This study will examine how nitrite infusions affect exercise tolerance (how much a person
can exercise before having to stop). Exercise ability is limited by how fast oxygen can be
delivered to the body and how fast the body can produce energy. Both of these processes are
affected by nitric oxide (NO), a gas produced by cells that line blood vessels. NO is
important in regulating blood vessel dilation, and consequently, blood flow. Nitrite may act
as a storehouse for nitric oxide and be able to improve exercise tolerance.
Healthy normal volunteers between 21 and 45 years of age who can use an exercise bicycle may
be eligible for this study. Candidates are screened with a medical history, physical
examination, electrocardiogram, breathing tests, blood tests, and a pregnancy test for women
who are able to bear children. Pregnant women are excluded from the study. The screening
session includes practice exercise on the bicycle.
Participants exercise on a stationery exercise bicycle for about 30 minutes on each of two
study days. During the test, they breathe in and out of a mouthpiece that allows inhaled and
exhaled respiratory gases to be measured. Before subjects begin to exercise, a small tube is
placed in the artery of their forearm inside the elbow. A longer tube called a central line
is placed in a deeper vein in the neck after the area has been numbed. A thinner tube, called
a pulmonary artery catheter, is placed through the central line and advanced into the
chambers of the heart, through the heart valve, and into the lung artery. This catheter
measures various pressures directly in the heart and lungs. Blood samples are drawn through
the catheter also, to avoid the need for multiple needle sticks. Another tube is placed in
the vein of the other arm to deliver medications.
Thirty minutes after all the tubes are placed, a blood sample is drawn for baseline
measurements. Then, either saline (sterile salt water) or nitrite is injected into the tube
in the arm vein. Thirty minutes after the injection, the subject starts exercising on the
bicycle. The work setting on the bicycle is increased every minute, and the subject pedals
until he or she is too tired to continue. During the test, a small blood sample is collected
every 2 minutes. Heart rate, blood pressure, and heart rhythms are continuously monitored.
After the test on the first day, participants are admitted to the hospital to rest for the
remainder of the afternoon and evening. The tubes are kept in place for the following
morning, when the procedure is repeated exactly as before, except that subjects who received
saline the first day are given nitrite the second day, and vice versa.
During exercise, there is a lag in the rate at which oxygen uptake (VO2) rises to meet energy
demand. It is uncertain whether this limitation is due to inadequate O2 delivery to working
muscle, limitations to the rate at which mitochondria can generate ATP to meet demand, or by
a combination of both. Both of these limitations may be modulated by nitric oxide. Nitric
oxide (NO) has been implicated in numerous physiological functions, including control of
skeletal muscle vasodilation and oxidative metabolism. During exercise, NO will both
vasodilate skeletal muscle and modulate (inhibit) mitochondrial respiration. The latter
effect could either decrease oxygen extraction by limiting the ability of mitochondria to
utilize oxygen, or paradoxically increase oxygen utilization by inhibiting mitochondria
proximal to blood vessels, an effect that facilitates oxygen diffusion to distal tissue and
mitochondria (NO dependent facilitated oxygen diffusion). Previous studies have demonstrated
that NO production increases during exercise and regional inhibition of NO production from
endothelial NO synthase reduces exercise-dependent blood flow by approximately 10%. NOS
inhibitors such as N(G) -nitro-L-arginine methyl ester (L-NAME) have been shown to decrease
exercise tolerance, and NO precursors (L-arginine) to increase exercise tolerance.
Administration of inhaled NO during exercise has not been shown to increase exercise
Considering the potential role of nitrite bioconversion to NO during hypoxia, is likely that
nitrite plays an important role in modulating exercise physiology. We therefore hypothesize
that during aerobic, and in particular anaerobic exercise, erythrocyte and plasma nitrite
will be converted to NO and modulate muscle blood flow, mitochondrial respiration, oxygen
diffusion and ultimately maximal oxygen consumption. While we expect these effects will
increase maximal oxygen consumption and increase work output, it is also distinctly possible
that NO production from nitrite during exercise will inhibit mitochondrial respiration and
decrease maximal oxygen consumption. The purpose of the present study is to investigate the
effects of aerobic-to-anaerobic exercise on circulating nitrite stores in erythrocytes and
plasma in the arterial and central venous circulation and the effects of systemic nitrite
infusion on aerobic and anaerobic exercise capacity. Physiological parameters including
maximal oxygen consumption (VO2max), maximal CO2 and NO production (VCO2max and VNOmax),
maximal work, rate of perceived exertion (RPE), anaerobic threshold, and pulmonary gas
exchange (VO2, VCO2, VE) will be monitored during exercise with and without nitrite
infusions. Our primary endpoint will be VO2 max with nitrite infusion compared to VO2 max
with saline placebo infusion.
- INCLUSION CRITERIA:
Subjects must be 21-45 years of age.
Subject must be in good health and able to perform cycle ergometry for the length of the
Subjects must provide informed, written consent for participation in this study.
Female subjects of childbearing age must have a negative pregnancy test.
Subjects with a history of cardiac, pulmonary, peripheral vascular, or mitochondrial
Subjects with abnormal EKG other than sinus bradycardia.
Individuals with a future cardiovascular risk greater than 1 % in the next 10 years will be
excluded from the study. Risk will be calculated using the Framingham risk calculator
published on the web site: http://hin.nhlbi.nih.gov/atpiii/calculator.asp?usertype=prof.
Subjects with any physical condition (for example, knee problems) that may impair their
performance during exercise testing.
Subjects who have a history of smoking within one year.
Subjects with anemia (defined as hemoglobin less than 10 g/dL).
Subjects with a history of reaction to a medication or other substance characterized by
dyspnea and cyanosis will not participate in this study.
Subjects with G6PD deficiency.
Subjects with a baseline methemoglobin level greater than 1%.
Lactating females who are breastfeeding, will not participate since nitrite crosses into
breast milk and could cause methemoglobinemia in the infant.
No volunteer subject will be allowed to take any medication (oral contraceptive agents are
allowed), vitamin supplements, herbal preparations, nutriceuticals or other alternative
therapies for at least one month prior to study and will not be allowed to take aspirin or
NSAIDs for one week prior to study.
Subjects with a blood pressure of less than 100/70 mmHg on the study day will be excluded
from the protocol.