In the present study we investigated whether hypocapnia that accompanies hypoxic hyperventilation might affect the biphasic, stimulatory/depressant, ventilatory response to hypoxia. The experiments were carried out in anesthetized, vagotomized, spontaneously breathing, and poikilocapnic rats. The animals were subjected to acute steady-state hypoxia consisting of 12% O(2) in N(2) in inspiratory mixture. Ventilation and its frequency and volume components were assessed from the integrated

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... yographic activity of the diaphragm. We found that despite the development of significant hypocapnia, the hypoxic ventilatory response consisted of rapid stimulation followed by a gradual decline. The frequency component contributed more to the ventilatory increase than that of volume. The results indicate that the hypoxic ventilatory profile in the anesthetized poikilocapnic rat resembles that known to be present during isocapnia. We conclude that hypocapnia neither hampers the hypoxic ventilatory reactivity nor alters the biphasic hypoxic ventilatory profile. These observations may aid planning experimental rat model studies.