Characterization of [125I]ZM 241385 binding to adenosine A2A receptors in the pineal of sheep brain.

Adenosine is a ubiquitous neuromodulator and homeostatic regulator that exerts its physiologic actions through activation of A(1), A(2A), A(2B) and A(3) adenosine receptor subtypes. In the central nervous system, adenosine's action in neurons is manifested in its modulation of tonic inhibitory control. Adenosine released in the brain during hypoxia has critical depressant effects on breathing in fetal and newborn mammals, an action suggested to be mediated by A(2A) receptors in the posteromedial thalamus. In an effort to more accurately define the spatial distribution of adenosine A(2A) receptors in fetal sheep diencephalon, we have used a receptor autoradiographic technique utilizing an iodinated radioligand [(125)I]ZM 241385, which has greater sensitivity and resolution than the tritiated compound. The distribution of ligand binding sites in the fetal sheep diencephalon indicated that the highest levels of binding were in select thalamic nuclei, including those implicated in hypoxic depression of fetal breathing, and the pineal. Given the high density of labeled A(2A) receptors in the pineal, these sites were characterized more fully in homogenate radioligand binding assays. These data indicate that [(125)I]ZM 241385 binding sites display a pharmacological signature consistent with that of adenosine A(2A) receptors and are expressed at similar levels in fetal, lamb and adult ovine brain. The adenosine A(2A) receptor pharmacologic signature of the [(125)I]ZM 241385 binding site in pineal cell membranes generalized to the site characterized in membranes derived from other portions of the lamb thalamus, including the sector involved in hypoxic inhibition of fetal breathing. These results have important implications for the functional roles of adenosine A(2A) receptors in the thalamus and pineal of sheep brain.

Pregnancy outcomes after the Fontan repair.

OBJECTIVES: This study sought to determine risks and outcome of pregnancy and delivery after the modified Fontan operation. BACKGROUND: Increasingly, female Fontan patients reaching child-bearing years are interested in having children. To date, the number of reported pregnancies is small, and pregnancy has therefore been discouraged. METHODS: One hundred ten of 126 female patients from the Fontan registries of the Mayo Clinic and University of California Los Angeles Medical Center responded to a mailed questionnaire. An additional six patients with a reported pregnancy from other centers were identified and reviewed to assess pregnancy outcomes. RESULTS: Among the participating centers, a total of 33 pregnancies after Fontan operation for various types of univentricular heart disease were reported. There were 15 (45%) live births from 14 mothers, with 13 spontaneous abortions and 5 elective terminations. In the 14 women with live births, the median number of years between operation and pregnancy was 4 (range 2 to 14). Reported prepregnancy problems in these gravidas included atrial flutter in one patient and ventricular dysfunction, aortic regurgitation and atrioventricular valve regurgitation in another. One patient developed supraventricular tachycardia during pregnancy and had conversion to sinus rhythm. No maternal cardiac complications were reported during labor, delivery or the immediate puerperium. There were six female and nine male infants (mean gestational age 36.5 weeks; median weight 2,344 g). One infant had an atrial septal defect. At follow-up, mothers and infants were alive and well. CONCLUSIONS: Pregnancy after the Fontan operation appears to have been well tolerated in 13 to 14 gravidas. There does appear to be an increased risk of miscarriage. The tendency to routinely discourage pregnancy may need to be reconsidered.

Placental calcification: a metastatic process?

Placental calcification commonly increases with gestational age. The mechanism of apatite mineralization probably involves one of three known mechanisms of tissue calcification: physiological (like bone), dystrophic (ischaemia-related) or metastatic (mineralization in a supersaturated environment). This study was designed to determine the mechanism of calcification by examining (1) the mineral content of placental calcifications in comparison to other physiological and pathological apatites, and (2) the expression of bone morphogenetic proteins (BMPs), which are important in physiological calcification, across gestational age. By energy-dispersive x-ray analysis (EDXA), the Ca/P weight ratio for apatitic mineral from mature calcifications was 2.00+/-0.05 (s.e.), which is similar to that for stones formed in a metastatic, supersaturated environment and lower than that observed in physiological calcification. Biologically active BMP, which was determined by bioassay, was demonstrated in mature and postmature placentae. The BMPs PLAB, PDF and related protein INSL-4 were identified by semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR), but their mRNA expression was independent of gestational age (7-41 weeks of gestation). We conclude that (1) the identified BMPs were not related directly to placental calcification, which argues against physiological calcification, and (2) the chemical composition of the apatitic mineral was suggestive of rapid formation in a supersaturated environment, which is consistent with a metastatic mechanism of calcification.

Fetal breathing, sleep state, and cardiovascular responses to adenosine in sheep.

The possibility that adenosine mediates hypoxic inhibition of fetal breathing and eye movements was tested in nine chronically catheterized fetal sheep (0.8 term). Intracarotid infusion of adenosine (0.25 +/- 0.03 mg.min-1.kg-1) for 1 h to the fetus increased heart rate and hemoglobin concentration but did not significantly affect mean arterial pressure or blood gases. As with hypoxia, adenosine decreased the incidence of rapid eye movements by 55% and the incidence of breathing by 77% without significantly affecting the incidence of low-voltage electrocortical activity. However, with longer (9 h) administration, the incidence of breathing and eye movements returned to normal during the adenosine infusion. Intravenous infusion of theophylline, an adenosine receptor antagonist, prevented most of the reduction in the incidence of breathing and eye movements normally seen during severe hypoxia (delta arterial PO2 = -10 Torr). It is concluded that 1) adenosine likely depresses fetal breathing and eye movements during hypoxia and 2) downregulation of adenosine receptors may contribute to the adaptation of breathing and eye movements during prolonged hypoxia.

Fetal breathing, sleep state, and cardiovascular responses to graded hypoxia in sheep.

Graded isocapnic hypoxemia was produced in unanesthetized fetal sheep by varying the inspired O2 concentration (21, 12, 10.5, and 9%) of the ewe. This produced corresponding mean preductal arterial O2 tension (PaO2) values of 25.2 +/- 1.1 (control), 20.1 +/- 1.0 (mild hypoxia), 17.8 +/- 0.9 (moderate hypoxia), and 16.8 +/- 1.4 Torr (severe hypoxia). These were associated with mean arterial O2 contents (CaO2) of 7.18 +/- 0.44, 5.19 +/- 0.34, 4.24 +/- 0.33, and 3.27 +/- 0.20 ml/dl, respectively. The most severe hypoxia was associated with metabolic acidosis and fetal bradycardia. Hypoxia did not reduce significantly the incidence of low-voltage electrocortical activity. The incidence of breathing and rapid eye movements was not affected by mild hypoxia; however, the incidence of both was significantly reduced during moderate and severe hypoxia. It is concluded that 1) acute reductions in the mean PaO2 of 5.9 +/- 0.6 Torr and CaO2 of 2.00 +/- 0.23 ml/dl are critical in that greater reductions inhibit fetal eye and breathing activity and 2) hypoxia probably inhibits eye and breathing movements by altering sleep state.

Fetal breathing, sleep state, and cardiovascular responses to graded anemia in sheep.

Graded anemia was produced for 2 h in 10 unanesthetized fetal sheep by infusing plasma in exchange for fetal blood. This reduced the mean fetal hematocrits during the 1st h of anemia to 19.7 +/- 0.5% [control (C) = 28.2 +/- 1.1%] for mild anemia, 17.4 +/- 0.9% (C = 30.0 +/- 1.1%) for moderate anemia, and 15.1 +/- 1.0% (C = 29.2 +/- 1.3%) for severe anemia. The respective mean arterial O2 contents (CaO2) were 4.46 +/- 0.20, 3.89 +/- 0.24, and 3.22 +/- 0.19 ml/dl. Mean arterial PO2 was reduced significantly (by 2 Torr) only during moderate anemia, and mean arterial pH was decreased only during severe anemia. No significant changes occurred in arterial PCO2. Fetal tachycardia occurred during anemia. Mean arterial pressure was reduced by 2-3 mmHg during mild anemia; however, no significant blood pressure changes were observed for moderate or severe anemia. The incidence of rapid-eye movements and breathing activity was not affected by mild anemia, but the incidence of both was reduced significantly during moderate and severe anemia. It is concluded that 1) a reduction in CaO2 of greater than 2.48 +/- 0.22 ml/dl by hemodilution inhibits rapid-eye movements and breathing activity, and 2) the PO2 signal for inhibition does not come from arterial blood but from lower PO2 in tissue.

Fetal breathing and sleep state responses to graded carboxyhemoglobinemia in sheep.

To investigate CO effects on brain oxygenation, graded carboxyhemoglobinemia (HbCO) was produced in nine unanesthetized fetal sheep by infusing CO-laden erythrocytes in exchange for fetal blood. For the 1st h after this procedure, the mean fetal carboxyhemoglobin levels were 16.5 +/- 0.4% [control (C) = 1.4 +/- 0.4%] for mild HbCO, 22.7 +/- 0.6% (C = 1.8 +/- 0.4%) for moderate HbCO, and 27.8 +/- 0.5% (C = 2.1 +/- 0.7%) for severe HbCO. This induction of HbCO significantly reduced mean preductal arterial PO2 values to 4.3 Torr below control for mild HbCO, 4.6 Torr below control for moderate HbCO, and 5.5 Torr below control for severe HbCO. The respective arterial O2 contents were decreased by 17, 21, and 29%. Mean arterial pH was lowered only during severe HbCO, and arterial PCO2 values were unchanged. HbCO produced a fetal tachycardia. Mean arterial blood pressure was only increased during severe HbCO. The incidences of rapid eye movements and breathing activity were decreased by HbCO in a dose-dependent manner. When related to calculated brain tissue PO2, these decreases were similar to those measured during hypoxic hypoxia and anemia, suggesting that carboxyhemoglobin effects result solely from diminished oxygenation. It is concluded that 1) the peripheral arterial chemoreceptors in the fetus apparently have little effect on hypoxic inhibition of breathing and 2) the carboxyhemoglobin concentrations required to inhibit fetal breathing are greater than those likely to be encountered clinically.

Adenosine stimulates breathing in fetal sheep with brain stem section.

Breathing responses to adenosine were determined in 12 chronically catheterized fetal sheep (greater than 0.8 term) in which hypoxic inhibition of breathing had been eliminated by brain stem section. The caudal extent of transection varied from the rostral midbrain to the pontomedullary junction. Isocapnic hypoxia [delta arterial PO2 (PaO2) of -12 Torr] doubled the incidence and depth of breathing activity and increased the incidence of eye movements. Intra-arterial infusion of adenosine (0.30 +/- 0.03 mg.min-1.kg fetal wt-1) increased the incidence and amplitude of breathing without affecting blood gases. Adenosine did not significantly alter the incidence of eye activity. Intra-arterial injection of oligomycin (120 +/- 26 micrograms/kg fetal wt), an inhibitor of mitochondrial oxidative phosphorylation, also stimulated breathing activity. In four fetuses with brain stem section, peripheral arterial chemodenervation blunted the stimulatory effects of hypoxia on breathing activity and abolished altogether the excitatory effects of adenosine. It is concluded that 1) hypoxia and adenosine likely inhibit breathing in normal fetuses by affecting similar areas of the brain stem and 2) in fetuses with brain section, hypoxic hyperpnea depends on peripheral and central mechanisms, whereas adenosine stimulates breathing via the peripheral arterial chemoreceptors.

Fetal breathing and cardiovascular responses to graded methemoglobinemia in sheep.

Graded methemoglobinemia (MetHb) was produced in unanesthetized fetal sheep to determine the effects on brain oxygenation. MetHb was induced by infusing methemoglobin-containing erythrocytes in exchange for fetal blood. During the hour after MetHb was established, fetal methemoglobin concentrations averaged 1.23 +/- 0.12 (mild MetHb), 1.71 +/- 0.13 (moderate MetHb), and 2.27 +/- 0.17 g/dl (severe MetHb). MetHb reduced mean arterial O2 content by approximately 19 (mild MetHb), 29 (moderate MetHb), and 39% (severe MetHb). The average preductal arterial PO2 fell by 1.6 (-7%), 2.8 (-11%), and 4.0 Torr (-16%) for mild, moderate, and severe MetHb, respectively. Fetal heart rate increased significantly during mild and moderate MetHb, and mean arterial pressure fell slightly during moderate and severe MetHb. The incidences of fetal breathing and eye movements were reduced in a dose-dependent manner when the calculated brain end-capillary PO2 was less than 14 Torr. We conclude that: 1) the effective capillary PO2 in the fetal brain can be significantly reduced by increasing the distance between non-methemoglobin-laden erythrocytes in capillaries and 2) hypoxic inhibition of fetal breathing probably arises from discrete areas of the brain having a PO2 less than 3 Torr.

Adenosine A(1) and A(2A) receptors modulate sleep state and breathing in fetal sheep.

This study was designed to determine the adenosine (Ado) receptor subtype that mediates the depressant effects of Ado on fetal breathing and rapid eye movements (REM). In chronically catheterized fetal sheep (>0.8 term), intra-arterial infusion of N(6)-cyclopentyladenosine (CPA), an Ado A(1)-receptor agonist, increased the incidence of high-voltage electrocortical (ECoG) activity while virtually abolishing low-voltage activity, REM, and breathing. These effects were blocked by 9-cyclopentyl-1,3-dipropylxanthine (DPCPX), an Ado A(1)-receptor antagonist. Infusion of DPCPX alone increased breath amplitude but had no significant effect on inspiratory duration, breath interval, incidence of REM, or incidence of low-voltage activity. Ado A(2A)-receptor blockade with ZM-241385 increased the incidence of low-voltage ECoG activity, REM, and breathing but had no effect on breath amplitude or respiratory cycle. Both DPCPX and ZM-241385 eliminated the inhibitory effects of Ado on REM and breathing. We conclude that 1) Ado A(1) receptors tonically inhibit fetal respiratory drive, 2) Ado A(2A) receptors tonically inhibit REM-like behavioral state, and 3) both Ado A(1) and A(2A) receptors mediate the depressant effects of Ado on REM and breathing.

Adenosine in the treatment of maternal paroxysmal supraventricular tachycardia.

Paroxysmal supraventricular tachycardia is the most common sustained cardiac arrhythmia in pregnant women. Because nearly 50% of these supraventricular tachyarrhythmias fail to respond to vagal maneuvers, other therapies are used, including electrocardioversion and pharmacologic agents. Propranolol, verapamil, and adenosine have Food and Drug Administration-approved labeling for acute termination of supraventricular tachycardia. Verapamil has been the most commonly used agent in the general population but it has several shortcomings, such as its potential to cause or exacerbate systemic hypotension, congestive heart failure, bradyarrhythmias, and ventricular fibrillation. In addition, verapamil readily crosses the placenta and has been shown to cause fetal bradycardia, heart block, depression of contractility, and hypotension. Adenosine has several advantages over verapamil, including rapid onset, brevity of side effects, theoretical safety, and probable lack of placental transfer. Adenosine ultimately may prove to be the preferred agent for termination of paroxysmal supraventricular tachycardia in the gravid woman.

Hypoxic inhibition of breathing in fetal sheep: relationship to brain adenosine concentrations.

Because hypoxic inhibition of fetal breathing may be caused by a rise in central adenosine levels, the effects of O2 deficiency on fetal brain adenosine concentrations were determined at levels of hypoxia that inhibited fetal breathing. Under halothane anesthesia, the brains of fetal sheep (0.8 term) were implanted with guide cannulas exteriorized through a Silastic rubber window in the uterus and flank of the ewe. At least 4 days after surgery, a microdialysis probe was inserted into a cannula with the membrane tip placed in the rostral brain stem. During 1 h of isocapnic hypoxia, mean fetal arterial PO2 was reduced from 24.0 +/- 0.9 Torr (control) to 13 +/- 0.6 Torr and arterial pH fell progressively from 7.354 +/- 0.007 to 7.273 +/- 0.023. Hypoxia decreased the incidence of fetal breathing movements from 33 +/- 5.2 to 5 +/- 2.2 min/h, with a normal incidence (29 +/- 3.5 min/h) during the hour after arterial PO2 returned to control values. Adenosine concentrations in microdialysis perfusate under control conditions averaged approximately 35 nM, increased up to 2.3-fold during the hour of O2 deficiency, and fell toward control values when normoxia was restored. We conclude that fetal brain adenosine levels are increased at levels of O2 deficiency that inhibit fetal breathing, which are results consistent with a role for adenosine in hypoxic inhibition of fetal breathing.

Anemia up-regulates pontine A1 adenosine receptors in fetal sheep.

A rise in central adenosine concentrations most likely triggers the inhibitory effects of acute hypoxia on fetal breathing movements. During prolonged O2 deficiency, the incidence of fetal breathing increases over time, an adaptation which may involve down-regulation of adenosine receptors. Therefore, isovolemic anemia was induced in chronically catheterized fetal sheep to determine the effects of acute O2 deprivation on the affinity of brainstem A1 adenosine receptors. Compared to control values, the mean hemoglobin concentration in 4 fetuses was lowered by 54% (to 3.7 +/- 0.4 g/dl) for 1 h and in 3 fetuses by 58% (to 3.8 +/- 0.6 g/dl) for 4 h. Mean preductal arterial pH during anemia was significantly reduced to values as low as 7.123 +/- 0.090, but mean paO2 and paCO2 were not significantly affected. The average incidence of fetal breathing movements was decreased by 80% during the first hour and by 50% during the fourth hour of anemia. In 3 fetuses with normal hemoglobin levels, about 32 +/- 6.0% of the A1 adenosine receptors in the brainstem were in the high affinity state. After 1 h of anemia, the percentage of high affinity receptors in the pons (but not medulla or midbrain) had significantly increased to 47 +/- 2.9%, but after 4 h of anemia all 3 brainstem regions had A1 receptor affinity within the range of control values. It is concluded that acute anemia induces a short-lived up-regulation of pontine A1 adenosine receptors, but anemia does not alter A1 receptor coupling in the midbrain where A1 receptors related to breathing inhibition may be located.

Source of extracellular brain adenosine during hypoxia in fetal sheep.

Microdialysis was performed to determine whether hypoxia increases fetal brain adenosine (ADO) concentration through dephosphorylation of extracellular 5'-adenosine monophosphate (5-AMP). Hypoxia (fetal PaO2 approximately 14 Torr) increased fetal brain ADO levels approximately two-fold when the probes were perfused with synthetic cerebrospinal fluid (CSF) containing inhibitors of the nucleoside transporter but not with this solution plus a blocker of ecto-5'-nucleotidase (AOPCP). The hypoxia-induced rise in fetal brain ADO concentrations depends critically upon the hydrolysis of extracellular 5'-AMP.

Enhancing adenosine A1 receptor binding reduces hypoxic-ischemic brain injury in newborn rats.

Hypoxia increases brain adenosine concentrations, which provides neuroprotection through activation of central adenosine A1 receptors. This study was carried out to determine whether PD 81,273, which increases adenosine's binding to A1 receptors, would reduce hypoxia-induced brain injury. PD 81,273 (3 mg/kg, i.p.) decreased by about 50% the weight loss of the left cerebral hemisphere caused by hypoxia-ischemia in neonatal rats. Thus, enhancing adenosine's binding to the A1 receptor decreases hypoxic brain damage.

Predictive value of HemoCue capillary whole blood glucose measurements in pregnancy.

OBJECTIVE: We determined the ability of capillary whole blood glucose concentrations to predict venous plasma and whole blood glucose levels. METHODS: During a standard oral glucose tolerance test in 29 pregnant women, paired capillary and venous blood samples were collected for analysis of glucose concentrations by the HemoCue photometer and by central laboratory methods. RESULTS: Glucose concentrations determined serially in a single blood sample by the HemoCue method were highly reproducible, with a coefficient of variation of 2.3%. However, glucose levels in blood from two different fingersticks from the same patient varied on average by 3 mg/dL, with a maximum difference of 14 mg/dL. Although capillary whole blood glucose results obtained by the HemoCue method correlated well with venous plasma or whole blood glucose measurements (r = 0.98 and r = 0.97, respectively) over the range investigated (60-250 mg/dL), individual capillary whole blood glucose measurements were only a fair predictor of venous values, with 95% of measured venous levels within +/- 26 mg/dL and +/- 20 mg/dL for concentrations predicted for plasma and whole blood, respectively. CONCLUSION: Sampling factors rather than measurement accuracy limit the ability of capillary whole blood glucose measurements to predict venous concentrations.

Central stimulation of breathing movements in fetal lambs by prostaglandin synthetase inhibitors.

In unanaesthetized fetal lambs at 125-135 days gestation in utero central acidosis caused by perfusion of the cerebral ventricular system with a solution containing less than 1 mM-HCO3- (cerebrospinal fluid (c.s.f.) pH 6.98) or intravenous infusion of ammonium chloride (c.s.f. pH 7.1) produced an increase in the depth and frequency of episodic breathing but no change in electrocortical activity, heart rate or arterial pressure. Administration of prostaglandin synthetase inhibitors, sodium meclofenamate (0.8-10 mg/kg I.V. or 0.6-2.6 mg/kg intracerebrally) or acetylsalicylic acid (6.7 mg/kg I.V.) caused prolonged episodes of fetal breathing during low and high voltage electrocortical activity, with a large increase in breath amplitude. Blood gas values, heart rate, blood pressure, electrocortical activity and eye movements were not altered. In fetuses whose brain stems had been sectioned in the upper pons or the inferior colliculus, sodium meclofenamate induced prolonged deep breathing. Intravenous prostaglandin E2 abolished the continuous breathing induced by meclofenamate, but not breathing movements enhanced by hypercapnia or hypoxia. It is concluded that the central chemoreceptors respond to acidosis in near-term lamb fetuses qualitatively as in adult animals. Secondly, the results suggest that prostaglandin E2 and the inhibitors of prostaglandin synthesis also act centrally in the lower pons or medulla to modulate fetal breathing.

Adenosine A(2A) receptors mediate cardiovascular responses to hypoxia in fetal sheep.

Nonselective adenosine (ADO) receptor antagonists block hypoxia-induced bradycardia and hypertension in fetal sheep. This study was designed to determine the ADO receptor subtype that is involved in these cardiovascular responses. In chronically catheterized fetal sheep (>0.8 term), fetal hypoxemia was induced by having the ewe breathe a hypoxic gas mixture (9% O(2)-3% CO(2)-88% N(2)) for 1 h. Intra-arterial infusion of ZM-241385, an antagonist highly selective for ADO A(2A) receptors, to eight fetuses during normoxia significantly increased mean arterial pressure (MAP) from 42.5 +/- 2.0 to 46.1 +/- 2.0 mmHg without altering heart rate (HR). Infusion of a selective antagonist of ADO A(1) receptors [1, 3-dipropyl-8-cyclopentylxanthine (DPCPX)] elevated MAP and HR only after the infusion was terminated, although administration of the vehicle for ZM-241385 or DPCPX had no effect on MAP or HR. Isocapnic hypoxia with infusion of DPCPX or the vehicle for DPCPX or ZM-241385 produced a transient fall in HR, a rise in MAP, and a decrease in plasma volume. In contrast, ADO A(2A) receptor blockade abolished the hypoxia-induced bradycardia and hypertension and blunted the decline in plasma volume. We conclude that fetal ADO A(2A) receptors: 1) modulate AP during normoxia, and 2) mediate cardiovascular responses during acute O(2) deficiency.

Role of plasma adenosine in breathing responses to hypoxia in fetal sheep.

The importance of plasma adenosine in hypoxic inhibition of breathing movements was determined in chronically catheterized fetal sheep (greater than 0.8 term). Preductal arterial blood for adenosine measurements was withdrawn using a double lumen catheter to mix blood entering the catheter with a solution to stop adenosine metabolism. In 6 fetuses, isocapnic hypoxia (delta PaO2 congruent to -10 Torr) increased the average plasma adenosine concentration from 1.1 +/- 0.2 (SEM) to 2.0 to +/- 0.4 microM. During hypoxia, plasma levels of adenosine were inversely related to preductal arterial O2 content (CaO2) with values ranging between 1.6 and 4.0 microM when CaO2 was less than 3 ml/dl. Hypoxia also significantly reduced the incidence of fetal breathing and rapid eye movements. In other experiments, adenosine (0.36 +/- 0.03 mg/min/kg) was infused for one hour into the inferior vena cava of 5 fetuses. During this infusion, mean plasma concentration of adenosine was 2.8 +/- 0.3 microM, a value about 2.5 times the control average. Adenosine also significantly reduced the incidence of low voltage electrocortical activity, rapid eye movements and breathing activity. We conclude that hypoxic inhibition of fetal breathing most likely arises from an increase in central adenosine production, although during severe O2 deprivation (CaO2 less than 3 ml/dl) blood-borne adenosine could also contribute.

Maturation of respiratory responses to graded hypoxia in rabbits.

The respiratory effects of graded hypoxia were determined in 8 rabbits on postnatal days 1, 6 and 21. Ventilation was measured in the unanesthetized state by plethysmography. Graded hypoxia was produced by reducing the fraction of inspired O2 to 0.15 (mild hypoxia) for 15 min and then lowering it further to 0.10 (moderate hypoxia) for another 15 min. Mild hypoxia initially stimulated breathing to the same extent in all 3 groups; however, with moderate hypoxia, the percent increase in ventilation was greater for older rabbits. The rise in ventilation was due to an increase in tidal volume and frequency in 1- and 21-day-old rabbits, and it was almost entirely the result of changes in frequency in 6-day-old rabbits. After 2-3 min of hypoxia (mild or moderate), a decline in ventilation was observed in all 3 groups. This reduction of respiration was greater in younger animals and with moderate hypoxia. Decreases in frequency and tidal volume contributed significantly to this fall in ventilation for 1- and 2-day-old rabbits. However, the fall in ventilation for 6-day-old pups occurred almost entirely by changes in tidal volume. It is concluded that the magnitude of phase 1 (increased ventilation) and phase 2 (decreased ventilation) responses to hypoxia depend upon the level of hypoxia and the age of the rabbit.