Red Blood Cell Adenylate Energetics Is Related to Endothelial and Microvascular Function in Long COVID

Abstract

Adenine nucleotides play a critical role in maintaining essential functions of red blood cells (RBCs), including energy metabolism, redox status, shape fluctuations and RBC-dependent endothelial and microvascular functions. Recently, it has been shown that infection with the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) might lead to morphological and metabolic alterations in erythrocytes in both mild and severe cases of coronavirus disease (COVID-19). However, little is known about the effects of COVID-19 on the nucleotide energetics of RBCs nor about the potential contribution of nucleotide metabolism to the long COVID syndrome. This study aimed to analyze the levels of adenine nucleotides in RBCs isolated from patients 12 weeks after mild SARS-CoV-2 infection who suffered from long COVID symptoms and to relate them with the endothelial and microvascular function parameters as well as the rate of peripheral tissue oxygen supply. Although the absolute quantities of adenine nucleotides in RBCs were rather slightly changed in long COVID individuals, many parameters related to the endothelial and microcirculatory function showed significant correlations with RBC adenosine triphosphate (ATP) and total adenine nucleotide (TAN) concentration. A particularly strong relationship was observed between ATP in RBCs and the serum ratio of arginine to asymmetric dimethylarginine-an indicator of endothelial function. Consistently, a positive correlation was also observed between the ATP/ADP ratio and diminished reactive hyperemic response in long COVID patients, assessed by the flow-mediated skin fluorescence (FMSF) technique, which reflected decreased vascular nitric oxide bioavailability. In addition, we have shown that patients after COVID-19 have significantly impaired ischemic response parameters (IR max and IR index), examined by FMSF, which revealed diminished residual bioavailability of oxygen in epidermal keratinocytes after brachial artery occlusion. These ischemic response parameters revealed a strong positive correlation with the RBC ATP/ADP ratio, confirming a key role of RBC bioenergetics in peripheral tissue oxygen supply. Taken together, the outcomes of this study indicate that dysregulation of metabolic processes in erythrocytes with the co-occurring endothelial and microvascular dysfunction is associated with diminished intracellular oxygen delivery, which may partly explain long COVID-specific symptoms such as physical impairment and fatigue.

Keywords: endothelium; long COVID; microcirculation; nucleotides; red blood cells.

Figure 1

Adenine nucleotides’ concentration in the erythrocytes of long COVID patients is not different from that in healthy controls. The concentration of (A) adenosine triphosphate (ATP), (B) adenosine diphosphate (ADP), (C) adenosine monophosphate (AMP), (D) total adenine nucleotide (TAN), (E,F) adenine nucleotide ratios (ATP/ADP; ADP/AMP) and (G) adenylate energy charge (AEC) in red blood cells of post-COVID-19 participants (n = 19) compared with healthy control group (n = 20). Results are shown as mean ± SEM; * p < 0.05 by unpaired Student’s t-test (A,B,D–G) or Mann–Whitney test (C), ns—not significant.

Figure 2

Adenosine triphosphate (ATP) concentration in the erythrocytes of long COVID patients correlates with ischemic response parameters measured by flow-mediated skin fluorescence (FMSF) technique. Correlations of red blood cell adenosine triphosphate (ATP) concentration with (A) ischemic response parameters (IR index; IR max), (B) hyperemic response parameters (HR index; HR max), (C) reactive hyperemic response (RHR) and (D) hypoxia sensitivity (logHS) in post-COVID-19 participants. Results are shown as correlation plots with corresponding Pearson (A–C) or Spearman (D) coefficient (r) and p value (p). Solid line—regression line, dotted line—error bars.

Figure 3

Adenosine triphosphate (ATP) concentration in the erythrocytes of long COVID patients correlates with circulating endothelial function parameters. Correlations of red blood cell adenosine triphosphate (ATP) concentration with (A) L-arginine/ADMA (asymmetric dimethyl-L-arginine) ratio, (B) symmetric dimethyl L-arginine (SDMA), (C) arginine, (D) citrulline and (E) glycine concentration in long COVID participants. Results are shown as correlation plots with corresponding Pearson coefficient (r) and p value (p). Solid line—regression line, dotted line—error bars.

Figure 4

Adenosine triphosphate (ATP) concentration in the erythrocytes of long COVID patients correlates with inflammatory hematological ratios. Correlations of red blood cell adenosine triphosphate (ATP) concentration with (A) peripheral blood neutrophil-to-lymphocyte ratio (NLR), (B) lymphocyte-to-monocyte ratio (LMR), (C) lymphocyte-to-C-reactive protein ratio (LCR) and (D) platelet-to-lymphocyte ratio (PLR) in post-COVID-19 participants. Results are shown as correlation plots with corresponding Pearson (A–C) or Spearman (D) correlation coefficient (r) and p value (p). Solid line—regression line, dotted line—error bars.