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Am J Physiol Heart Circ Physiol 286: H222-H229, 2004. First published September 25, 2003; doi:10.1152/ajpheart.00532.2003
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Modulation of endothelial nitric oxide synthase expression by red blood cell aggregation

Oguz K. Baskurt,1 Ozlem Yalcin,1 Sadi Ozdem,2 Jonathan K. Armstrong,3 and Herbert J. Meiselman3

1Department of Physiology and 2Department of Pharmacology, Akdeniz University Faculty of Medicine, Antalya, Turkey 07070; and 3Department of Physiology and Biophysics, Keck School of Medicine, University of Southern California, Los Angeles, California 90033

Submitted 6 June 2003 ; accepted in final form 15 September 2003


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The effects of enhanced red blood cell (RBC) aggregation on nitric oxide (NO)-dependent vascular control mechanisms have been investigated in a rat exchange transfusion model. RBC aggregation for cells in native plasma was increased via a novel method using RBCs covalently coated with a 13-kDa poloxamer copolymer (Pluronic F-98); control experiments used RBCs coated with a nonaggregating 8.4-kDa poloxamer (Pluronic F-68). Rats exchange transfused with aggregating RBC suspensions demonstrated significantly enhanced RBC aggregation throughout the 5-day follow-up period, with mean arterial blood pressure increasing gradually over this period. Arterial segments ({approx}300 µm in diameter) were isolated from gracilis muscle on the fifth day and mounted between two glass micropipettes in a special chamber equipped with pressure servo-control system. Dose-dependent dilation by ACh and flow-mediated dilation of arterial segments pressurized to 30 mmHg and preconstricted to 45–55% of the original diameter by phenylephrine were significantly blunted in rats with enhanced RBC aggregation. Both responses were totally abolished by nonspecific NO synthase (NOS) inhibitor (N{omega}-nitro-L-arginine methyl ester) treatment of arterial segments, indicating that the responses were NO related. Additionally, expression of endothelial NOS protein was found to be decreased in muscle samples obtained from rats exchanged with aggregating cell suspensions. These results imply that enhanced RBC aggregation results in suppressed expression of NO synthesizing mechanisms, thereby leading to altered vasomotor tonus; the mechanisms involved most likely relate to decreased wall shear stresses due to decreased blood flow and/or increased axial accumulation of RBCs.

flow-mediated dilation; poloxamer coating

NITRIC OXIDE (NO) plays a key role in the control of circulatory function (21), and the importance of NO in the regulation of vascular smooth muscle tone and the maintenance of vascular resistance has been well established (13, 38, 44). In addition, several other physiological functions (e.g., apoptosis) have been attributed to this simple molecule (9, 25). Endothelial cells are the source of NO that modulates vascular resistance: NO diffuses to the underlying vascular smooth muscle where it activates guanylate cyclase to increase cGMP, thus resulting in smooth muscle relaxation (26) and decreased vascular resistance.

Endothelial NO is synthesized by the endothelial isoform of NO synthase (eNOS) using L-arginine as substrate (31). While eNOS is continuously expressed at a basal level in endothelial cells, its activation is dependent on Ca2+/calmodulin complex levels, and thus NO generation by endothelial cells is dependent on intracellular calcium concentration (41). It has been shown that this calcium-dependent regulation of NO synthesis in endothelial cells is closely related to the hemodynamic conditions to which they are exposed (39): mechanical forces acting on endothelial cells trigger calcium entry and activate eNOS (54). Mechanical forces acting on endothelial cells can also activate eNOS by phosphorylation through calcium-independent mechanisms (15, 29, 54). Additionally, eNOS expression in endothelial cells can be modulated by the magnitude of the mechanical forces to which they are exposed (46, 53, 58).

Mechanical forces acting on endothelial cells in vivo are due to blood flow over these cells (i.e., tangential forces) and can be quantitated as wall shear stress: the magnitude of the wall shear stress is determined by the product of the fluid velocity profile adjacent to the vessel wall (i.e., wall shear rate) and the apparent viscosity of the fluid in this marginal area (33, 42). It has been demonstrated in a number of studies that alterations in hemodynamic conditions and wall shear stress may affect NO-synthesizing mechanisms in the vascular endothelium. Cardiac failure induced by coronary artery ligation, and hence decreased blood flow rates, leads to significantly diminished eNOS expression in skeletal muscle samples (55). This alteration can be reversed by regular exercise, indicating the importance of blood flow rate in maintaining physiological eNOS expression levels (55). These observations are supported by studies demonstrating enhanced eNOS expression in various regions of the circulatory system by regular exercise (24, 28, 30) and by increased blood flow due to arteriovenous shunts (23, 39).

The above-mentioned definition of wall shear stress suggests a significant role for the physical (i.e., rheological) properties of the blood in close contact with the vessel wall (33, 42). It is well known that the composition of blood across the diameter of a blood vessel is not uniform (20). Rather, red blood cells (RBCs) tend to migrate toward the axis, resulting in a plasmarich, cell-poor marginal layer with relatively lower viscosity (16, 20). The extent of RBC axial migration is strongly affected by their tendency for aggregation, such that an enhanced tendency for RBC aggregation promotes axial migration (16) and, under the appropriate conditions, decreases flow resistance (6, 16, 47). Soutani et al. (50) clearly demonstrated the existence of this cell-poor layer and the influence of RBC aggregation on its thickness in an in vivo study using rabbit mesentery. They suggested that the widening of the cell-poor layer tends to reduce the flow resistance in the microvascular network (50). However, it should be noted that the effects of RBC aggregation on flow dynamics are highly complex and are modulated by a number of factors such as the orientation of blood vessels versus gravity (20) and transit times (i.e., flow times) through a given vessel segment (11).

The present study was designed to test the hypothesis that enhanced RBC aggregation would yield diminished eNOS expression as a result of decreased wall shear stress due to decreased blood flow rates and/or more effective RBC axial migration. In contrast with previous reports that employed the addition of high-molecular-weight water-soluble polymer solutions to circulating blood (e.g., Refs. 6, 11, 19, 35, and 48), enhanced RBC aggregation was achieved in the present study by covalent attachment of a 13-kDa poloxamer (Pluronic F-98) to the RBC surface (3). This technique allows controllable augmentation of RBC aggregation without diluting plasma proteins and does not involve chemical or viscosity changes of the plasma. Attachment of a 8.4-kDa poloxamer (Pluronic F-68) to the RBC does not alter aggregation for cells in plasma, thereby allowing control experiments that are closely matched to the group with enhanced aggregation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Animals

Adult rats of either sex weighing 250–300 g were used in the present study and were randomly assigned to each arm of the exchange transfusion protocol (see Modification of RBC Aggregation) or served as blood donors. Blood from donor animals was obtained from their abdominal aorta under light ether anesthesia, anticoagulated with EDTA (1.5 mg/ml), and used for preparing polymer-coated RBC suspensions. The experimental protocol was approved by the Animal Care and Usage Committee of Akdeniz University and was in accordance with the Declaration of Helsinki and International Association for the Study of Pain guidelines.

Modification of RBC Aggregation

Enhanced RBC aggregation was achieved by employing a novel technique whereby copolymers that possess the ability to self-associate above a specific temperature are covalently attached to the RBC surface. These copolymers, known as poloxamers, are triblock macromolecules composed of a central hydrophobic block of polypropylene glycol (PPG) flanked by two identical hydrophilic polyethylene glycol (PEG) chains (49). Poloxamers exhibit a critical micellization temperature (CMT) at which a phase transition occurs from predominantly single, fully hydrated copolymer chains to micelle-like structures characterized by association of the hydrophobic PPG blocks (2, 5, 8). The CMT decreases with increasing mass of the PPG block, and, by proper selection of the poloxamer (8), RBC aggregation can be controllably and predictably enhanced or inhibited after covalent attachment to the RBC surface (3). That is, at temperatures above the CMT, poloxamers attached to the RBC surface micellize with poloxamers on adjacent RBCs, thereby enhancing RBC aggregation–the strength of the association being dependent on the poloxamer type, concentration, and temperature (3). Conversely, below the CMT, poloxamers attached to the RBC surface inhibit RBC aggregation due to polymer-polymer repulsion.

Derivatization of poloxamers. Pluronic copolymers were a gift from BASF Performance Chemicals (Parsippany, NJ). These poloxamers contained 80% PEG by mass and had the following designations and molecular weights: Pluronic F-68, 8.4 kDa; and Pluronic F-98, 13.0 kDa. To facilitate covalent attachment of the poloxamers to the RBC surface, a reactive derivative of the copolymers was first prepared. This was achieved by converting the terminal hydroxyl groups of the copolymer chain to succinimidyl carbonate groups using a modification of the method described by Miron and Wilchek (36). In brief, 1 mmol of poloxamer was dissolved in 100 ml of dioxane (warmed slightly to aid dissolution) and then cooled to room temperature; the clear solution was stirred using a magnetic stirrer and purged with nitrogen. A threefold molar excess (6 mmol) of N,N'-disuccinimidyl carbonate was then slurried in 10 ml of dry acetone and added to the solution. 4-(Dimethylamino)pyridine (6 mmol) was dissolved in 20 ml of dry acetone and added slowly to the reaction mixture, and the reaction was allowed to proceed for 24 h at room temperature under nitrogen. The reaction mixture was filtered to remove any solid precipitate, and the filtrate was poured slowly under high shear into 300 ml of diethyl ether to precipitate the derivatized poloxamer and remove the 4-(dimethylamino)pyridine. The precipitate was filtered and washed with 200 ml of diethyl ether, resuspended in 200 ml of isopropanol, filtered, and washed with 100 ml of isopropanol to remove any unreacted N,N'-disuccinimidyl carbonate. The precipitate was then resuspended in 200 ml of cyclolhexane, filtered, and washed with 100 ml of cyclohexane. Finally, the poloxamer succinimidyl carbonate derivatives (Pluronic F-68-SC and Pluronic F-98-SC) were dried under a stream of dry nitrogen for 12 h.

RBC coating. To achieve an even coating of poloxamer, and to avoid the formation of cross-linked aggregates of coated RBC, all poloxamer-coating procedures were performed at 4°C (i.e., below the poloxamer CMT). Blood was centrifuged at 1,400 g for 6 min, and the plasma was separated and saved. RBCs were washed three times with isotonic PBS (pH 7.4) and resuspended in 30 mM triethanolamine buffer (290 mosm/kg, pH 8.60) at a hematocrit of ~0.1 l/l, and the suspension was cooled to 4°C. Immediately before use, the reactive poloxamer to be used for coating of RBCs (either Pluronic F-68-SC or Pluronic F-98-SC) was dissolved in a 4°C hypotonic phosphate buffer (50 mM NaH2PO4 and 60 mM NaCl; pH 5) at a concentration of 10 mg/ml. The poloxamer solution was then promptly added to the RBCs suspended in triethanolamine buffer to obtain a final concentration of 0.25 mg/ml, and the suspension was incubated for 2 h at 4°C with continuous gentle mixing on a tube rocker. Coated RBCs were separated, washed three times with PBS by gentle centrifugation (400 g, 5 min), and then resuspended in their autologous plasma at 0.4 l/l hematocrit. The viscosity of the RBC suspensions at 37°C over a range of shear rates (0.1–94.5 s–1) was measured using a Contraves LS-30 Couette viscometer (Contraves; Zürich, Switzerland).

Exchange Transfusions and Follow-up Procedures

Rats were anesthetized by xylazine-ketamine solution (10 mg/ml xylazine plus 50 mg/ml ketamine), 0.1 ml per 100 g body wt, and the left carotid artery and left jugular vein were catheterized using 20-gauge cannulas. Approximately 30% of total blood volume of each rat was exchanged with a poloxamer-coated RBC suspension by infusing the suspension slowly (0.28 ml/min) into the cannulated jugular vein using an infusion pump (model 975, Harvard Instruments; Holliston, MA) and withdrawing the same amount of blood from the carotid artery. Both the carotid artery and jugular vein were tied off at the end of the exchange transfusion procedure, and the surgical incision was closed by 3-0 silk sutures.

Blood pressure after the exchange transfusion was measured daily over a 4-day follow-up period using a tail-cuff method (BP HR200 Module plus MP100 system, Biopac Systems; Goleta, CA). Additionally, blood samples were obtained from tail tips of the rats every day during this period and used for microscopic examinations. Wet mount preparations of dilute RBC suspensions were examined and photographed using bright-field light microscopy. These photographs were used to assess RBC aggregation via the microscopic aggregation index (MAI) technique (45). In the MAI method, the total number of RBCs in a field is divided by the number of RBC units (i.e., sum of the number of individual RBC and aggregates) in the same field; the MAI has a value of unity in the absence of aggregation and increases with enhanced RBC aggregation (45).

In a separate series of experiments, 2 mg of reactive Pluronic F-68 or F-98 in 0.1 ml isotonic saline were infused into the tail vein of rats (n = 4 for each poloxamer) daily over a 4-day period. Arterial blood pressure and RBC aggregation were monitored during this period using the methods described above to test the effect of free polymers on these parameters. These experiments were repeated using inactivated polymers (i.e., polymers incubated overnight at pH 8 at room temperature) for daily injections.

In Vitro Vascular Studies

On the fourth day after the exchange transfusion, rats were anesthetized by urethane (1 g/kg, 20% in isotonic saline). The gracilis muscle from the left leg of the animal was excised and transferred to a dissecting dish containing oxygenated Krebs solution, and an arterial segment of ~300 µm in diameter was isolated from the muscle under a dissection microscope. The arterial segment was transferred to a vessel chamber (CH/1, Living Systems; Burlington, VT), mounted between two glass micropipettes, and superfused with oxygenated Krebs solution [containing (in mmol/l) 119 NaCl, 24 NaHCO3, 4.7 KCl, 1.18 KH2PO4, 1.17 MgSO4·7H2O, 1.6 CaCl2, and 5.5 glucose] at 37°C. The micropipettes were connected to a pressure servo-control system composed of two roller pumps, two pressure transducers that monitored the inflow and outflow pressures, and a control unit (PS/200/Q, Living Systems), and the vessel segment was perfused from a separate source of the oxygenated Krebs solution described above. The servo-control system allowed us to set the flow rate through the arterial segment from 0 to 370 µl/min. The arterial segment preparation was placed on an inverted microscope (TS100, Nikon) equipped with a charge-coupled device camera (XC73CE Sony). The camera was connected to a video dimension-analysis system (model V94, Living Systems), which allowed continuous recording of the vessel diameter; intraluminal and perfusion pressures, fluid flow rate through the arterial segment, and the vessel diameter were continuously recorded via a digital computer.

Before being studied, the length of the arterial segment was adjusted to its in vivo length and the arterial segment was equilibrated at 30-mmHg intraluminal pressure and zero flow rate for 30 min. The following three types of studies were carried out: ACh dilation, flow-mediated dilation (FMD), and FMD plus N{omega}-nitro-L-arginine methyl ester (L-NAME).

Acetylcholine dilation. At the end of the equilibration period, the segment was constricted to 45–55% of its original diameter by adding 10–7–10–5 M phenylephrine to the superfusion solution. When the phenylephrine-induced contraction reached the desired 45–55% plateau level, increasing concentrations of ACh (10–8–10–5 M) were added to the superfusion solution and the vessel diameter at each concentration was recorded.

Flow-mediated dilation. The arterial segment was washed with oxygenated Krebs solution and again constricted by phenylephrine to 45–55% of the basal diameter. FMD was assessed at a series of fluid flow rates between 8 and 35 µl/min while the intraluminal pressure was kept constant at 30 mmHg by the pressure servo-control system. Each flow rate was maintained for 2 min to obtain a steady vessel diameter.

Flow-mediated dilation with L-NAME. The arterial segment was washed again, and 10–3 M L-NAME was added to the vessel chamber. The procedure to assess FMD (i.e., phenylephrine to achieve 45–55% constriction, 8–35 µl/min flow rate) was repeated after 30 min of incubation with L-NAME.

Determination of eNOS Expression

Samples obtained from gracilis muscles were used to determine eNOS expression by Western blotting techniques; muscle samples from normal rats that did not receive exchange transfusions were also analyzed together with the samples from the Pluronic F-68 and F-98 groups. Tissue samples weighing ~300 mg were homogenized in 200 µl of 10% Triton X-100-HEPES buffer. The protein content of the homogenate was determined by Bradford reagent, and samples containing 100 µg protein were loaded onto 10% polyacrylamide gels; SDS-PAGE was carried out using a potential difference of 120 V. The proteins were transferred to polyvinylidene difluoride (PVDF) membranes by incubating them in Tris-glycine-methanol buffer at 4°C overnight. The membranes were blocked in 1% fat-free milk powder solution prepared in 0.1% Tween 20 containing phosphate buffer (pH 7.4) for 2 h at room temperature. The membranes were then incubated in the presence of eNOS primer antibody (sc654, Santa Cruz Biotechnology; Santa Cruz, CA) at a dilution of 1/2,000 for 1 h at room temperature, followed by 1 h of washing in Tween 20-phosphate buffer to remove nonspecific binding. The membrane was then exposed to horseradish peroxidase (HRP)-conjugated secondary antibody (anti-rabbit IgG-HRP, sc-2030, Santa Cruz Biotechnology) at a dilution of 1/200 for 1 h at room temperature. After being washed for 1 h, the membrane was treated by ECL detection reagent (RPN2106, Amersham Biosciences; Buckinghamshire, UK) and exposed to ECL-sensitive films. PVDF membranes were also used for the determination GAPDH expression, as the loading standard, as described above using GAPDH primer antibody (ab8245, Abcam; Cambridge, UK). The intensities of the bands were quantified by densitometric analysis using a gel scanner (Cobrascan CX 312 T, Radiographic Digital Imaging; Torrance, CA), with the results expressed as the ratio of densities of eNOS-specific bands to GAPDH bands.

Statistics

The results obtained from the Pluronic F-68 and F-98 groups were compared by Student's t-test where appropriate. Multiple comparisons were done by one-way ANOVA, followed by the Newman-Keuls post hoc test or two-way ANOVA, followed by the Bonferroni post hoc test. EC50 values (i.e., concentrations that caused 50% of maximal responses) were calculated by linear regression using statistical analysis software (GraphPad Software; San Diego, CA). Statistical significance was accepted at P values of <0.05.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
RBC Aggregation

Examination of dilute RBC suspensions in autologous plasma indicated significantly enhanced RBC aggregation in the group that underwent exchange transfusions with Pluronic F-98-coated RBC suspensions. Typical examples for both Pluronic F-68 and F-98 groups are presented in Fig. 1, where it can be seen that multicell aggregates and RBC clumping exist in blood from the Pluronic F-98 animals, whereas RBC aggregation is negligible in the Pluronic F-68 group. It should be noted that due to strong associations between PPG blocks on the surfaces of adjacent Pluronic F-98-coated RBCs, the appearance of RBC aggregates in the Pluronic F-98 group was not similar to the regular linear rouleaux and the branched arrays of linear aggregates observed in normal human blood under physiological conditions (45). Rather, the RBC aggregation observed in Pluronic F-98-infused animals is characterized by stronger associations between individual RBCs as indicated by greater forces needed to break the mostly irregular clumps (i.e., RBC aggregates are more resistant to dispersion due to movement of the coverslip over the microscope slide) as previously demonstrated in vitro using low shear viscometry (3). However, this clumping was clearly reversible when sufficiently large shearing forces were applied to the suspension between the microscope slide and coverslip. The observed degree of clumping was reduced in the days after exchange transfusions in the Pluronic F-98 group (Fig. 1).



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  		Fig. 1. Photomicrographs of red blood cells (RBCs) obtained from rats before and after exchange transfusions with Pluronic F-68-coated (A) or F-98-coated (B) RBC suspensions. RBCs are suspended in autologous plasma at low hematocrit ({approx}0.01 l/l).

 

MAIs calculated using microphotographs of blood samples taken daily after the exchange transfusions are shown in Fig. 2. RBC aggregation was found to be ~3.5 times higher immediately after exchange transfusions with Pluronic F-98-coated RBC suspensions compared with either samples obtained before exchange transfusion or to samples from animals receiving Pluronic F-68-coated RBCs. RBC aggregation in the Pluronic F-98 group decreased gradually over the 4-day period after the exchange transfusions, yet the aggregation index was still 2.4 times higher than preexchange or Pluronic F-68 values. Aggregation indexes did not change significantly at any time point after exchange transfusions with Pluronic F-68-coated RBC suspensions (Fig. 2).



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  		Fig. 2. Microscopic aggregation indexes (MAIs) of RBCs suspended in autologous plasma for the Pluronic F-68 and F-98 groups. Values are means ± SE; n = 8 rats/group. *Significant difference from the Pluronic F-68 group by two-way ANOVA, P < 0.001; {dagger}significant difference from before exchange by one-way ANOVA, P < 0.001.

 

Blood viscosity measurements of poloxamer-coated RBCs suspended in autologous plasma were performed at 37°C. Pluronic F-98-coated RBCs showed a 4.9-fold increase and Pluronic F-68-coated RBCs showed a 0.3-fold decrease in low shear blood viscosity (shear rate = 0.1 s–1) relative to control uncoated RBCs. No significant difference in high shear (94.5 s–1) viscosity was observed.

Blood Pressure

Mean arterial pressure values measured using a tail-cuff method are presented in Fig. 3, where it can be seen that there was no alteration in arterial pressure in the Pluronic F-68 group during the days after the exchange transfusions. Conversely, in the group that received exchange transfusions with Pluronic F-98-coated RBC suspensions, there was a gradual increase in mean arterial pressure. Significantly higher values were recorded in this group on and after the second day after exchange transfusions: mean arterial pressure was significantly higher than both the baseline values (i.e., the value before the exchange transfusion) and those for the Pluronic F-68 group (Fig. 3).



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  		Fig. 3. Mean arterial blood pressure (MAP) of rats after exchange transfusions with Pluronic F-68 or F-98-coated RBC suspensions. Values are means ± SE; n = 8 rats/group. *Significant difference from the Pluronic F-68 group by two-way ANOVA, P < 0.01; {dagger}signficant difference from day 1 by one-way ANOVA, P < 0.001.

 

Effects of Free Poloxamers on Arterial Blood Pressure and RBC Aggregation

Daily injections of 2 mg of reactive Pluronic F-68 or F-98 in 0.1 ml of isotonic saline over a 4-day period did not lead to an increase of arterial blood pressure (data not shown). Strong RBC aggregation was observed in peripheral blood samples within 15 min of injection of reactive poloxamers, with return to normal aggregation by 4 h. Infusion of inactivated poloxamers according to the same protocol did not affect either RBC aggregation or arterial blood pressure.

ACh-Mediated Dilation

ACh-induced dilations of arterial segments pressurized to 30 mmHg at zero flow and preconstricted to 45–55% of the original diameter are shown in Fig. 4 for the Pluronic F-68 and F-98 groups. The shape of the ACh concentration-dilation curve obtained in the Pluronic F-68 group was similar to that obtained using the arterial segments obtained from normal animals that had not received exchange transfusions (data not shown). However, there was a rightward shift of the concentration-response curve in the Pluronic F-98 group versus the Pluronic F-68 group such that the concentration-response curve of the Pluronic F-98 group indicated a significantly blunted response (Fig. 4) with the dilation at 10–6 M ACh being significantly lower (72.1 ± 7.5 for Pluronic F-68 vs. 46.5 ± 9.5 for Pluronic F-98). Accordingly, the EC50 value for ACh was found to be 2.07 x 10–7 M in the Pluronic F-68 group, whereas in the Pluronic F-98 group the EC50 value was ~17 times higher (3.48 x 10–6 M).



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  		Fig. 4. Percent dilation induced by 10–8–10–4 M ACh in ~300-µm-diameter arterial segments preconstricted to 45–55% of the original diameter with phenylephrine. Values are means ± SE; n = 8 segments/group. *Significant difference from the Pluronic F-68 group by two-way ANOVA, P < 0.05.

 

Flow-Mediated Dilation

Responses of pressurized and preconstricted arterial segments to alterations in fluid flow rate through the segments are presented in Fig. 5. FMD at flow rates of 8 and 17 µl/min were significantly greater in the Pluronic F-68 group compared with the Pluronic F-98 group. FMD reached its maximum at 17 µl/min in the Pluronic F-68 group, whereas in the Pluronic F-98 group the maximum response to flow was observed at 26 µl/min (Fig. 4). The flow rate-dilation curve in the Pluronic F-68 group was similar to that obtained using arterial segments isolated from normal, nonoperated rats (data not shown). Prior L-NAME incubation of the arterial segments for 30 min abolished the FMD responses in both groups.



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  		Fig. 5. Flow-mediated dilation (FMD; in %) induced by flow rates between 8 and 35 µl/min in ~300-µm-diameter arterial segments preconstricted to 40–50% of the original diameter with phenylephrine. Values are means ± SE; n = 8 segments/group. *Significant difference from the Pluronic F-68 group by two-way ANOVA, P < 0.05.

 

eNOS Expression

Western blotting results revealed that eNOS expression was markedly reduced in the Pluronic F-98 group 4 days after the exchange transfusion (Fig. 6): the ratio of eNOS band density to GAPDH band density was 3.2 times higher in the Pluronic F-68 group compared with the Pluronic F-98 group. There was no significant difference between the values of the Pluronic F-68 group and nonoperated control animals that did not receive exchange transfusions.



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  		Fig. 6. Expression of endothelial nitric oxide synthase (eNOS) protein. A: typical pattern of Western blot results. B: eNOS-to-GAPDH ratio for control (nonoperated) animals and Pluronic F-68 and F-98 groups. Results are presented as means ± SE. *Significant difference from the Pluronic F-68 group, P < 0.001.

 


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
With the use of a rat exchange transfusion protocol involving RBCs coated with a proaggregating poloxamer (i.e., Pluronic F-98), the results of the present study demonstrate that 1) increased RBC aggregation leads to a gradual increment of arterial blood pressure; and 2) the increment of blood pressure is accompanied by impaired NO-mediated vasodilatory responses together with a diminished expression of eNOS in vessels of skeletal muscle. Control experiments using RBCs coated with a nonaggregating poloxamer (i.e., Pluronic F-68) indicated no changes of arterial blood pressure or eNOS expression. The relationship between NO synthesis and blood pressure is well known: interventions to suppress NO synthesis by endothelial cells have been used as hypertension models (12, 58), and increased arterial pressure is known to alter NO-related vascular control mechanisms (40). The relationship between RBC aggregation and arterial blood pressure is also well known, at least from clinical observations (1, 32). It is thus suggested that the present study provides some insight to these complex relationships that are based on different types of observations.

Arterial blood pressure is directly proportional to the product of cardiac output and peripheral vascular resistance, with peripheral vascular resistance determined by vascular geometry, primarily blood vessel diameter, and the rheological properties of blood (14). RBC aggregation is one of the main determinants of blood's rheological behavior, with increased aggregation resulting in elevated blood viscosity at low to medium rates of shear (7). Thus, based on in vitro viscometric data, enhanced RBC aggregation would increase blood viscosity and flow resistance in large blood vessels. RBC aggregates should be dispersed (i.e., disaggregated) as they approach the microcirculation, where the size of blood vessels approaches that of individual RBCs; the energy cost of such disaggregation would increase if the aggregation tendency of RBC is enhanced (56). Therefore, increased hemodynamic resistance might be expected to be coincident with the stepwise increase of RBC aggregation resulting from the exchange transfusion with Pluronic F-98-coated RBCs: enhanced RBC aggregation would thus be the direct cause of the observed increase of arterial blood pressure (Fig. 3). Given such a mechanism, arterial blood pressure should have reached a maximum immediately after the rapid increase of RBC aggregation and then gradually returned toward control as RBC aggregation tended to decrease (Fig. 2). However, our experimental data do not support this prediction: mean arterial blood pressure rose gradually over the 4-day follow-up period, whereas RBC aggregation decreased ~30% (Figs. 2 and 3).

The lack of a close temporal linkage between increases of RBC aggregation and arterial blood pressure suggests the need to consider other mechanisms. For example, it could be suggested that the gradual increase of arterial blood pressure might be independent of RBC aggregation and rather related to delayed release of poloxamer from the RBC surface. If true, then an infused poloxamer should be expected to have an effect on arterial blood pressure. However, a separate series of studies in rats indicated that daily injections of reactive Pluronic F-68 or F-98 over a 4-day period did not lead to an increase of arterial blood pressure, although strong RBC aggregation was observed in peripheral blood samples during the first 4 h after infusions. This short-term increase in RBC aggregation can be explained by covalent attachment of poloxamers to plasma proteins and cross-linking of plasma proteins by the reactive poloxamer (both Pluronic F-68 and F-98), thereby inducing strong aggregation by markedly increasing the effective macromolecular size of plasma proteins. Note, however, that these short-term increases of aggregation had no meaningful effects on arterial pressure. Furthermore, in the event that the poloxamers did detach from the RBC surface in vivo, these "free" copolymers would be inactive and would not covalently attach to plasma proteins. Inactivated poloxamers were shown to be ineffective on both RBC aggregation and arterial blood pressure. These observations thus exclude the possibility of an effect of poloxamers (Pluronic F-68 and F-98) on blood pressure that is independent of long-term enhancement of RBC aggregation.

Although the in vitro viscometric data suggest increased vascular resistance to blood flow due to increased RBC aggregation, the effects of RBC aggregation on blood flow in vivo are controversial. Dispersion of strongly aggregated RBC at the microvascular level requires additional energy, yet widening of the cell-poor marginal layer near the vessel wall due to aggregation tends to decrease in vivo flow resistance (52). Therefore, the net effect of increased RBC aggregation at a given time point would reflect the interplay between these two opposite influences. Furthermore, it is now well established that the flow conditions in the vascular system (i.e., fluid shear stress) also affect vascular geometry, thereby modulating the vascular component of flow resistance. The possible effects of enhanced RBC aggregation on NO-related vascular control most likely relate to altered blood flow and blood composition near vessel walls. Given increased RBC aggregation and hence increased hemodynamic resistance, blood flow in the peripheral circulation would be diminished. It has been previously demonstrated that blood flow and wall shear stress in the vascular system are important determinants of NO synthesis by endothelial cells (15, 39, 46, 53, 59), with decreased blood flow shown to suppress eNOS expression and FMD responses in small arteries (55). Enhanced RBC aggregation would also tend to promote axial accumulation of RBC in blood vessels, resulting in a less-viscous, plasma-rich region near vessel walls (16). Diminished wall shear stress resulting from this nonuniform radial composition of blood should be expected to influence the NO-related mechanisms in blood vessels (16, 20, 33). Note that these two suggestions (i.e., decreased blood flow and a less-viscous region near vessel walls) are not mutually exclusive and most likely are synergistic in decreasing mechanical forces acting on endothelial cells (10, 11, 59).

Regardless of the specific mechanism(s) responsible for reduced fluid forces at vessel walls, such reductions should result in shifted vasomotor balance and increased peripheral resistance due to diminished NO generation by endothelial cells. Altered wall shear stress may affect NO synthesis by constitutive NOS existing in endothelial cells, and the molecular mechanisms of eNOS activation by mechanical shear stress acting on endothelial cells have been the subject of several studies (15, 29, 52). It has also been demonstrated that wall shear stress modulates the expression of eNOS protein (17, 18, 53). However, the time scales for these two effects of shear stress differ. That is, while the modulation of existing eNOS activity follows rapidly after hemodynamic alterations, alteration of eNOS protein expression is known to occur over a longer span of time (23, 46). For example, while NO-related mechanisms in skeletal muscle are known to play a significant role in vascular adjustments during exercise (27, 34), regular physical exercise requires several weeks to induce increments in eNOS expression (24, 28, 30). It thus seems likely that the gradual increase of arterial blood pressure seen in the present study (Fig. 3) reflects the gradual downregulation of eNOS protein expression due to decreased shear forces on endothelial cells. This suggestion is supported by the suppressed NO-mediated dilation (both ACh and flow induced) responses (Figs. 4 and 5) and the markedly decreased eNOS expression in skeletal muscle samples on the day when arterial pressure was significantly higher than the control group (Fig. 6).

It should be noted that the vascular endothelium might not be the only source of eNOS in the muscle tissue samples used herein for Western blot analysis. It has been demonstrated that eNOS is expressed by rat skeletal muscle cells and is most probably localized to mitochondria (51). Although the level of eNOS expression in skeletal muscle cells was reported to be low (51), the relative contribution of eNOS from endothelial cells to the total eNOS determined in this study is somewhat uncertain. Unfortunately, it was technically impossible to isolate enough resistance blood vessel tissue from rat skeletal muscles and total total muscle tissue samples were employed. However, it seems highly unlikely that the skeletal muscle cell eNOS expression would be modified by the altered hemodynamic conditions: the existing literature clearly indicates a vascular origin for altered eNOS expression due to altered hemodynamics (30, 37, 55). Furthermore, decreased eNOS expression in the Pluronic F-98 group was associated by blunted eNOS-mediated vasomotor responses (Figs. 4 and 5) in resistance vessels isolated from the same muscles. Accordingly, altered eNOS protein content in the skeletal muscle tissue samples were accepted as a strong evidence for the effect of altered shear stresses on endothelial eNOS expression.

The poloxamer-coating method used herein to enhance RBC aggregation in vivo is unique in that it does not involve the addition of foreign polymers to circulating blood: RBCs are suspended in unaltered native plasma. In previous studies designed to assess the hemodynamic effects of RBC aggregation, high-mass-weight polymers such as 500-kDa dextran have been used to increase RBC aggregation (6, 11, 19, 35, 48). The use of these large polymers has been proven to be effective for modifying RBC aggregation, yet has potential disadvantages: 1) plasma or suspending phase viscosity is increased [e.g., for 500-kDa dextran, 1.5 g/dl yields a 80% increase of suspending medium viscosity (43)] and hence the fluid viscosity near the vascular wall is modified; 2) plasma proteins are diluted via the added polymer solution; 3) in some experimental animals (e.g., the rat), dextran and similar polymers can lead to anaphylactic-type reactions and thus induce a pathophysiological state (10); 4) the elevated colloid osmotic pressure caused by macromolecules in solution could alter intraversus extravascular fluid volumes; and 5) due to metabolism/excretion, polymer concentration decreases with the time after infusion (22), thus precluding long-term temporal studies of RBC aggregation effects. The latter point is also somewhat relevant to the coated cells used herein, although it is notable that the degree of aggregation for Pluronic F-98-coated cells decreased by <30% over the 4-day period after exchange transfusion (Figs. 1 and 2). The poloxamers used herein are not toxic at the concentrations that the experimental animals were exposed to but can be nephrotoxic at very high concentrations (57). The use of poloxamer-coated RBCs (3, 4) thus appears to represent an improved approach for investigating the in vivo effects of RBC aggregation.


   ACKNOWLEDGMENTS
 
We thank Murat Uyuklu, Erol Nizamoglu, and Sakir Atalay for technical assistance.

GRANTS

This study was supported by Turkish Scientific and Technical Council Grant SBAG-2435, National Institutes of Health Research Grants HL-15722, HL-48484, and Fogarty International Research Collaboration Award IR03 TW-01295, and the Akdeniz University Research Projects Unit.


   FOOTNOTES
 

Address for reprint requests and other correspondence: O. K. Baskurt, Dept. of Physiology, Akdeniz Univ. Faculty of Medicine, Kampus, Antalya, Turkey 07070 (E-mail: baskurt{at}akdeniz.edu.tr).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 DISCUSSION
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