Regulation of arterial oxygen levels is critically important in mammals, particularly during early life. Peri-and postnatal hypoxia may lead to impaired cognitive development, abnormalities in cardiovascular function, breathing control maturation and lung function, and death (Okubo & Mortola, 1988;Nyakas et al. 1996;Hudlicka & Brown, 1996). The main sensors of arterial Oµ tension are the carotid body chemoreceptors, which are located bilaterally at the bifurcations of the common carotid arteries. Carotid chemoreceptor sensory afferents, via the carotid sinus nerves (CSN), project to the nucleus tractus solitarii and other brainstem nuclei, providing the major source of Oµ-mediated ventilatory drive. Neural signals from the carotid chemoreceptors to brainstem cardiorespiratory control nuclei also mediate critically important respiratory reflexes such as arousal from sleep during hypoxia and cardiovascular reflexes that modulate heart rate and blood pressure (Marshall, 1987). The primary site of oxygen sensing in the carotid body is thought to be the type I cell (Gonzalez et al. 1994). Type I cells are specialized sensory neurons which depolarize in response to low Oµ, resulting in Ca¥ entry via voltage-gated calcium channels, and exocytosis of neurotransmitters and modulators onto apposed CSN terminals (Gonzalez et al. 1994). Although further study is needed to define their precise role, there is little question that type I cells play a crucial role in carotid chemoreceptor oxygen sensing. Carotid denervation, which is well tolerated by adults, in neonates leads to profound abnormalities of respiratory control and high mortality rates. In piglets and in lambs the

The site of postnatal maturation of carotid body chemoreception is unclear. To test the hypothesis that maturation occurs synchronously in type I cells and the whole carotid body, the development of changes in the intracellular Ca2+ concentration responses to hypoxia, CO2, and combined challenges was studied with fluorescence microscopy in type I cells and compared with the development of carotid sinus nerve (CSN) responses recorded in vitro from term fetal to 3-wk animals. Type I cell responses to all challenges increased between 1 and 8 days and then remained constant, with no multiplicative O2-CO2interaction at any age. The CSN response to hypoxia also matured by 8 days, but CSN responses to CO2 did not change significantly with age. Multiplicative O2-CO2interaction occurred in the CSN response at 2–3 wk but not in younger groups. We conclude that type I cell maturation underlies maturation of the CSN response to hypoxia. However, because development of responses to CO2 and combined hypoxia-CO2 challenges differed between type I cells and the CSN, responses to these stimuli must mature at other, unidentified sites within the developing carotid body.

The O2 sensitivity of carotid chemoreceptor type I cells is low just after birth and increases with postnatal age. Chronic hypoxia during postnatal maturation blunts ventilatory and carotid chemoreceptor neural responses to hypoxia, but the mechanism remains unknown. We tested the hypothesis that chronic hypoxia from birth impairs the postnatal increase in type I cell O2 sensitivity by comparing intracellular Ca2+ concentration ([Ca2+]i) responses to graded hypoxia in type I cell clusters from rats born and reared in room air or 12% O2. [Ca2+]ilevels at 0, 1, 5, and 21% O2, as well as with 40 mM K+, were measured at 3, 11, and 18 days of age with use of fura 2 in freshly isolated cells. The [Ca2+]iresponse to elevated CO2/low pH was measured at 11 days. Chronic hypoxia from birth abolished the normal developmental increase in the type I cell [Ca2+]iresponse to hypoxia. Effects of chronic hypoxia on development of [Ca2+]iresponses to elevated K+ were small, and [Ca2+]iresponses to CO2 remained unaffected. Impairment of type I cell maturation was partially reversible on return to normoxic conditions. These results indicate that chronic hypoxia severely impairs the postnatal development of carotid chemoreceptor O2sensitivity at the cellular level and leaves responses to other stimuli largely intact.

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Upper airway collapsibility may be influenced by both muscular and nonmuscular factors. Because mucosal blood volume (and therefore vascular tone) is an important determinant of nasal airway patency, vascular tone may be an important nonmuscular determinant of pharyngeal collapsibility. This hypothesis was tested in two experimental models. First, upper airway closing (CP) and opening (OP) pressures and static compliance were measured in nine anesthetized, sinoaortic-denervated, paralyzed cats with isolated upper airways. Vascular tone was decreased with either papaverine or sodium nitroprusside (NTP), and increased with phenylephrine (PE), whereas blood pressure and end-tidal CO2 were maintained constant. Vasodilation increased CP (control = -10.4 +/- 1.3, NTP = -7.3 +/- 1.2 cm H2O; p less than 0.05) and OP (control = -7.9 +/- 1.5, NTP = -3.3 +/- 1.8 cm H2O; p less than 0.05). In contrast, vasoconstriction tended to decrease CP (control = -10.7 +/- 1.5, PE = -11.7 +/- 1.4 cm H2O; p less than 0.09) and OP (control = -8.1 +/- 1.2, PE = -9.9 +/- 1.9 cm H2O; p less than 0.1). Thus, vasodilation increased and vasoconstriction tended to decrease upper airway collapsibility. Upper airway static compliance was unchanged during either drug infusion. In order to assess changes in pharyngeal cross-sectional area (CSA) that occurred during vasodilation, magnetic resonance imaging was utilized in seven cats. During vasodilation with NTP, pharyngeal CSA was reduced from 0.44 +/- 0.10 to 0.30 +/- 0.09 cm2 (p less than 0.05), and pharyngeal volume was reduced from 15.3 +/- 2.4 to 13.9 +/- 2.7 cm3 (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

]i) response to hypoxia in type 1 cells from 1, 3, and 11-to 16-day-old rats. Using fura-2, we studied the effects of quinpirole, a D2 receptor agonist, on type 1 cell [Ca 2ϩ ]i response to 90-s hypoxia challenges (PO2 ϳ1-2 mmHg). Cells were sequentially exposed to the following challenges: 1) hypoxia control, 2) hypoxia plus quinpirole, and 3) hypoxia plus quinpirole plus sulpiride (D2 receptor antagonist). In the 11-to 16-day-old group, type 1 cell [Ca 2ϩ ]i increased ϳ3 to 4-fold over resting [Ca 2ϩ ]i in response to hypoxia. Quinpirole (10 M) significantly blunted the peak [Ca 2ϩ ]i response to hypoxia. Repeat challenge with hypoxia plus 10 M quinpirole in the presence of 10 M sulpiride partially restored the hypoxia [Ca 2ϩ ]i response. In sharp contrast to the older aged group, 10 M quinpirole had minimal effect on hypoxia response of type 1 cells from 1-day-olds and a small but significant effect at 3 days of age. We conclude that stimulation of dopamine D2 receptors inhibits type 1 cell [Ca 2ϩ ]i response to hypoxia, consistent with an inhibitory autoreceptor role. These findings suggest dopamine-mediated inhibition and oxygen sensitivity increase with age on a similar time course and do not support a role for dopamine as a major mediator of carotid chemoreceptor resetting. dopamine receptors; hypoxia; development; chemoreceptor THE MAIN SENSORS of arterial oxygen level in mammals are the carotid body (CB) chemoreceptors (25). It is widely believed that hypoxia leads to CB type 1 cell depolarization and Ca 2ϩ influx through voltage-gated calcium channels. The rise in intracellular calcium ([Ca 2ϩ ] i ) releases neurotransmitter(s), which are believed to cause firing of action potentials in the adjacent carotid sinus nerve terminals and to act presynaptically on type 1 cell autoreceptors (25). Carotid sinus nerve (CSN) input to brain stem respiratory control nuclei drives the ventilatory response to hypoxia and mediates, at least in part, other defensive responses to hypoxic stress (12, 13, 21-23, 30, 31).In neonates, but not adults, carotid denervation leads to high mortality rates and abnormalities of respiratory control (13,15,18,30,31), suggesting a vulnerable period during mammalian postnatal maturation during which the CB is important for survival and normal maturation of breathing control. Despite their importance in the developing infant, the carotid chemoreceptors have low sensitivity to hypoxia at birth and become more sensitive over the first few days or weeks of life (9,10,14,17,33,36,38), a process termed "resetting." The mechanism(s) underlying resetting are unknown, but some have postulated involvement of inhibitory neuromodulators such as dopamine (DA) (28,29).In rats and rabbits, an age-related increase in O 2 sensitivity occurs at the type 1 cell level (41, 45). Both the [Ca 2ϩ ] i rise and the neurotransmitter (catecholamine) release in response to hypoxia increase with maturation, exhibiting the same time course as CSN response maturation, strongly suggesting that a s...

To determine if depression of central respiratory output during progressive brain hypoxia (PBH) can be generalized to other brain stem outputs, we examined the effect of PBH on the tonic (tSCS) and inspiratory-synchronous (iSCS) components of preganglionic superior cervical sympathetic (SCS) nerve activity. Peak phrenic and SCS activity were measured in nine anesthetized, paralyzed, peripherally chemodenervated, vagotomized cats. PBH was produced by inhalation of 0.5% CO in 40% O2 while blood pressure and end-tidal CO2 were maintained constant. A progressive reduction in arterial O2 content from 14.3 +/- 0.6 to 4.5 +/- 0.3 vol% caused a 79 +/- 7% depression of peak phrenic activity and an 84 +/- 10% reduction of iSCS activity, but tSCS activity increased 42 +/- 21%. During CO2 rebreathing, iSCS activity increased in parallel with peak phrenic activity while tSCS activity was unchanged. The slopes of the CO2 responses of both phrenic (6.3 +/- 1.2%max/mmHg) and iSCS (4.6 +/- 0.8%max/mmHg) activity were unaffected by PBH. In four of nine hypocapnic and three of nine hypoxic studies, inspiratory activity in the SCS nerve was observed even after completely silencing the phrenic neurogram.(ABSTRACT TRUNCATED AT 250 WORDS)

Normal lung morphogenesis is dependent on chloride-driven fluid transport. The molecular identity of essential fetal lung chloride channel(s) has not been elucidated. CLC-2 is a chloride channel, which is expressed on the apical surface of the developing respiratory epithelium. CLC-2-like pH-dependent chloride secretion exists in fetal airway cells. We used a 14-day fetal rat lung submersion culture model to examine the role of CLC-2 in lung development. In this model, the excised fetal lung continues to grow, secrete fluid, and become progressively cystic in morphology (26). We inhibited CLC-2 expression in these explants, using antisense oligonucleotides, and found that lung cyst morphology was disrupted. In addition, transepithelial voltage (V(t)) of lung explants transfected with antisense CLC-2 was inhibited with V(t) = -1.5 +/- 0.2 mV (means + SE) compared with -3.7 +/- 0.3 mV (means + SE) for mock-transfected controls and -3.3 +/- 0.3 mV (means + SE) for nonsense oligodeoxynucleotide-transfected controls. This suggests that CLC-2 is important for fetal lung fluid production and that it may play a role in normal lung morphogenesis.