The effect of short post-apnea time on plasma triglycerides, lipoprotein and cholesterol derived oxysterols levels

Introduction

During dives, apnea or breath hold divers undergo extreme hypercapnia and hypoxemia [1]. While large inter-individual variations in apneic capabilities exist, the transition from physiological to pathophysiological reactions to apneic diving are often not fully understood [2]. Studies have reported, inter alia, changes in cerebral and peripheral tissue oxygenation as part of the diving response [3], changes in, e.g., hemoglobin and blood cell concentration levels [4] or higher long-term risks for the prevalence of chronic kidney disease [5]. Since apnea diving as a recreational sport is becoming increasingly popular, so will possible associated health risks [2]. Therefore, studies have been initiated to assess possible pathophysiological processes in non-elite apnea divers concerning inflammatory processes and their burden on the cardiovascular system [6], [7], [8]. Hypoxemia as a key point in apnea diving is also a phenomenon seen in illness conditions, such as obstructive sleep apnea (OSA) syndrome [9]. In OSA patients, the risk of developing cardiovascular disease is increased. However, for apneic divers, long term studies are missing [10]. Morin et al. reported that hypoxia may have effects on triglyceride metabolism via production of very low-density lipoproteins (VLDL) and chylomicrons and highlights the effects of hypoxia on circulating blood lipids, especially triglycerides [11]. Morin et al. further reviewed that simulated hypoxia led either to increased or to no differences in triglyceride levels [11]. Hypoxia exposition times of 6 h led to no differences in fasting healthy young men [12, 13], to increased blood triglyceride levels in prandial participants (15%) [14] and in postprandial healthy young men, to higher increased blood triglyceride levels (45%) [15]. In OSA patients, no differences in blood triglyceride levels were observed after a hypoxia exposition time of 6 h [15]. However, no short-time hypoxia expositions, especially with regard to plasma triglycerides, lipoprotein and cholesterol derived oxysterol levels, have been performed in voluntary apnea divers to date. In general, triglycerides are transported within the core of lipoproteins [16]. Postprandially, they are transported with chylomicrons in the intestine and during fasting episodes, the liver secretes VLDL transporting triglycerides [17]. VLDL are inter alia metabolized to low density lipoproteins (LDL) [16]. Hypertriglyceridemia and elevated LDL levels are associated with an increased risk of cardiovascular diseases, especially coronary heart disease and stroke [18]. Autoxidation of cholesterol, e.g., in LDL leads to formation of oxysterols (e.g., 7 ketocholesterol) [19]. Oxysterols are oxygenated derivates of cholesterol and are not only generated via autooxidation, but also via enzymatic formation involving different enzymes [20, 21]. Oxysterols are linked to several biological functions and pathologies, such as cardiovascular, metabolic, and neurodegenerative diseases as well as cancer [22]. Focusing on the cardiovascular pathologies, cholesterol and oxysterols are involved in atherosclerosis, oxidative stress, and inflammatory processes [23]. To date, several investigations based on protein coding messengers and endothelial cell-derived microparticles (EMPs) as well as magnetic resonance imaging have shown that apnea diving could adversely affect the cardiovascular system [6, 24].

To our knowledge, no investigations have been performed regarding cholesterol catabolism in voluntary apnea divers. We hypothesize that short time hypoxemia leads to altered triglyceride, cholesterol, and oxysterol plasma levels in voluntary apnea divers.

Aim of this explorative single-center prospective study is to identify effects on cholesterol and oxysterols after a short-time maximal apnea, to compare results with long-term data of hypoxia on circulating blood lipid levels and to further improve knowledge of pathophysiological processes affecting the cardiovascular system in apneic diving.

Materials and methods

Results

Mean age of divers was 41 ± 1 years. The mean total apnea time was 317 s [±111 SD] and the mean minimal peripheral oxygen saturation (SpO₂) was 79 [±12 SD] [7]. Mean values, Standard error of the mean (SEM), mean differences, 95%- confidence intervals (CI), p-values and slopes for different time points for (apo)lipoproteins, triglycerides and total cholesterol (enzymatically) are presented in Table 1, for cholesterol (GC-FID) and absolute oxysterol concentrations in Table 2 and for oxysterols corrected for cholesterol measured by GC-FID in Table 3.

Spearman’s correlation analysis between apnea time and differences in lipoprotein cholesterol concentrations between the individual time points showed statistically significant correlations for HDL (T2-T1; R=0.716, p=0.024), LDL (T2-T1; R=0.787, p=0.009) and LDL (T3-T2; R=0.699, p<0.029).

Discussion

To our knowledge, no short-time hypoxia expositions, especially with regard to plasma triglycerides, lipoprotein and cholesterol derived oxysterol levels, have been performed in voluntary apnea divers to date. After a single maximal dry apnea, we observed significantly reduced levels of apolipoprotein B, lipids (triglycerides, cholesterol, HDL, LDL) and oxysterols (7β-OHC, 24-OHC, 27-OHC and 7-KC). Nevertheless, GC-cholesterol corrected plasma levels showed no significant changes after a short post-apnea time except for a statistically significant elevation of the 3β5a, 6β-cholestantriol ratio.

In contrast to 6 h hypoxia in fasting healthy young men as published by Morin et al. we indeed found a reduction in triglycerides and low-density lipoproteins after 0.5 h, eventually resulting in lower plasma sterols and oxysterols [15]. In ten elderly male subjects with chronic stable angina, no changes in fasting apolipoprotein, lipoprotein, and lipid levels before and after carbon monoxide hypoxia exposures were observed [27]. Data from rodent studies also showed a wide range of increasing triglyceride levels after different hypoxia times [11]. In contrast to these literature observations, we used a different hypoxia model while studying short post-apnea times after a single maximal dry apnea in healthy trained divers. Interestingly, experiments with single hypoxia exposures in mice resulted in increased triglyceride levels via decreasing plasma lipoprotein clearance, but experiments were carried out below thermoneutrality [28]. When performing experiments under thermoneutral conditions, no effects on plasma triglyceride levels were observed [29]. It is known that under acute hypoxia, sympathetic nervous system arousal induces adipose tissue lipolysis, but whether hypoxia affects fasting triglyceride levels in humans is still controversially discussed [13]. In a rodent model, rats exposed to continuous low-pressure oxygen showed reduced levels of, inter alia, LDL-cholesterol, triglycerides and cholesterol, whereas LDL-cholesterol values were higher than in the control group [30]. In human monocyte-derived macrophages under hypoxia, the intracellular metabolism of LDL cholesterol was affected, and the authors speculate that hypoxia might adjust cholesterol/protein transport in the cytosol [31].

In our short post-apnea time setting, we could show that hypoxemia lead to reduced plasma levels of apolipoprotein B, triglycerides, cholesterol, HDL, LDL and oxysterols and an increased cholestane-3β, 5α, 6β-triol ratio in voluntary apnea divers. Future follow-up studies should expand these results after repetitive apnea times. Long term follow-up with specific focus on the cardiovascular system could improve recreational apnea divers care and deepen knowledge of long term effects of apneic diving.

Conclusions

Our study deepens insights into triglyceride, cholesterol and oxysterol plasma levels after a single short time hypoxemia under dry conditions in voluntary apnea divers. In physical examinations, cardiovascular risk factors should be carefully evaluated.