Fig. 3. Effect of systemic DAMGO on the responsiveness of minute ventilation (V^E), respiratory frequency (f), and tidal volume (V^T) to progressive hypercapnia.  A represents the examples of the V^Eresponsiveness to end-tidal pressure of carbon dioxide (Petco²) before, 5 min after, and 30 min after DAMGO administration in an anesthetized rat, and the linear regression equations before, 5 min after, and 30 min after intravenous injection of DAMGO: (1) for V^E, y = 23.2x − 778.8, y = 14.2x − 503.1, and y = 19.8x − 655.1, respectively; (2) for f, y = 0.85x + 62.02, y = 0.95x + 50.16, and y = 0.78x + 60.04, respectively; and (3) for V^T, y = 0.19x − 5.29, y = 0.13x − 3.58, and y = 0.17x − 4.43, respectively. The range of  R ²in all equations was 0.94–0.99.  B –  D illustrate the group data of the slope for V^E, f, and V^T, respectively. “0” on the x-axis indicates the onset of intravenous administration of DAMGO. Data are presented as mean ± SE; n = 14. **  P < 0.01 compared with control by repeated one-way analysis of variance.