Loss of a large intravascular volume of blood results in hemorrhagic shock, a state of hypotension associated with hemodynamic abnormalities that lead to the collapse of homeostasis (Gutierrez et al., 2004
). In the microcirculation, the functional capillary density decreases significantly during hemorrhage, reducing the oxygen supply to the surrounding tissues (Salazar Vazquez et al., 2008
; Torres et al., 2008
). In the absence of timely and proper treatment, severe hemorrhagic shock is associated with high morbidity and mortality.
The primary goal in treating hemorrhagic shock is to stop the bleeding and restore the volume of the circulating blood. Blood transfusion is widely considered the best treatment for patients who have lost a large volume of blood, although some studies have suggested that blood transfusion itself may sometimes be problematic (Sugerman et al., 1970
; Rao et al., 2004
). The whole blood transfusion is often impractical, particularly in the case of accidents or on the battlefield, when the injured are at high risk of losing blood but have no immediate access to red blood cells.
Current fluid resuscitation methods include the administration of colloid or crystalloid solutions (Falk et al., 1992
), followed by the transfusion of oxygen-carrying blood. These resuscitation methods are aimed at restoring circulating blood volume, even at the cost of a lower-than-normal hematocrit or hemoglobin concentration (). Alternative resuscitation methods have been developed, including the use of blood substitutes, which are usually hemoglobin-based oxygen carriers. Unlike whole red blood cell suspensions, hemoglobin-based blood substitutes can be carried in emergency trauma bags and injected into any recipient without the need for the time-consuming process of blood type matching. However, significant side effects have been reported for the blood substitutes currently under development (Alayash, 2004
; Buehler & Alayash, 2008
; Natanson et al., 2008
). Commonly used resuscitation fluids, such as crystalloids or colloids, are designed to restore intravascular volume. Administration of a resuscitation fluid restores the circulation and maintenance of systemic blood pressure and metabolic balance, but the circulating blood now has a lower hematocrit after treatment. This hemodilution can cause problems, one of which is vasoconstriction, which further decreases functional capillary density.
Figure 1 Blood flow and release of NO released from NOS3 in erythrocytes. The region shown can be divided into five layers: intraluminal region, endothelium and interstitial space, smooth muscle cells, non-perfused tissue, and tissue perfused by capillaries. (A) (more ...)
One of the hypotheses to explain the hemorrhage-induced collapse of homeostasis is that the production of nitric oxide (NO) is significantly impaired because of the reduced blood viscosity and flow (Salazar Vazquez et al., 2008
). NO is a gas molecule that dilates blood vessels and is necessary for matching O2
delivery to the metabolic activity of the local tissue. Once formed, NO can diffuse into the vascular smooth muscle, where it reacts with soluble guanylate cyclase (sGC), which catalyzes the formation of 3’,5’-cyclic guanosine monophosphate (cGMP) to induce vasodilation.
NO synthesized by endothelial NOS (eNOS or NOS3) in endothelial cells is widely considered to be the major source of this signaling molecule in the vasculature (Lancaster, 1994
; Buerk, 2007
; Tsoukias, 2008
). However, experimental evidence and theoretical analyses have suggested that NO from non-endothelial cells, including those from intraluminal sources, can play a significant role in a variety of physiological functions in the vasculature (reviewed in (Chen et al., 2008b
)). In addition to its presence in endothelial cells, NOS3 has been shown to be expressed in erythrocytes as a potential NO source. Chen and Mehta (Chen & Mehta, 1998
) have reported the presence of NO-producing enzymes in erythrocytes, but later studies have suggested that the identified NOS enzymes were inactive proteins (Kang et al., 2000
). Recently, Kleinbongard et al. (Kleinbongard et al., 2006
), using multiple experimental approaches, found that erythrocytes express a functional NOS3 that potentially can serve as an intraluminal NO source. This NOS3 is located both on the plasma membrane and in the cytoplasm of the erythrocytes. However, although some subsequent studies have confirmed the presence or function of NOS3 in erythrocytes (Webb et al., 2007
; Ulker et al., 2008
), others have questioned its existence (Hilarius et al., 2007
). NO from this source has been thought to regulate the function of erythrocytes and platelets (Gladwin, 2006
; Kleinbongard et al., 2006
; Yalcin et al., 2008
). Thus, it is still unclear whether a significant amount of NO from this intraluminal source can reach the vascular wall and cause blood vessel relaxation.
Certain intraluminal sources of NO (e.g. S
-nitrosohemoglobin, intraluminal nitrite) have been proposed to exert a hypoxic vasodilation effect (Singel & Stamler, 2005
; Kim-Shapiro et al., 2006
; Chen et al., 2008b
). If the NO synthesized by intraerythrocytic NOS3 can escape scavenging by hemoglobin as other blood-born NO, it could be another important source regulating vascular tone. During hemorrhagic shock, the loss of blood must result in a concomitant loss of erythrocytic NOS3. If this source is indeed important for NO-mediated vasodilation, hemorrhage would not only result in the loss of erythrocytes that carry oxygen, but also of the vasodilating NO that they “carry”.
We have therefore hypothesized that the loss of erythrocyte-based NOS3 during hemorrhage contributes to NO deficiency observed in shock. Because of the technical difficulties involved in attempting to obtain quantitative measurements of intraluminal NO, there are no published data concerning how much NO from this source is available to smooth muscle. In the present study, we have constructed a multicellular computational model that simulates NO production from NOS3 in erythrocytes and its transport through an arteriole. Our simulations indicate that whether a significant amount of NO from this source can be delivered to vascular smooth muscle is strongly dependent on the existence of a protected mechanism for NO delivery.