Effect of long-term intermittent and sustained hypoxia on hypoxic ventilatory and metabolic responses in the adult rat

The effects of chronic sustained hypoxia (SH) on ventilation have been thoroughly studied. However, the effects of intermittent hypoxia (IH), a more prevalent condition in health and disease are currently unknown. We hypothesized that the ventilatory consequences of SH and IH may differ and be related to changes in N-methyl-D-aspartate (NMDA) glutamate receptor subunit expression. To examine these issues, Sprague-Dawley adult male rats were exposed to 30 days of either SH (10% O2) or IH (21% and 10% O2 alternations every 90 s) or to normoxia (RA), at the end of which ventilatory and O2 consumption responses to a 20-min acute hypoxic challenge (10% O2) were conducted. In addition, dorsocaudal brain stem tissue lysates were harvested at 1 h, 6 h, 1 day, 3 days, 7 days, 14 days, and 30 days of SH and IH and analyzed for NR1, NR2A, and NR2B NMDA glutamate receptor expression by immunoblotting. Normoxic ventilation was higher after both SH and IH (P < 0.001). Peak hypoxic ventilatory response was higher after SH but not after IH compared with RA. However, hypoxic ventilatory decline was more prominent after SH than IH (P < 0.001). NR1 expression showed a biphasic pattern of expression over time that was essentially identical after IH and SH (P value not significant). However, NR2A and NR2B expression was higher in IH compared with SH and RA (P < 0.01). We conclude that long-lasting exposures to SH and IH enhance normoxic ventilation but are associated with different time domains of ventilation during acute hypoxia that may be accounted in part by changes in NMDA glutamate receptor subunit expression.

Responses to hypoxia during early development

Sustained hypoxia evokes a predictable cascade of ventilatory, neurochemical, and metabolic responses. Responses in immature animals are characterized by earlier and more marked depression of ventilation than fully mature animals. Ventilation during hypoxia reflects a collective system output, incorporating a number of compensatory mechanisms (stimulation or depression) from multiple systems. The time course of these responses is clearly developmentally regulated. When hypercapnia interacts with hypoxia, the ventilatory responses are enhanced but other responses are apparently unchanged. We propose a model in which responses to intermittent stimuli vary according to the point within the sequence of a single response where the stimulus interruption occurs. An intermittent stimulus may be seen as 'continuous' if the recurrence frequency exceeds a certain threshold, whereas application of slower cycles below such threshold may elicit discordant recruitment of the compensatory responses. Indeed, experimental observations on intermittent (hypercapnic or poikylocapnic) hypoxia show excitatory or depressant effects that are dictated by the cycle duration. Subject to further testing, this model may help explain how detrimental effects of hypoxic events in infancy only affect selected groups.

Effect of stimulus cycle time on acute respiratory responses to intermittent hypercapnic hypoxia in unsedated piglets

To determine whether stimulus frequency affects physiological compensation to an intermittent respiratory stimulus, we studied piglets (n = 43) aged 14.8 +/- 2.4 days. A 24-min total hypercapnic hypoxia (HH) (10% O(2)-6% CO(2)-balance N(2) = HH) was delivered in 24-, 8-, 4-, or 2-min cycles alternating with air. Controls (n = 10) breathed air continuously. Minute ventilation and temperature were not different between the 2-min and 24-min groups, with neither different from controls during recovery. Piglets exposed to 8-min cycles had ventilatory stimulation, whereas those exposed to 4-min cycles had significant depression of ventilation. Despite this, piglets in these intermediate intermittent HH (IHH) groups (8- and 4-min cycles) showed more severe acidosis and attenuated temperature changes (P < 0.001 and P < 0.01 for pH and temperature vs. 24 min, respectively). Cycle time affected the ability of young piglets to tolerate IHH. More severe respiratory acidosis developed when IHH was delivered in intermediate (4 min or 8 min) cycles compared with the same total dose as a single episode or in short (2 min) cycles.

Respiratory effects of gestational intermittent hypoxia in the developing rat

Intermittent hypoxia (IH), one of the hallmarks of obstructive sleep apnea, occurs more frequently during pregnancy. We hypothesized that IH may lead to persistent postnatal changes in respiratory responses to acute hypoxia and may also lead to adverse effects on spatial function learning as revealed by the Morris water maze. To examine this issue, time-pregnant Sprague-Dawley rats were exposed to IH and room air (IHRA; 21 and 10% O2 alternations every 90 seconds) or to normoxia (RARA) until delivery. Ventilatory and metabolic responses to a 20-minute acute hypoxic challenge (10% O2) were conducted at postnatal ages 5, 10, 15, and 30 days. In addition, spatial task learning was assessed in the water maze at 1 and 4 months of age. Normoxic ventilation was higher at all time points in IHRA rats than in RARA rats (p < 0.01). Peak hypoxic ventilatory responses were attenuated in IHRA rats at 5 days of age and hypoxic ventilatory depression was accentuated at this age as well. However, ventilatory equivalents (minute ventilation/oxygen consumption) revealed significant reductions in peak hypoxic ventilatory responses of IHRA rats and hypoxic ventilatory depression at all postnatal ages (p < 0.01). Acquisition and retention of a spatial task were similar in the IHRA and RARA groups at both 1 and 4 months of age. We conclude that gestational intermittent hypoxia elicits long-lasting alterations in the control of breathing. We postulate that such IH-induced respiratory plasticity may create selective vulnerability to hypoxia during development.