Re-enrollment: Subjects that fully complete the study will have the choice to re-enroll and complete the wing of the study they did not participate in. Subjects will be re-screened to determine eligibility and will be separated by at least 1 week from completion of the study. Furthermore, they only participate in the L-NMMA or BQ 123 (which ever they have not completed in the other wing of the study), they will not need to repeat the control/saline visit.