Erythrocytes primarily function to transport and distribute oxygen to body cells and return carbon dioxide back to the lungs from the cells [27]. In achieving this, erythrocytes undergo membrane deformations in order to be able to pass through the microcirculation and deliver oxygen as well as needed nutrients to the cells [28]. Erythrocyte deformability has also been observed to influence oxygen delivery, acid-base balance functions and the survival rate of erythrocyte in circulating plasma [11]. It also aids the exit of erythrocytes from the bone marrow, reduces bulk viscosity in larger vessels and prevents phagocytic clearance of erythrocytes by macrophages [29]. Disruption in erythrocyte deformability contributes to alterations in microcirculatory blood flow that play a pivotal role in multiple organ failure and death arising from impairment in tissue oxygenation [30, 31].
The deformability of the erythrocyte and its functions has been reported to be influenced by its plasma membrane compositional elements and their interactions [9, 32]. Dietary conditions, especially high fat diet, has been reported to affect the cholesterol content, fatty acid and protein matrix of erythrocyte membrane resulting in alterations of its membrane cholesterol-phospholipids ratio [33, 34], increased erythrocyte phagocytic susceptibility and lipid peroxidation [6], reductions in erythrocyte membrane p55 and band 4.2 skeletal proteins [8], and alter erythrocyte shape and deformability index [35]. In this study, increased osmotic fragility and mean corpuscular fragility, was observed in the HFD group. This suggests increased susceptibility of erythrocytes to undergo haemolysis in this treatment group, which is in accordance with the reports of Kalmath et al. [7] who in an in-vitro study showed that HFD induces hypo-osmotic fragility in rat erythrocytes.
Several studies have indicated that high-fat consumption causes overproduction of circulating free fatty acids and systemic inflammation, which can result cellular and molecular damage via oxidative stress [36, 33, 10]. This study also shows an increase in erythrocyte lipid peroxidation, and sedimentation rate suggesting not only increased systemic inflammatory conditions in the HFD group but also increased oxidative damage to erythrocyte membrane lipids and hence compromised cholesterol-phospholipids ratio and structural integrity of the cell. Oxidative stress has been implicated in the alteration of membrane integrity and fragility of erythrocytes resulting in a dysfunctional propensity for them to flow into microcirculatory vascular beds, adhere to endothelial monolayers and hemolyse [37]. Furthermore ICAM-4, an erythrocyte-specific inter-cellular adhesion molecule that binds to αVβ3 integrins on endothelial cells [26] was increased in the HFD group. This suggests and increased susceptibility of these cells to bind to macrophages in the spleen and undergo haemolysis. High fat diet feeding has been reported to increase circulating chemokine level, promote its binding to erythrocyte Duffy antigen receptor for chemokine (DARC) [38, 6] and trigger macrophage attack of erythrocyte by increasing externalization of phosphatidylserine in erythrocyte membrane [6]. Hence, up-regulation of cell adhesion molecule (ICAM-4) as observed in this study suggests and activation of pro-inflammatory responses in erythrocytes [5, 39] which can increased its susceptibility to be phagocytized my macrophages.
Gas exchange, the primary function of erythrocytes, is a passive diffusional process that presents with no direct metabolic demand, but requires a rheologically competent cell [40]. The disco-like shape of erythrocytes allows it to deform, fold, and squeeze against the endothelial walls of capillaries, thus exposing maximal surface area and offering minimal diffusional distances for rapid oxygen and carbon dioxide exchanges across the capillary walls. For maximum efficiency, the optimal rheology of erythrocytes is maintained by its ability to regulate cell volume below the maximum spherical volume that can be accommodated by the membrane area of the cell [14]. The volume control in mature erythrocytes is depended on the presence of active and passive membrane transporter on their membranes. The ability of the erythrocytes to regulate its volume in association with its net cation content is also important to its function and life span. A large number of membrane proteins including Na+-K+ ATPase and Ca2+-Mg2+ ATPase are involved in cation homeostasis and water content regulation [14, 11]. Na+-K+ ATPase catalyzes the energy-dependent transport of Na+ and K+ across membrane thus regulating cell volume and ionic uptake [11] and its activity can be modulated by membrane lipid content [41]. This study shows a reduction in the total protein content of the membrane in the HFD treatment group possibly as a result of protein oxidation [42], arising from high fat diet-induced oxidative stress [36, 6]. This reduction in membrane protein level may also account for the reduction in Na+-K+ ATPase activity observed in this study, an observation that has been reported to decrease erythrocyte deformability, and predispose to rheological induced cardiovascular disease conditions [43].
The membrane-bound Ca2+ ATPase has been reported to maintain intracellular calcium level within narrow range compared to extracellular free Ca2+, and regulate erythrocyte membrane fluidity [11] while Mg2+ ATPase regulates the membrane aminophospholipid translocase activity which can affect Ca2+ ATPase [44, 45]. The results from this study show a slight increase in the activity of Ca2+ Mg2+ATPase in the HFD treatment group, which suggest likely increases in erythrocyte intracellular Ca2+ levels in this group. Increases in erythrocyte intracellular calcium levels have been associated with increased osmotic fragility [46] and hence may also contribute to the increased erythrocyte osmotic fragility observed in the HFD treatment group.
This study suggests that prolonged high fat dietary intake may compromise the rheology and integrity of erythrocytes through increased systemic inflammation, cellular oxidative stress, membrane proteins oxidation, and decreased activity of membrane proteins necessary for the maintenance of erythrocytes volume and stability in male Wistar rats.