Vasodilator responses (ΔFVC) to incremental adenosine administration during saline, l-NMMA, aminophylline, and combined l-NMMA-aminophylline administration. In protocol 1 (A), nitric oxide synthase inhibition (l-NMMA) reduced ΔFVC at all doses of exogenous adenosine (ADO) compared with control (saline). Additionally, there was a significant main effect of drug during combined l-NMMA-aminophylline on the responsiveness to exogenous ADO compared with l-NMMA alone. In protocol 2 (B), adenosine receptor inhibition (aminophylline) reduced ΔFVC at all doses of exogenous ADO compared with control (saline). Combined aminophylline-l-NMMA did not reduce ΔFVC compared with aminophylline alone. *P < 0.01 vs. l-NMMA (protocol 1) or aminophylline (protocol 2) and combined l-NMMA-aminophylline; †P < 0.01 main effect combined l-NMMA-aminophylline vs. l-NMMA alone.