Fig. 1.

Progressive augmentation of the short-term hypoxic phrenic response during severe (light gray bars) but not moderate (dark gray bars) hypoxic episodes or time control groups (normoxia; black bars) where 1, 2, and 3 represent hypoxic or normoxic episodes 1, 2, and 3. In A, the hypoxic response after no drug was administered and during severe acute intermittent hypoxia (AIH) progressively increases with each episode. [Asterisk (*) indicates 3 is significantly greater than 1 and 2]. Neither intrathecal MSX-3 (B) nor methysergide (C) alters this effect, demonstrating that progressive augmentation is independent of spinal serotonin or A2A receptor activation. Moderate AIH did not elicit progressive augmentation, similar to previous reports from our laboratory. There were no changes in time control experiments. Plus (+) indicates a greater response relative to time control experiments at equivalent times. (#) indicates that 1, 2, and 3 during moderate AIH and time control experiments are significantly lower than severe 1, 2, and 3, respectively. (<) indicates 2 is significantly higher than 1 in B and C. Values are means ± 1 SE; significance is P < 0.05.