Effects of hypoxia and atrial natriuretic peptide on aldosterone secretion in healthy subjects

Functional state of the students’ cardiovascular system under the conditions of mountain hypoxia

The article presents the functional state of the cardiovascular system of students under conditions of high-altitude hypoxia. From the analysis of the results of our research, it can be argued that due to the fact that students live under conditions of a reduced oxygen content in the atmospheric air, there is a significant increase in the systolic output of their hearts. Thus, the systolic blood volume of students under the conditions of mountain hypoxia increases, reaching the maximum values of 63.4 ml (P <0.05) in males and 64.4 in females in the middle-level mountains. The range of pulse pressure between the groups of males is 1.5%, and females 1.7. The level of average dynamic blood pressure in males in the middle mountains is 3.5% lower, and in females of the same age is 3.9% lower, respectively, compared to the plains. The drop in the minute blood volume at an altitude of 1600 m among female students was 2.1%, and among peers - 2.2, compared to the 170 m level. The decrease in total peripheral vascular resistance (TPVR) at the maximum altitude was 1.5% in females, and 2.7% in males, compared to the plain-related data. The value of blood circulation efficiency coefficient (CEC) and endurance factor (EF) in the middle-level mountains among female students decreased by 6.2 and 2.7%, among peers - by 4.2 and 5.4%, compared to the plain. Thus, the depth of the changes caused is directly proportional to the level of the acting hypoxia.

Role of Hypobaria in Fluid Balance Response to Hypoxia

To estimate the separate and combined effects of reduced P(B) and O2 levels on body fluid balance and regulating hormones, measurements were made during reduced PB (altitude, ALT; P(B) = 432 mm Hg, F(I(O2)) = 0.207), reduced inspired O2 concentration (normobaric hypoxia, HYX; P(B) = 614 mm Hg, F(I(O2)) = 0.142), and lowered ambient pressure without hypoxia (normoxic hypobaria HYB; P(B) = 434 mm Hg, F(I(O2)) = 0.296). Nine fit and healthy young men were exposed to these conditions for 10 h in a decompression chamber. Lake Louise AMS scores, urine collections, and blood samples were obtained every 3 h, with recovery measurements 2 h after exposure. AMS was significantly greater during ALT than HYX, as previously reported (J. Appl. Physiol. 81:1908-1910. 1996), because the combination of reduced P(B) and P(O2) over the 10 h favored fluid retention by reducing urine volume, while plasma volume (PV) remained higher than during HYX. At ALT the plasma Na+ fell significantly at 6 h, probably from dilution of extracellular fluid, and antidiuretic hormone (ADH) was highest (p = 0.006 versus HYB). The PV, urine flow, free water clearance, and plasma renin activity (PRA) rose significantly during recovery from ALT as AMS symptoms subsided, suggesting increased intravascular fluid and reduced adrenergic tone. During HYB, the plasma aldosterone (ALDO) and K+ levels were significantly elevated, and PRA was highest and ADH lowest, without fluid retention. During HYX, fluid balance was similar to HYB, but PV and ALDO were significantly lower, and ALDO increased significantly in recovery from HYX. The fluid retention at ALT in AMS-susceptible subjects appears related to a synergistic interaction involving reduced P(B) and ADH and ALDO.

Early fluid retention and severe acute mountain sickness

Field studies of acute mountain sickness (AMS) usually include variations in exercise, diet, and environmental conditions over days and development of clinically apparent edemas. The purpose of this study was to clarify fluid status in persons developing AMS vs. those remaining without symptoms during simulated altitude with controlled fluid intake, diet, temperature, and without exercise. Ninety-nine exposures of 51 men and women to reduced barometric pressure (426 mmHg = 16,000 ft. = 4,880 m) were carried out for 8–12 h. AMS was evaluated by Lake Louise (LL) and AMS-C scores near the end of exposure. Serial measurements included fluid balance, electrolyte excretions, and plasma concentrations, regulating hormones, and free water clearance. Comparison between 16 subjects with the lowest AMS scores near the end of exposure (“non-AMS”: mean LL = 1.0, range = 0–2.5) and 16 others with the highest AMS scores (“AMS”: mean LL = 7.4, range = 5–11) demonstrated significant fluid retention in AMS beginning within the first 3 h, resulting from reduced urine flow. Plasma Na+ decreased significantly after 6 h, indicating dilution throughout the total body water. Excretion of Na+ and K+ trended downward with time in both groups, being lower in AMS after 6 h, and the urine Na+-to-K+ ratio was significantly higher for AMS after 6 h. Renal compensation for respiratory alkalosis, plasma renin activity, aldosterone, and atrial natriuretic peptide were not different between groups, with the latter tending to rise and aldosterone falling with time of exposure. Antidiuretic hormone fell in non-AMS and rose in AMS within 90 min of exposure and continued to rise in AMS, closely associated with severity of symptoms and fluid retention.

Physiology and pathophysiology of blood volume regulation

Adequate cardiac output and tissue perfusion is dependent on intravascular blood volume and its adequate return to the heart. Considering the overall functions of the cardiovascular system in ensuring appropriate flow to the peripheral microcirculation, it is not surprising that conflicts of interest may occur when an organism is exposed to stresses. Of particular importance are the stresses in which there are increased oxygen demands, decreased oxygen availability and concomitant requirements for thermoregulation. When there is depletion of intravascular blood volume, the splanchnic circulation is in effect an "autologous blood bank" for maintaining venous return until trans-capillary refill and haemodilution occurs. With the acute haematological stress response centralisation of blood, secondary contraction of the venous capacitance occurs, as seen with acute hypoxia. This results in overfilling of the heart, activation of atrial volume receptors, release of atrial natriuretic peptide and subsequent reduction of the plasma volume by rapid shifting of plasma into the lymphatic capacitance (via spleen) and transcapillary efflux throughout the circulation. In this overview the physiology and pathophysiology of blood volume, red cell mass and plasma volume regulation is reviewed.

Sustained systemic arterial hypertension induced by extended hypobaric hypoxia

Regular administration of recombinant erythropoietin (EPO) is frequently complicated by a rise in arterial blood pressure. We therefore asked if prolonged stimulation of endogenous EPO production has the same effect. To this end, male Sprague-Dawley rats were placed in a hypobaric chamber (390 mm Hg) for 24 days. The control (NL) group was placed in the chamber at normobaric condition. The animals were then removed from the chamber and monitored through day 108. Plasma EPO peaked within 24 hours and returned to baseline by day 7 and remained so thereafter. Hematocrit rose steadily during the hypoxic phase and declined steadily during the normobaric phase, reaching the baseline on day 45. This was accompanied by parallel changes in erythrocyte mass and blood volume. The rise in hematocrit during hypoxia was accompanied by a parallel rise in blood pressure which peaked on day 24. Despite the restoration of normal hematocrit, erythrocyte mass and blood volume following resumption of normoxia, blood pressure remained elevated throughout the observation period. To dissect the role of hypoxia from that of the associated rise in hematocrit, the experiments were repeated using a group of rats whose hematocrits were kept constant by repeated phlebotomies. These animals exhibited a sustained rise in blood pressure identical to that found in the original group. Thus, prolonged hypobaric hypoxia leads to a severe hematocrit-independent systemic hypertension (HTN) that persists long after the restoration of normoxia. Given the transient nature of the rise in its plasma concentration, endogenous EPO does not appear to play a role in the genesis of the observed systemic HTN. The authors believe that this animal model can be used for future studies of the mechanism, consequences and treatment of acquired HTN.