Atrial natriuretic peptide during and after maximal and submaximal exercise under normoxic and hypoxic conditions

Relationship between Blood Volume, Blood Lactate Quantity, and Lactate Concentrations during Exercise

We wanted to determine the influence of total blood volume (BV) and blood lactate quantity on lactate concentrations during incremental exercise. Twenty-six healthy, nonsmoking, heterogeneously trained females (27.5 ± 5.9 ys) performed an incremental cardiopulmonary exercise test on a cycle ergometer during which maximum oxygen uptake (V·O_2max), lactate concentrations ([La^-]) and hemoglobin concentrations ([Hb]) were determined. Hemoglobin mass and blood volume (BV) were determined using an optimised carbon monoxide-rebreathing method. V·O_2max and maximum power (P_max) ranged between 32 and 62 mL·min^-1·kg^-1 and 2.3 and 5.5 W·kg^-1, respectively. BV ranged between 81 and 121 mL·kg^-1 of lean body mass and decreased by 280 ± 115 mL (5.7%, p = 0.001) until P_max. At P_max, the [La^-] was significantly correlated to the systemic lactate quantity (La^-, r = 0.84, p < 0.0001) but also significantly negatively correlated to the BV (r = -0.44, p < 0.05). We calculated that the exercise-induced BV shifts significantly reduced the lactate transport capacity by 10.8% (p < 0.0001). Our results demonstrate that both the total BV and La^- have a major influence on the resulting [La^-] during dynamic exercise. Moreover, the blood La^- transport capacity might be significantly reduced by the shift in plasma volume. We conclude, that the total BV might be another relevant factor in the interpretation of [La^-] during a cardio-pulmonary exercise test.

Cardiac stroke volume in females and its correlation to blood volume and cardiac dimensions

We aimed to continuously determine the stroke volume (SV) and blood volume (BV) during incremental exercise to evaluate the individual SV course and to correlate both variables across different exercise intensities. Twenty-six females with heterogeneous endurance capacities performed an incremental cycle ergometer test to continuously determine the oxygen uptake (V̇O2), cardiac output (Q̇) and changes in BV. Q̇ was determined by impedance cardiography and resting cardiac dimensions by 2D echocardiography. Hemoglobin mass and BV were determined using a carbon monoxide-rebreathing method. V̇O2max ranged from 32 to 62 mL·kg-1·min-1. Q̇max and SVmax ranged from 16.4 to 31.6 L·min-1 and 90-170 mL, respectively. The SV significantly increased from rest to 40% and from 40% to 80% V̇O2max. Changes in SV from rest to 40% V̇O2max were negatively (r = -0.40, p = 0.05), between 40% and 80% positively correlated with BV (r = 0.45, p < 0.05). At each exercise intensity, the SV was significantly correlated with the BV and the cardiac dimensions, i.e., left ventricular muscle mass (LVMM) and end-diastolic diameter (LVEDD). The BV decreased by 280 ± 115 mL (5.7%, p = 0.001) until maximum exercise. We found no correlation between the changes in BV and the changes in SV between each exercise intensity. The hemoglobin concentration [Hb] increased by 0.8 ± 0.3 g·dL-1, the capillary oxygen saturation (ScO2) decreased by 4.0% (p < 0.001). As a result, the calculated arterial oxygen content significantly increased (18.5 ± 1.0 vs. 18.9 ± 1.0 mL·dL-1, p = 0.001). A 1 L higher BV at V̇O2max was associated with a higher SVmax of 16.2 mL (r = 0.63, p < 0.001) and Q̇max of 2.5 L·min-1 (r = 0.56, p < 0.01). In conclusion, the SV strongly correlates with the cardiac dimensions, which might be the result of adaptations to an increased volume load. The positive effect of a high BV on SV is particularly noticeable at high and severe intensity exercise. The theoretically expected reduction in V̇O2max due to lower SV as a consequence of reduced BV is apparently compensated by the increased arterial oxygen content due to a higher [Hb].

Effect of Exercise-Induced Reductions in Blood Volume on Cardiac Output and Oxygen Transport Capacity

We wanted to demonstrate the relationship between blood volume, cardiac size, cardiac output and maximum oxygen uptake (V.O_2max) and to quantify blood volume shifts during exercise and their impact on oxygen transport. Twenty-four healthy, non-smoking, heterogeneously trained male participants (27 ± 4.6 years) performed incremental cycle ergometer tests to determine V.O_2max and changes in blood volume and cardiac output. Cardiac output was determined by an inert gas rebreathing procedure. Heart dimensions were determined by 3D echocardiography. Blood volume and hemoglobin mass were determined by using the optimized CO-rebreathing method. The V.O_2max ranged between 47.5 and 74.1 mL⋅kg^–1⋅min^–1. Heart volume ranged between 7.7 and 17.9 mL⋅kg^–1 and maximum cardiac output ranged between 252 and 434 mL⋅kg^–1⋅min^–1. The mean blood volume decreased by 8% (567 ± 187 mL, p = 0.001) until maximum exercise, leading to an increase in [Hb] by 1.3 ± 0.4 g⋅dL^–1 while peripheral oxygen saturation decreased by 6.1 ± 2.4%. There were close correlations between resting blood volume and heart volume (r = 0.73, p = 0.002), maximum blood volume and maximum cardiac output (r = 0.68, p = 0.001), and maximum cardiac output and V.O_2max (r = 0.76, p < 0.001). An increase in maximum blood volume by 1,000 mL was associated with an increase in maximum stroke volume by 25 mL and in maximum cardiac output by 3.5 L⋅min^–1. In conclusion, blood volume markedly decreased until maximal exhaustion, potentially affecting the stroke volume response during exercise. Simultaneously, hemoconcentrations maintained the arterial oxygen content and compensated for the potential loss in maximum cardiac output. Therefore, a large blood volume at rest is an important factor for achieving a high cardiac output during exercise and blood volume shifts compensate for the decrease in peripheral oxygen saturation, thereby maintaining a high arteriovenous oxygen difference.

Limitation of Maximal Heart Rate in Hypoxia: Mechanisms and Clinical Importance

The use of exercise intervention in hypoxia has grown in popularity amongst patients, with encouraging results compared to similar intervention in normoxia. The prescription of exercise for patients largely rely on heart rate recordings (percentage of maximal heart rate (HR_max) or heart rate reserve). It is known that HR_max decreases with high altitude and the duration of the stay (acclimatization). At an altitude typically chosen for training (2,000-3,500 m) conflicting results have been found. Whether or not this decrease exists or not is of importance since the results of previous studies assessing hypoxic training based on HR may be biased due to improper intensity. By pooling the results of 86 studies, this literature review emphasizes that HR_max decreases progressively with increasing hypoxia. The dose–response is roughly linear and starts at a low altitude, but with large inter-study variabilities. Sex or age does not seem to be a major contributor in the HR_max decline with altitude. Rather, it seems that the greater the reduction in arterial oxygen saturation, the greater the reduction in HRmax, due to an over activity of the parasympathetic nervous system. Only a few studies reported HR_max at sea/low level and altitude with patients. Altogether, due to very different experimental design, it is difficult to draw firm conclusions in these different clinical categories of people. Hence, forthcoming studies in specific groups of patients are required to properly evaluate (1) the HR_max change during acute hypoxia and the contributing factors, and (2) the physiological and clinical effects of exercise training in hypoxia with adequate prescription of exercise training intensity if based on heart rate.

Extracellular bicarbonate and non-bicarbonate buffering against lactic acid during and after exercise

Defense of extracellular pH constancy against lactic acidosis can be estimated from changes (Delta) in lactic acid ([La]), [HCO(3)(-)], pH and PCO(2) in blood plasma because it is equilibrated with the interstitial fluid. These quantities were measured in earlobe blood during and after incremental bicycle exercise in 13 untrained (UT) and 21 endurance-trained (TR) males to find out if acute and chronic exercise influence the defense. During exercise the capacity of non-bicarbonate buffers (beta(nbi) = -Delta[La] . DeltapH(-1) - Delta[HCO(3)(-)] . DeltapH(-1)) available for the extracellular fluid (mainly hemoglobin, dissolved proteins and phosphates) amounted to 32 +/- 2(SEM) and 20 +/- 2 mmol l(-1) in UT and TR, respectively (P < 0.02). During recovery beta(nbi) decreased to 14 (UT) and 12(TR) mmol l(-1) (both P < 0.001) corresponding to values previously found at rest by in vivo CO(2) titration. Bicarbonate buffering (beta(bi)) amounted to 44-48 mmol l(-1) during and after exercise. The large exercise beta(nbi) seems to be mainly caused by an increasing concentration of all buffers due to shrinking of the extracellular volume, exchange of small amounts of HCO(3)(-) or H(+) with cells and delayed HCO(3)(-) equilibration between plasma and interstitial fluid. Increase of [HCO(3)(-)] during titration by these mechanisms augments total beta and thus the calculated beta(nbi) more than beta(bi) because it reduces DeltapH and Delta[HCO(3)(-)] at constant Delta[La]. The smaller rise in exercise beta(nbi) in TR than UT may be caused by an increased extracellular volume and an improved exchange of La(-), HCO(3)(-) and H(+) between trained muscles and blood.

Causes of differences in exercise-induced changes of base excess and blood lactate

It has been concluded from comparisons of base excess (BE) and lactic acid (La) concentration changes in blood during exercise-induced acidosis that more H+ than La- leave the muscle and enter interstitial fluid and blood. To examine this, we performed incremental cycle tests in 13 untrained males and measured acid-base status and [La] in arterialized blood, plasma, and red cells until 21 min after exhaustion. The decrease of actual BE (-deltaABE) was 2.2 +/- 0.5 (SEM) mmol l(-1) larger than the increase of [La]blood at exhaustion, and the difference rose to 4.8 +/- 0.5 mmol l(-1) during the first minutes of recovery. The decrease of standard BE (SBE), a measure of mean BE of interstitial fluid (if) and blood, however, was smaller than the increase of [La] in the corresponding volume (delta[La](if+blood)) during exercise and only slightly larger during recovery. The discrepancy between -deltaABE and delta[La]blood mainly results from the Donnan effect hindering the rise of [La]erythrocyte to equal values like [La]plasma. The changing Donnan effect during acidosis causes that Cl- from the interstitial fluid enter plasma and erythrocytes in exchange for HCO3(-). A corresponding amount of La- remains outside the blood. SBE is not influenced by ion shifts among these compartments and therefore is a rather exact measure of acid movements across tissue cell membranes, but changes have been compared previously to delta[La]blood instead to delta[La](if+blood). When performing correct comparisons and considering Cl-/HCO3(-) exchange between erythrocytes and extracellular fluid, neither the use of deltaABE nor of deltaSBE provides evidence for differences in H+ and La- transport across the tissue cell membranes.

Fluid-Regulatory Hormone Responses during Cycling Exercise in Acute Hypobaric Hypoxia

PURPOSE

This study was designed to describe the responses of fluid-regulating hormones during exercise in acute hypobaric hypoxia and to test the hypothesis that they would be dependent on the relative intensity of exercise rather than the absolute workload.

METHODS

Thirteen men cycled for 60 min on four occasions in the same individual hydration status: in normoxia at 55% and 75% of normoxia maximal aerobic power (N55 and N75, respectively), in hypoxia (PB = 594 hPa) at the same absolute workload and at the same relative intensity as N55 (H75 and H55, respectively). VO2, heart rate, and rectal and mean skin temperatures were recorded during exercise. The total water loss was measured by the difference in nude body mass adjusted for metabolic losses. Venous blood samples were drawn before and 15, 30, 45, and 60 min after the beginning of exercise to measure variations in plasma volume, osmolality, and concentrations in arginine vasopressin (AVP), atrial natriuretic factor (ANF), plasma renin activity (ARP), aldosterone (Aldo), and noradrenaline (NA).

RESULTS

During N55 and H55, AVP, Aldo and ARP did not change, whereas ANF increased slightly. Increases in AVP, Aldo, ARP, and NA were greater during N75 than during H75, whereas the increase in ANF was greater during H75 than N75.

CONCLUSION

Plasma levels of AVP, Aldo, and ARP increase during exercise when a threshold is reached and thereafter are dependent on the absolute workload, without any specific effect of hypoxia. The time course of ANF appears to be different from that of the other hormones.

Blood volume and hemoglobin mass in endurance athletes from moderate altitude

PURPOSE

To determine whether total hemoglobin (tHb) mass and total blood volume (BV) are influenced by training, by chronic altitude exposure, and possibly by the combination of both conditions.

METHODS

Four groups (N = 12, each) either from locations at sea level or at moderate altitude (2600 m) were investigated: 1) sea-level control group (UT-0 m), 2) altitude control group (UT-2600 m), 3) professional cyclists from sea level (C-0 m), and 4) professional cyclists from altitude (C-2600 m). All subjects from altitude were born at about 2600 m and lived all their lives (except during competitions at lower levels) at this altitude. tHb and BV were determined by the CO-rebreathing method.

RESULTS

VO2max (mL x kg(-1) x min(-1)) was significantly higher in UT-0 m (45.3 +/- 3.2) than in UT-2600 m (39.6 +/- 4.0) but did not differ between C-0 m (68.2 +/- 2.7) and C-2600 m (69.9 +/- 4.4). tHb (g x kg(-1)) was affected by training (UT-0 m: 11.0 +/- 1.1, C-0 m: 15.4 +/- 1.3) and by altitude (UT-2600 m: 13.4 +/- 0.9) and showed both effects in C-2600 m (17.1 +/- 1.4). Because red cell volume showed a behavior similar to tHb and because plasma volume was not affected by altitude but by training, BV (mL x kg(-1)) was increased in C-0 m (UT-0 m: 78.3 +/- 7.9; C-0 m: 107.0 +/- 6.2) and in UT-2600 m (88.2 +/- 4.8), showing highest values in the C-2600 m group (116.5 +/- 11.4).

CONCLUSION

In endurance athletes who are native to moderate altitude, tHb and BV were synergistically influenced by training and by altitude exposure, which is probably one important reason for their high performance.

Long-term physical exercise and atrial natriuretic peptide in obese Zucker rats

Endurance training increases natriuretic peptide synthesis in the hypertrophied myocardium of spontaneously hypertensive rats. We examined the effects of 22-week-long treadmill exercise on plasma and tissue atrial natriuretic peptide in Zucker rats, a model of genetic obesity and moderate hypertension without clear cardiac hypertrophy. The blood pressures of the animals were measured by the tail-cuff method, and plasma and tissue samples for the peptide determinations were taken at the end of the study. The training increased heart weight to body weight ratio, while atrial natriuretic peptide contents in the right and left atrium, ventricular tissue, and plasma did not change. The exercise prevented the elevation of blood pressure, which was observed in non-exercised obese Zucker rats, and also reduced blood pressure in the lean rats. In conclusion, these results suggest that in the absence of preceding myocardial hypertrophy, the long-term exercise-induced workload is not deleterious to the heart in experimental obesity, since no changes in plasma and tissue atrial natriuretic peptide were detected.

Plasma-electrolytes in natives to hypoxia after marathon races at different altitudes

PURPOSE

It is well known that altitude natives differ from sea level natives in aspects of fluid and electrolyte homeostasis.

METHODS

To evaluate exercise and environmental influences on the electrolyte and water status in hypoxia adapted subjects, we investigated 11 well-trained marathon runners (33.7 +/- 0.7 yr, 60.5 +/- 1.9 kg), native to an altitude above 2600 m, before and after two marathon races. One competition was held at moderate altitude (AM, 2650 m, 14 degrees C, 55% RH, running time 3 h 6 min +/- 22 min) and another under tropical conditions (HM, 470 m, 28 degrees C, 70% RH, running time 2 h 54 min +/- 30 min). Blood samples were taken 3 d before, immediately after, 1 h after, and 24 h after the races.

RESULTS

The loss in body fluid was calculated to be 2.15 L during AM and 5.05 L during HM, respectively. It was compensated mostly by ingested fluids without electrolyte content and by metabolically produced water, which led to hyponatremia during AM (plasma [Na+] from 144.3 +/- 0.7 to 131.7 +/- 2.1 mmol x L(-1)). Severe dehydration without significant changes in plasma [Na+] could be detected after HM. Serum antidiuretic hormone concentrations and serum aldosterone concentrations significantly increased during both races and remained at a high level for at least 1h after both competitions. Serum atrial natriuretic peptide (ANP) concentrations were at a high level at rest, increasing during HM, and decreasing during AM.

CONCLUSION

Under tropical conditions, we found a severe state of dehydration characterized by an extended ANP-response, which was not prevented by water intake during the race. Under hypoxic conditions, however, we found that hyponatremia had developed. This can be partly explained by pure water intake and metabolically produced water, and also, possibly, by a special hypoxia-induced effect.

Is urodilatin the missing link in exercise-dependent renal sodium retention?

The purpose of the present study was to investigate the behavior of plasma atrial natriuretic peptide [ANP-(99-126)] concentration ([ANP]) and renal urodilatin [Uro; ANP-(95-126)] excretion during and after exercise and their possible effects on renal Na+ retention. Ten male subjects performed a cycle ergometer test for 60 min at 60% of maximum workload. Blood and urine samples were collected before, during, and up to 24 h after exercise. During exercise, plasma [ANP] and renal Uro excretion were oppositely affected: whereas [ANP] increased from 46.5 +/- 5.1 to 124.1 +/- 10.6 pg/ml, urinary Uro excretion decreased from 120.8 +/- 16.0 to 49.5 +/- 9.8 fmol/min and remained at a lower level until 1 h after exercise. Glomerular filtration rate showed lowest values during exercise (from 164.9 +/- 15.3 to 75.8 +/- 10.1 ml/min), and urine flow and the fractional excretion rate of Na+ (FENa+) and Cl- (FECl-) had their nadir during the first hour after exercise. Positive relationships were observed between Uro excretion and FENa+ (P < 0.05) and FECl-, whereas a tendency toward a negative correlation was obtained between [ANP] and FENa+. It seems possible that Uro may be, among other factors, involved in the exercise-related regulation of renal Na+ retention. The specific roles Uro and ANP play during exercise, however, remain to be investigated.

Gas exchange and neurohumoral response to exercise: influence of the exercise protocol

Maximal oxygen uptake varies with the exercise protocol, but the extent to which hormonal and metabolic responses to exercise are influenced by the exercise protocol has not been precisely defined. Twelve healthy subjects underwent maximal exercise testing using two incremental bicycle tests with individualized, identical work rate increments between 40 and 70 W. One protocol employed a 1-min and the other a 3-min duration per stage. Expiratory gas and venous blood were sampled at regular intervals for metabolic and hormonal analysis. Exercise duration for the 1-min and 3-min protocols was 6.0 +/- 0.1 and 14.3 +/- 0.3 min, respectively (P < 0.001). Significantly higher values were observed for peak VO2 and maximal ventilation during the 3-min protocol compared with the 1-min protocol (41.1 +/- 1.8 vs 38.3 +/- 1.6 ml.kg-1.min-1, P < 0.001; and 104.9 +/- 8.0 vs 97.2 + 5.7 l.min-1, P < 0.05, for peak VO2 and peak ventilation, respectively). However, the maximal workload achieved was higher during the 1-min versus the 3-min protocol (330 + 24 vs 280 + 21 W, P < 0.01). No differences were observed for maximal heart rate or blood pressure, whereas maximal plasma lactate was roughly twice as high during the 3-min compared with the 1-min protocol (7.5 +/- 0.8 vs 3.8 +/- 0.5 mmol.l-1, P < 0.001). Norepinephrine, epinephrine, dopamine, and growth hormone levels were generally higher throughout exercise during the 3-min compared with the 1-min protocol. When expressed as a percentage of peak VO2, however, differences in catecholamine levels were not observed. Endothelin levels did not change. We conclude that the exercise protocol profoundly influences exercise capacity as well as the metabolic and hormonal response to exercise and should be considered when using these variables to evaluate an intervention.