Dopaminergic Properties of Cultured Rat Carotid Body Chemoreceptors Grown in Normoxic and Hypoxic Environments
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Using dissociated carotid body (CB) cultures prepared from neonatal (postnatal days 5-7; P7) or juvenile (postnatal day 19-20; P20) rats, we compared catecholaminergic properties and mechanisms of O2 sensing in glomus cells grown in normoxic (Nox; 20% O2) and chronically hypoxic (CHox; 6% O2) environments for up to 2 weeks. In Nox cultures, basal dopamine (DA) release, determined by HPLC and normalized to the number of tyrosine hydroxylase-positive glomus cells present, was similar for P7 and P20 cultures (approximately 0.3 pmol/1,000 cells/15 min) and was unaffected by culture duration (2 vs. 12 days). Acute hypoxia (5 and 10% O2) caused a dose-dependent stimulation (6x and 3x basal, respectively) in DA release, that was inhibited by nifedipine (10 microM). DA release was also stimulated by high extracellular K+ (30 mM) and iberiotoxin (200 nM), a selective blocker of PO2-regulated, Ca-dependent K+ channel in glomus cells. The stimulatory effect of iberiotoxin was similar to 5% O2 in P20 cultures, but substantially less (about one-half) in P7 cultures. In contrast, in CHox cultures, basal DA release was substantially elevated, approximately 8x Nox levels, although this did not correlate with significant differences in stores. Further, whereas acute hypoxia (5% O2) and high K+ also stimulated DA release in CHox cultures (approximately 2x and approximately 3x basal), iberiotoxin (200 nM) did not. Thus, after chronic hypoxia in vitro, there is an enhanced basal catecholamine release and an apparent down-regulation of functional Ca-dependent K+ channels in CB chemoreceptors. These cellular adaptations may relate to changes in CB chemosensitivity during chronic hypoxemia.
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