Figure 3. The influence of morphine (1 micro gram/ml), followed by morphine + nitro-L-arginine (L-NA; 10 sup -6 and 10 sup -5 M), and then morphine + L-NA (10 sup -5 M) + S-nitroso-acetylpenicillamine (SNAP; 10 sup -4 M) (n = 5) on PaCO2(top; expressed as the change from baseline), on dp/dt60(middle; expressed as percentage of baseline), and on nociceptive threshold (bottom; expressed as percentage of baseline). As a control, the time-dependent effects of continuous morphine perfusion (n = 4) are also presented. Values are means +/- SE *P < 0.05 versus baseline value (see Table 2); *P < 0.05 versus morphine alone.