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Gozal, David, José E. Torres, Yair M. Gozal, and Sanford M. Littwin. Effect of nitric oxide synthase inhibition on cardiorespiratory responses in the conscious rat. J. Appl. Physiol. 81(5): 2068–2077, 1996.—Nitric oxide synthase (NOS) blockade was used to test the cardioventilatory responses to hypercapnia and hypoxia in freely behaving animals. Chronically instrumented adult Sprague-Dawley rats were studied before and after intravenous administration of either 100 mg/kg of NG-nitro-l-arginine methyl ester (l-NAME), a nonspecific NOS blocker, or 10 mg/kg of S-methyl-l-thiocitrulline (SMTC), a selective neural NOS inhibitor.l-NAME injection induced sustained blood pressure (BP) elevation with transient tachycardia and increased minute ventilation (V˙e), which returned to baseline within minutes. SMTC elicited similar, although transient, BP increases; however, heart rate andV˙e decreased.l-NAME and SMTC did not modify overall steady-state hypercapnic responses. In control conditions, hypoxia induced early V˙eincreases with further V˙e enhancements at 30 min. l-NAME increased the early V˙e response to 10% O2but induced lateV˙e reductions in hypoxia. SMTC did not change early V˙e responses but induced marked reductions in the later V˙ehypoxic responses. In control animals, hypoxia induced a significant heart rate increase. This increase was absent during the early response after SMTC and was followed in bothl-NAME- and SMTC-treated animals by significant heart rate reductions to values below room air. Similarly, the sustained BP response to hypoxia in control animals was absent after administration of NOS inhibitors. These findings suggest that NOS activity exerts excitatory influences on respiration and cardiac chronotropy and sustained vasomotor tone during hypoxia. We speculate that NOS-mediated mechanisms may play an important role in hypoxia-induced ventilatory roll-off during wakefulness.