullary chromaffin cells suggest that hypoxia may close Ca²⁻-dependent K⁻ channels, leading to membrane depolarization, entry of extracellular Ca²⁻ and catecholamine release. However, unlike glomus cells, adrenomedullary chromaffin cells possess a developmentally regulated oxygen-sensing mechanism, since hypoxia had no significant effect on catecholamine release in juvenile adrenomedullary chromaffin cells.</p>