I. NOS inhibition: L - NMMA is a potent non-selective NOS inhibitor used to study vascular physiology. L-NMMA will be infused at 3 mg/kg body weight/hr bolus (over 10min ) followed by a maintenance infusion of 1 mg/kg/hr for the duration of the experiment. Systemic delivery of L - NMMA has been shown to elevate blood pressure ~8-15 mmHg that is within the range observed during exercise.