John A. Krasney

Effect of graded exercise on nitric oxide in expired air in humans

This study was performed to determine the influence of graded dynamic exercise and of voluntary hyperventilation on both the concentration ([NO]) and the amount of nitric oxide per unit time (VNO) in exhaled air. Young human subjects (n = 8) of varying fitness levels having peak O2 consumption (VO2) values ranging between 25.7 and 50.9 ml/min/kg were studied during graded levels of treadmill exercise. Expired [NO] determined by chemiluminescence was 26.3 +/- 6.7 SE parts per billion (ppb) at rest ranging between 11 and 66 ppb. Although variable, [NO] was maintained as work rate increased. VNO rose significantly in most subjects from a mean resting value of 12.3 +/- 3.5 nmol/min. VNO correlated linearly and significantly with ventilation (VE) and CO2 output in 6 of 8 subjects, with VO2 in 4 of 8 subjects, and with heart rate in 5 of 8 subjects. Increases of VNO per unit increase of VE were significantly higher in subjects having higher peak VO2 levels. Voluntary hyperventilation (two-fold of the control VE) for 1 min in 6 subjects decreased expired [NO] from 9.5 (+/- 2.5) to 4.8 (+/- 2.8) ppb and VNO was unchanged, while hyperventilation at 3 x control VE increased VNO by 50% and [NO] decreased to 4.7 +/- 1.8 ppb. VNO appeared to be related to VO2 during hyperventilation. The results suggest that VNO can be correlated with ventilation and heart rate during exercise and with VO2 during both exercise and hyperventilation. [NO] is influenced by the flow rate of the expired air whereas VNO is influenced by NO clearance at the alveolus.

Cerebral Blood Flow and Metabolic Responses to Sustained Hypercapnia in Awake Sheep

This investigation determined the effects of sustained hypercapnia on cerebral blood flow (CBF; radiolabeled microspheres), cerebral metabolic rates for O2 and glucose (CMRO2 and CMRglc), and brain water content in conscious sheep instrumented with aortic, left ventricular, vena cava, and brain sagittal sinus catheters. PaCO2 was elevated from 38 ± 3 to 53 ± 3 (mean ± SD) mm Hg and PaO2 from 109 ± 7 to 131 ± 4 mm Hg for 96 h in an environmental chamber. Hypercapnia did not alter sheep behavior, food and water intake, arterial pressures, core temperature, or brain lactate release. Total and regional CBF and CBF/CMRO2 reached peak values at 1 h and then readjusted, to stabilize at lower, but still elevated levels at 24 h and thereafter. CMRO2 and CMRglc increased at 6 h and thereafter during hypercapnia. PaCO2, CBF, CMRO2, and CMRglc remained elevated at 3 h after restoration to room air, while CBF/CMRO2 returned to the control value. Frontal and occipital lobe wet-to-dry weight ratios increased modestly but significantly after hypercapnic exposure. It is concluded that sustained hypercapnia induces stable and nonadapting increases in both CBF and brain metabolism that persist for at least 3 h after restoration to room air in association with hypoventilization and modest elevations of brain water.

Cardiac output and regional oxygen transport in the acutely hypoxic conscious sheep.

We have studied the effects of severe acute hypoxemia (PaO2 = 25 torr) on cardiac output (Q), heart rate (HR), left ventricular contractility ((dP/dt)max/P), intravascular pressures and blood flow to the heart, brain, abdominal viscera, skin and respiratory and non-respiratory muscles in twelve conscious ewes that breathed a mixture of 8% O2 and 92% N2 for 20 min. Q, HR, (dP/dt)max/P) and systemic and pulmonary arterial pressures increased. Total peripheral resistance decreased while pulmonary vascular resistance remained unchanged. Coronary, cerebral, respiratory and nonrespiratory muscle and adrenal flows increased, in association with a decrease in regional vascular resistances, while the flows to the kidney and other abdominal viscera remained unchanged. The concentration of total plasma catecholamines doubled, indicating that the sympathetic nervous system plays a major role in the hemodynamic response to this level of hypoxia. Increased oxygen delivery to the heart (31%) and respiratory muscles (44%) were brought about by increases in both the magnitude and the redistribution of Q, the latter being the more important of the two mechanisms. In contrast, both mechanisms contributed equally to the amount of oxygen delivered to the brain and nonrespiratory muscles. We concluded that in acute hypoxemia, both the increase in Q and its regional redistribution contribute to the delivery of oxygen to the various tissues.

Effect of gravity on lung exhaled nitric oxide at rest and during exercise.

Exhaled nitric oxide (NO) from the lungs (VNO) in nose-clipped subjects increases during exercise. This may be due to endothelial shear stress secondary to changes in pulmonary blood flow. We measured VNO after modifying pulmonary blood flow with head-out water immersion (WI) or increased gravity (2 Gz) at rest and during exercise. Ten sedentary males were studied during exercise performed in air and WI. Nine subjects were studied at 1 and 2 Gz. Resting NO concentrations in exhaled air ([NO]) were 16.3 +/- 8.2 ppb (air). 15 +/- 8.2 ppb (WI) and 17.4 +/- 5 ppb (2 Gz). VNO (ppb/min) was calculated as [NO]VE and was unchanged at rest by either WI or 2 Gz. VNO increased linearly with Vo2, VE and fii during exercise in air, WI and at 2 Gz. These relationships did not differ among the experimental conditions. Therefore, changes in pulmonary blood flow failed to alter the output of NO exhaled from the lungs at rest or during exercise.

Effects of immersion in cool water on lung-exhaled nitric oxide at rest and during exercise

Lung nitric oxide (NO) has been postulated to relax airway and vascular smooth muscle at rest and during exercise. As a cold environment is a common cause of respiratory distress, lung exhaled NO was measured during skin and core body cooling at rest and during a progressive cycle exercise. Ten healthy male subjects were immersed in water at a water temperature (Tw) which was thermal neutral (35 degrees C) at 30 degrees C Tw, at which only skin temperature is decreased; and at 20 degrees C Tw, at which the core temperature is decreased (0.05 degrees C). At rest, V(O), and V(E) increased while exhaled NO concentration [NO] and the rate of expiration of NO (V(NO)) decreased with decreased Tw. V(O2) and ventilation (V(E)) increased with workload (W) and the values at all Tw were not different, whereas, [NO] decreased with W and the values during exercise were progressively less at all Ws as Tw declined. These results indicate that lung NO output is reduced in a graded fashion during body cooling at rest and during exercise. This suggests that lower lung NO may contribute to airway obstruction in cold environments and NO may contribute to regulation of lung heat and water exchange.

Head-Out Water Immersion: A Critical Evaluation of the Gauer-Henry Hypothesis

In recent years, there has been a renewed interest in the study of the physiological responses elicited during head-out water immersion (WI) in both humans and animals. This interest is based on the observation that WI leads to stereotyped cardiovascular, renal, fluid shift, and endocrine responses which develop rapidly. The homeostatic basis or rationale for this response pattern which involves the integrated responses of multiple systems is unclear at the present time. Therefore one goal of this chapter is to attempt to identify certain critical physiologic variables which may be regulated during the course of this complex response pattern which is generally elicited during the simple act of returning to the aquatic environment from whence we came. The major focus of this analysis will be on the hypothesis originally proposed by Gauer and Henry (34) concerning a physiologic link between heart and kidney and the particular role this link may play in governing the hormonal responses elicited during WI. In this regard, a number of excellent reviews representing several points of view related to this issue have been published (25,34,35,74).

Regional circulatory responses to 96 hours of hypoxia in conscious sheep

Exposure of adult ewes to normobaric hypoxia (PaO2 40 mm Hg) for 96 h led to increases of VE (+ 54%), while VO2 decreased by 48%. PaCO2 declined progressively to stabilize at 24 (+/- 1.5 SE) mm Hg by 24-48 h. Cardiac output (thermodilution) was elevated temporarily for 24 h (23-34%) but then returned to normoxic levels, while heart rate (28-42%) and pulmonary artery pressure (38-56%) were increased for the duration of hypoxia. Cerebral blood flow (radiolabelled microspheres) increased transiently for 48 h from 65.9 (+/- 4.4) to 100.4 (+/- 9.9) ml X min-1 X 100 g-1 with no change in its regional distribution. Coronary flow was elevated for the duration of hypoxia from 181 (+/- 15) to between 280 (+/- 33) and 350 (+/- 37) ml X min-1 X 100 g-1 with a more pronounced increment in right heart flow, and a decline in the endocardial/epicardial flow ratio. These regional flow increases resulted from a sustained decrease in pancreatic flow from 234 (+/- 11) to 125 (+/- 13) ml X min-1 X 100 g-1 for 96 h, with persisting decreases in splenic flow from 249 (+/- 30) to 100 (+/- 18), and in renal cortical flow from 787 (+/- 70) to 540 (+/- 31) ml X min-1 X 100 g-1, occurring at 48 and 72 h, respectively. Therefore, there is a redistribution of cardiac output during 96 hours of hypoxia with increased flows to heart and brain, and decreased flows to abdominal viscera.

Control of ventilation during graded exercise in the dog

We analyzed the time courses of the VE, VT, and f responses to graded levels of exercise produced by increases in treadmill speed at preset inclines in 207 experiments on 15 tracheostomized dogs. At the onset of work, VE increased within 1-2 respiratory cycles (VE fast), and then either remained constant, or decreased. Following this time delay (TD), VE rose more slowly (VE slow) to attain a stable plateau (Dejours). The amplitude of VE fast, VT, and f during the TD were independent of the work load. However, the duration of the TD and the amplitude of the component mediating VE slow were workload dependent. The VE fast and the VE during the TD are the major components of the total VE response at low work levels (VO2 = 30-40 ml . min-1 . kg-1) and are mediated primarily by increased f, whereas, at higher VO2 (70-90 ml . min-1 . kg-1), VE slow is mediated largely by increased VT and this component is engaged earlier to make a greater contribution to the total VE response. In the conscious dog, the total VE response to exercise appears to be comprised of both neural and humoral components when thermal stress is minimal.

Methemoglobin production by nitric oxide in fresh sheep blood.

As nitric oxide (NO) inhalation is used therapeutically, we studied the production of methemoglobin (metHb) by NO in fresh adult sheep blood. NO solutions were prepared by bubbling a 10% NO-90% N2 gas mixture in phosphate-buffered saline (pH = 7.41 at 20 degrees C) for at least 60 min. Fresh blood samples were obtained from catheterized femoral arteries or veins just prior to mixing with NO solution. Measurements of metHb were made at times 0, 30 sec, 2 min and 5 min after mixing of NO-containing buffer and blood using a Radiometer OSM-3 hemoximeter. Mixing was performed using two syringes connected via a stopcock. The reaction of NO with blood occurred rapidly after mixing since data values for each of the time points after 30 sec were unchanged for all mixtures. The mixing volume ratio of NO-containing buffer to blood was either 1:1 (protocol A) for comparisons of arterial vs venous blood, or the ratios were randomized (protocol B) to investigate effects of Hb oxygenation. Protocol A elicited only slight increases of metHb in arterial and venous blood which did not differ significantly. In protocol B, an increase of metHb was associated with a relative decline of deoxyhemoglobin (deoxyHb). A higher molar ratio of NO/deoxyHb yielded greater amounts of metHb. Therefore, in fresh sheep blood, deoxyHb is converted to metHb in the presence of NO. This reaction is not affected by the presence of oxyhemoglobin.

Peripheral circulatory responses to 96 hours of eucapnic hypoxia in conscious sheep

Conscious sheep acclimatizing to hypoxia (PaO2 40 mm Hg, PaCO2 24 mm Hg) respond with increases in cardiac output (Qco) and cerebral blood flow lasting for 24 and 48 h, respectively. Coronary flow increases in a sustained fashion, while there are progressive decreases in renal, splenic and pancreatic flows. In the present study, 5 adult ewes were exposed to similar levels of normobaric hypoxia (PaO2 40 mm Hg) but the PaCO2 was maintained at eucapnic levels (32 mm Hg). VE increased (+210%) while VO2 decreased by 35%. Ventilatory sensitivity to CO2 was unchanged. Qco (thermodilution) was elevated for 96 h (+20%) as stroke volume was maintained at normoxic levels and heart rate increased (+36%). Pulmonary artery pressure increased (+35%) along with plasma catecholamine levels (+116-196%). There were sustained elevations of cerebral flow (radiolabelled microspheres) from 79.1 (+/- 9.2 SEM) to 121.6 ml X min-1 X 100 g-1 (+/- 10.8), coronary flow from 183 (+/- 22.1) to 373 ml X min-1 X 100 g-1 (+/- 46.3), diaphragm flow (+400%) and intercostal muscle flow (+186%) with no apparent redistribution of Qco. Therefore, the cardiac and peripheral circulatory response patterns are altered significantly in eucapnic hypoxia. The rate of O2 delivery to brain and several abdominal viscera is higher.