Hemorheology in experimental research: is it necessary to consider blood fluidity differences in the laboratory rat?

Abstract

This study was designed to identify whether blood fluidity differs between commercially available laboratory rat strains. The hemorheological profiles of seven clinically healthy wild-type rat strains were analyzed to determine whether any diversity in blood fluidity might affect the outcome of cardiovascular studies. Study 1: 65 healthy adult rats (Lewis, Long–Evans, Hairless, Wistar and Fisher; mixed gender and comparable ages) were compared. In order to determine the greatest possible difference, the two strains with the greatest hematocrit (HCT) differences were selected for more detailed evaluation. Red blood cell (RBC) deformability (maximum elongation index, shear stress for half-maximal deformation of RBC; both P < 0.0001), and the effect of plasma protein concentration upon plasma viscosity (P < 0.0001) were different between Lewis and Long–Evans strains. Whole blood viscosity – although different at native HCT (P < 0.004) – was unaltered following HCT standardization of samples. Differences in RBC aggregation were statistically significant but these were small and may not be clinically relevant. Study 2: these 65 animals were compared with 21 animals (10–16 weeks old; both sexes) from mutant strains (Dahl SS/JrHsdMcwiCrl, n = 10; ZDF-Lepr^fa/Crl, n = 11). In both mutant strains, plasma and whole blood viscosity were increased compared with commonly used strains at native and standardized HCT (P < 0.001). Unusually high RBC aggregation values were seen in the ZDF rat strain (P < 0.001). It was concluded that the variability in blood fluidity among clinically healthy adult laboratory rat strains was both statistically and clinically significant. A hemorheological profile should be added to a routine phenotyping process, since both variables can significantly influence study outcomes.

Keywords

hemorheology rat phenotyping RBC cardiovascular

The fluidity of blood is based on the quantitative and qualitative properties of blood cells and the constitution of blood plasma. Compared with other particular systems or dispersions, the viscosity of whole blood is low, even in the presence of a 50% particulate fraction. This is due to the ability of red blood cells (RBCs) to contort according to hemodynamic forces. The cell membrane folds or aligns in the direction of flow, owing to the flexibility of the membrane cytoskeleton and the rotation of the phospholipid membrane around the cytoplasm. In addition to RBC size, the deformability of erythrocytes is also a crucial factor for microvascular perfusion. In association with the actual shear forces, blood cells aggregate or disaggregate when they pass through the vascular tree. Not every mammalian species, however, possesses RBCs with pronounced cell aggregability.^1,2 If RBCs can form rouleaux, aggregation takes place at low shear rates (<10/s), whereas RBCs are singularly suspended in plasma at higher shear rates (>20/s).^3,4 In vitro flow curves simulate this assembly and disassembly of RBCs. By contrast, there is also evidence for RBC aggregation under high shear forces, measured in vivo by ultrasound backscatter.⁵ RBC aggregation has been shown to affect the quantity of RBCs in subsequent vessels in the microvasculature.^6–8 The modulation of endothelial shear stresses by the flowing blood has been proposed as an underlying causative mechanism. The cross-sectional shear rate gradient in a given vascular segment⁹ generates a phase separation of blood in the vessel – provided that RBC aggregation takes place. In this instance, the majority of RBCs would be located in the central layer, whereas the marginal layer would be cell-depleted. Using a numerical mode, a recent study has shown that both RBC aggregation and RBC deformability influence the organization of RBCs in the flow field in microvessels, resulting in phase separation.¹⁰ Such phase separation was also shown to be pronounced at low shear rates.¹¹ Phase separation creates a marginal cell-free layer in the vessel lumen¹² and the width of this cell-free layer affects the endothelial shear stress. As a result, vasoactive substances (nitric oxide, prostacyclin, endothelin) are expressed which adjust the arteriolar diameter.^13,14 It should be considered, however, that cyclic blood flow instabilities may also happen through the pulsatility of blood flow and the arteriolar wall movement. The effect of RBC aggregation on blood flow has been summarized previously.¹⁵ This review shows that RBC aggregation has an impact on peripheral perfusion. It is therefore hypothesized that in the choice of animal for a specific experimental setting, differences in physiological RBC aggregability and deformability could bias the gained results.

The laboratory rat is an established species in experimental research and there are several strains available for different applications. The aim of the present study was to investigate the merits of using a particular strain in experimental applications with respect to hemorheology. Firstly, hemorheological values of arbitrarily chosen, frequently used wild-type rat strains were collected. These strains do not typically develop clinical illnesses during their adult lifespan. The data obtained from these animals were then compared with the values of two mutant wild-type rat strains (Dahl SS/JrHsdMcwiCrl rat; ZDF-Lepr^fa/Crl rat). All animals were clinically healthy at the time of blood collection. The results from this study showed differences in blood fluidity among wild-type rat strains, which are relevant to experimental applications.

Animals

A total number of 86 adult rats were included in the study.

Five strains (65 animals) of the wild-type genome were subsequently identified in the text as ‘commonly used’ strains. Both sexes were used and the animals were 10–16 weeks old (see Table 1 for the demographic data). All animals were kept conventionally in polysulfone cages mounted by filter tops in the animal house of the Medical University of Vienna. The Lewis rat strain was housed in a specific pathogen free (SPF) enclosure. The animals were kept on a 12 h day/12 h night cycle starting at 06:00 h and 18:00 h, respectively. All the animals except the Lewis strain had been inbred over a long time period throughout the animal house. These strains had been bred for maintenance and training purposes for the education of animal keepers. By contrast, the Lewis rats were purchased from Charles River Inc (Sulzfeld, Germany) at intervals of 2–3 years (which equals 8–12 generations) in order to maintain the LEW/Crl phenotype. For this reason, F3–F12 generations were used for this study. The animals were fed conventional rat diets (treated by autoclave: V1324–300, or extruded: V1326–000; ssniff, Soest, Germany) and tap water ad libitum. The health status of the animals was recorded on a regular basis (the last complete parasitology, bacteriology and virology tests had been undertaken three months prior to the experiment). The Lewis rats inside the SPF barrier followed the Federation of European Laboratory Animal Science Associations (FELASA) SPF recommendations and were free of parasites, viruses, and bacteria. The rats, which were kept conventionally in filter top cages, were free of ectoparasites and worms, but they were positive for protozoa (Spironucleus sp.). These rats were negative for viruses specified on the FELASA list, but colonized by Pasteurella pneumotropica, Staphylococcus sciuri, and Corynebacterium bovis. All the rats were examined by the veterinarian prior to the experiment and were considered clinically healthy. They showed no clinical signs of infection.

Table 1.

Demographic data of rat strains.

Name	Sex (m/f)	Age (weeks) median (25%/75%)	Inbred since*	Source	Nomen-clature	Numbers used
Hairless	4/6	14 (12/17)	1982	Unknown	–	10
Lewis	6/14	12 (11/15)	–	CRL	LEW/Crl	20
Long–Evans	9/11	13 (10.5/16)	1979	ZfV^†	LE:Han	20
Wistar	5/5	12 (11.5/16)	1979	ZfV^†	Han:WIST	10
Fisher	3/2	11 (10/12.5)	1979	ZfV^†	F344/NHan	5

Inbred in the entire animal house of the Medical University of Vienna, Austria, only. ^†Central Institute for Laboratory Animal Breeding, Hannover, Germany. m: male; f: female.

Two mutant strains (21 animals) were used: Dahl SS/JrHsdMcwiCrl (n = 10, 5 females + 5 males) and ZDF-Lepr^fa/Crl (n = 11, 6 females + 5 males). The animals were 10 weeks old when they arrived in the animal house and were kept conventionally in polysulfone cages mounted by filter tops. The mutant strains were fed the same diet (V1326–000; ssniff) and tap water ad libitum. These animals were tested within the first four weeks of arrival in the animal house; with the exception of one female ZDF rat, which was tested at 16 weeks of age.

The rats were kept in the animal facilities of the Medical University of Vienna in accordance with institutional policies and federal guidelines.

Materials and methods

Experimental procedure

The rats were bled by cardiac puncture, under general anesthesia with 100 mg/kg ketamine and 10 mg/kg xylazine intraperitoneally. Blood (5–10 mL) was withdrawn smoothly from each animal using a 10 mL syringe mounted by a 16 G needle. Blood samples were collected into tubes containing sodium EDTA, sodium heparin, and sodium citrate. Animals were killed by cervical dislocation following the sampling procedure. In order to eliminate circadian influences on blood fluidity, blood samples were withdrawn in the morning between 08:00 and 10:00 h.¹⁶ The procedure was approved by the Animal Experimental Ethics Committee of the Medical University of Vienna and the Austrian Federal Ministry of Science and Research (animal license number: GZ-66.009/0158-II/3b/2013).

Hematology and blood chemistry

A routine hematological profile was obtained from each animal using the Cell-Dyn 3500 (Abbott, Abbott Park, IL, USA). Hematological parameters included mean corpuscular volume (MCV, in fL), mean corpuscular hemoglobin (MCH, in pg/mL), mean corpuscular hemoglobin concentration (MCHC, in %) and red and white blood cell count (RCC, in cells·10¹²/L, WCC, in cells·10⁹/L). Hematocrit (HCT, in %) was measured by centrifugation (Hettich, Tuttlingen, Germany).

The routine blood chemistry profile of each animal (Hitachi 904; Hitachi, Tokyo, Japan) was performed to show the health status of the animals. Plasma fibrinogen concentration (FIBR, in mg/dL) was measured manually according to the Clauss method.

Hemorheology

Whole blood viscosity (WBV, in mPa·s) was carried out using the LS30 viscometer (Contraves AG, Zürich, Switzerland) at 37℃. Blood samples were firstly measured at native HCT values, then centrifuged for HCT (40 ± 2%) adjustment with autologous plasma as described¹ and analyzed again.

Principle of measurement: A one millilitre sample of blood was placed into the modified Couette cylinder system of the device. The viscosity of blood exerts a mechanical strain on the free movable cylinder, which was measured electromagnetically. Shear rates of 0.7, 2.4 and 94/s were used in this study. Three consecutive readings were recorded for each value.

RBC aggregability was measured using the Myrenne aggregometer MA1 (Myrenne, Roetgen, Germany) at room temperature and native HCT (aggregation index: M0, M1). Principle of measurement: A 30 µL sample of blood was placed between the transparent cone-plate shearing device and spread to a cellular layer. This layer was first sheared at 600/s for 10 s to disaggregate all pre-existing aggregates, then abruptly stopped in M0 mode or rotated at 3/s in M1 mode for another 10 s to allow aggregation. The extent of plasma gaps inside the whole blood monolayer allowed light transmission through the red cell suspension. The instrument computes an aggregation index (M0 at stasis and M1 at 3/s) proportional to the area under the light transmission curve.¹⁷ At least five readings were taken for each value of M0 and M1.

Erythrocyte deformability was measured by laser diffraction analysis, using an ektacytometer (LORCA; Mechatronics, Hoorn, The Netherlands). For various fluid shear stresses, elongation indices (EI) were calculated from the diffraction image as the (length – width)/(length + width) of the image at 37℃. The shear stress for half-maximal deformation (SS_½) and the EI at the highest shear stress (EI_max) were estimated as described elsewhere.¹⁸

Principle of measurement: RBC deformability was measured at fluid shear stresses between 0.3 and 53.06 Pa. Twenty-five microlitres of blood was diluted in a phosphate buffered saline (pH 7.4) – polyvinylpyrrolidone (PVP, M = 360,000 Sigma-Aldrich, Germany) medium of known viscosity (25.7 mPa·s). The sample was sheared in the Couette system with a 0.3 mm gap between the two cylinders. A laser beam traverses the suspension and gets diffracted by the RBCs in that area. EI were calculated for each shear stress.

Plasma viscosity (PV, in mPa·s) was measured by OCR-D (Anton Paar, Graz, Austria) at 25°C.

Principle of measurement: one millilitre of plasma was filled into a plastic cup and mounted at the bottom of a glass capillary (diameter: 0.9864 mm, length: 100 mm). The plastic cup was compressed at 2 Hz exerting shear rates inside the capillary. The viscosity at the shear rate of 10/s was read for PV.

Experimental design and statistics

In the first study, the two commonly used rat strains with the greatest deviation in HCT were selected for further exploration. This was done in order to define the greatest possible differences in hemorheological values between laboratory rats of comparable age. The sample size calculation for the number of rats was based on pilot data for the Long–Evans (lowest HCT) and Lewis (highest HCT) groups of the variable WBV_0.7*s–1 available from three rats per group. The pilot data showed a mean of 34.4 mPa·s in the Lewis group and 27.5 mPa·s in the Long–Evans group with a common standard deviation of 7.2. Using a two-sided t-test (with significance level 0.05), a sample size of 20 per group was found necessary to detect the given difference with 80% power. The 20 animals per group were drawn randomly from the given pool. Unpaired t-tests were used to investigate the differences in hemorheological parameters (WBV_0.7*s–1, WBV_2.4*s–1, WBV_94*s–1, PV, M0, M1, EI_max, SS_½), MCV and total protein (TP). To correct for multiplicity, all P values less than 0.005 (= 0.05/10) were considered as statistically significant (Bonferroni adjustment). In addition, a similar analysis was calculated to investigate sex-related differences among the commonly used rat strains. To investigate the correlation between TP, PV and WBV_0.7*s–1, regression analysis for TP accounting for rat-group, PV, and WBV_0.7*s–1 was performed. A similar analysis was calculated to investigate the coherence between M1 and fibrinogen. Pearson correlation coefficients were calculated. The other strains were investigated by descriptive statistics only.

In the second study, comparisons between commonly used and mutant strains were made. All data collected in Study 1 (65 animals) were pooled and compared with the ZDF group (11 animals) and the Dahl/SS group (10 animals) separately by analysis of variance (ANOVA). The following parameters were tested: WBV_0.7*s–1, WBV_2.4*s–1, WBV_94*s–1, PV, M0, M1, EI_max, SS_½, TP and MCV. To correct for multiplicity, again, all P values less than 0.005 were considered statistically significant. Again, as an addition, t-tests were used to compare parameters between male and female rats. Furthermore, a two-way ANOVA with factors rat strain and sex was performed. To correct for multiplicity, all P values less than 0.005 (= 0.05/10) were considered as statistically significant.

In both studies, descriptive statistics were calculated using the median, 25% and 75% percentiles. Statistical analysis was performed using SAS (release 9.1; SAS Institute Inc, Cary NC, USA).

Results

Figure 1 shows the RBC EI–shear stress curves of Lewis and Long–Evans rats. Figure 2a and b shows the aggregation indices M0 and M1, Figure 3 shows the PV and Figure 4a and b show the WBV of the rat strains.

Figure 1.

Red blood cell elongation index (EI)–shear stress curves of Lewis and Long–Evans rats.

Figure 2.

Aggregation indices M0 (a) and M1 (b) of the rat strains.

Figure 3.

Plasma viscosity (PV) of the rat strains.

Figure 4.

Whole blood viscosity [WBV_0.7*s-1 (a) and WBV_94*s-1 (b)] of the rat strains.

Table 2 shows the hemorheological data, HCT, MCV, TP, cholesterol (CHOL) and fibrinogen (FIBR) of those rat strains that were included into the statistical evaluation (Lewis, Long–Evans, ZDF and Dahl/SS rat strains). The complete data-set of all commonly used rats is provided in the supplementary file (supplementary material is available online with this article, http://lan.sagepub.com).

Table 2.

Hemorheological values at standardized hematocrit (HCT) (40%, centrifuged cells in autologous plasma).

	Lewis	Long–Evans	ZDF	Dahl/SS
WBV_0.7*s–1 (mPa·s)	30.81 (23.45/35.93)	27.95 (21.86/29.92)	42.30 (37.05/48.68)^§	39.63 (25.51/60.78)^§
WBV_2.4*s–1 (mPa·s)	18.49 (15.12/20.83)	17.62 (15.26/20.56)	23.46 (19.54/25.89)^§	21.49 (16.22/29.23)
WBV_94*s–1 (mPa·s)	5.84 (5.18/6.17)	5.55 (5.11/6.39)	7.56 (7.06/8.13)^§	6.30 (5.74/6.74)
PV (mPa·s)	1.22 (1.19/1.24)	1.23 (1.19/1.26)	1.48 (1.39/1.56)^§	1.31 (1.28/1.53)^§
M0	0.9 (0.5/2.2)*	0.3 (0.1/0.6)*	3 (1.5/5)^§	1.5 (0.5/5)^§
M1	5.6 (3.5/8.2)	4 (2/8)	13 (9.5/15)^§	8 (4/19)^§
EI_max	0.67 (0.66/0.69)*	0.59 (0.57/0.59)*	0.64 (0.63(0.64)	0.63 (0.63/0.64)
SS_½	2.84 (2.54/3.39)*	1.71 (1.56/2.15)*	2.85 (2.61/3.35)	2.07 (1.86/2.20)
HCT (%)	45 (42/48)	37 (34/39)	41 (38/48)	41.5 (41/44)
TP (g/dL)	5.9 (5.5/6.4)	5.7 (5.5/6.0)	7.7 (6.8/8.5)^§	5.9 (5.6/6.6)
MCV (fL)	55.8 (54/56.8)	56.8 (54.3/58.8)	52.3 (51.7/53.5)^§	57.4 (55.8/61.1)
CHOL (mg/dL)	82 (77/94)	61.5 (54/65)	135 (114/151)	91.5 (80/149)
FIBR (mg/dL)	184 (169/211)	226 (174/260)	276 (236/340)	286 (242/342)

The table also shows the native HCT, as well as total protein (TP), mean corpuscular volume (MCV), cholesterol (CHOL), and fibrinogen (FIBR) of blood samples of the tested rat strains. Values are expressed as median and 25%/75% percentile in parenthesis. WBV: whole blood viscosity, PV: plasma viscosity, M0, M1: aggregation indices, EI_max: elongation index at the highest shear stress, SS_½: shear stress for half-maximal deformation. *Values different (P < 0.005) between commonly used rat strains (Lewis versus Long–Evans); Study 1. ^§Values of mutant rat strains different (P < 0.005) from the pooled data of all other rats; Study 2.

Hemorheological differences in the commonly used rat strains (Study 1)

The greatest differences in HCT were observed between the Lewis and Long–Evans rats. These two strains were therefore selected for the statistical analysis. HCT was chosen as a key parameter as it is a major determinant of blood viscosity. By comparing native blood at different HCT values, WBV_2.4*s–1 was found to differ (P = 0.004) between the two rat strains. This observation necessitated the standardization of all samples to a specific HCT to diminish the effect of HCT and RCC on blood fluidity. Therefore, only HCT standardized blood samples were included into the analysis (WBV at native HCT is shown in the supplementary file, Table B).

At the standardized HCT values, no significant differences were observed in WBV between Lewis and Long–Evans rat strains. No significant differences in PV were observed between these strains. TP was found to have a significant influence on values of PV (P < 0.0001; Pearson correlation coefficient: 0.80) for these two rat groups. No similar influence on the values of WBV_0.7*s–1 could be detected. Aggregation index M0 (P = 0.005) was increased in the Lewis rat strain, however, the aggregation indices M1 did not differ significantly. RBC deformability parameters (SS_½ and EI_max, both P < 0.0001) were increased in the Lewis rat strain. Since geometric factors (for instance, cell size) influence the EI, as measured by ectacytometry, it is important to control MCV. For rat strains in this study, no significant differences (P = 0.17) in MCV were seen, so the influence of cell size on RBC deformation may be disregarded. The ‘maximum’ RBC deformation (EI_max) was higher in Lewis rats, although the cells required a higher shear stress to reach half of this ‘maximum’ deformation. Figure 1 shows that the ‘maximum’ deformation did not reach a plateau value; so the highest EI in this chart does not provide a real ‘maximum’ deformation of the RBC. This effect has also been observed previously for the elephant.¹⁹ In our method, the shear stress range that was presented in the sample was fixed. However, extrapolation of the EI–shear stress curve towards higher shear stresses resulted in a plateau at higher EI values in the Lewis rat. By considering both indices together (SS_½ and EI_max) we suggest that a higher shear stress is needed for initial RBC deformation in Lewis rats. When those RBCs have undergone 50% of their deformation, their further elongation is alleviated compared with that of Long–Evans rats.

Hemorheological differences between the commonly used and the mutant rat strains (Study 2)

In this study, the hemorheological values of Dahl/SS and ZDF rats were compared with the pooled values of all other rat strains. WBV was increased in the ZDF rat strain at all shear rates (all P < 0.0001). In the Dahl/SS rat strain, WBV was elevated at the lowest shear rate (0.7/s) only (P < 0.001). PV was increased in both mutant strains (both P < 0.0001). Aggregation indices M0 and M1 were also increased (all P < 0.001). SS_½ and EI_max of the mutant strains did not differ significantly from those of commonly used strains. MCV, however, was lower in the ZDF rat strain (P < 0.001). A smaller cell requires a higher shear stress for the same deformation than a larger cell if the membrane has the same flexibility. Therefore, the lack of response does not implicate that RBC deformability in the ZDF rat strain was unaltered compared with commonly used rat strains.

In addition, the effect of sex on the variables has been tested. There was a sex-related difference in WBV_94*s–1 in the Lewis rat (P < 0.0001). Significant larger parameters were found for female rats. There were no differences in the Long–Evans rats. However, WBV_94*s–1 did not show a significant difference between the Lewis and Long–Evans rats in our main study. A two-way ANOVA with factors rat strain and sex confirmed no significant differences between female and male rats within the rat strains ZDF, Dahl/SS, and the pooled values of all other rat strains. Medians and interquartile distances of variables, separated by sex are provided in Table C of the supplementary file.

Other laboratory values

Commonly used strains

Differences in blood chemistry were observed between strains. The plasma CHOL (mg/dL) concentration, for example, was higher in the Lewis rats than in the Long–Evans rats. Although the aggregation index M0 differed between the two strains, FIBR (mg/dL) was similar. Hematology was similar between the two strains, with the exception of RCC. In principle, hematology of the commonly used strains was within the range of our laboratory specific reference values for the ‘rat’ species. These were also comparable with reference values from the literature.^20,21 Deviations from reference values (laboratory reference values, and Ref. 22) concerning the plasma concentration of liver transaminases and specific ions were observed in the Hairless rat strain (see Table A in the supplementary file). These differences existed despite the animals having the same diet as the other strains of this study and their being kept in the conventional part of the animal house.

Mutant strains

The plasma concentrations of CHOL and FIBR were higher than in the commonly used strains. TP was increased in the ZDF rat strain (P < 0.0001) only. All other values were comparable with the reference values of the vendor.

Discussion

Cardiovascular physiology and fluid dynamics investigations may require precise blood fluidity values. The relevant parameters for obtaining blood fluidity in the experimental animal of choice should therefore be known. The laboratory rat is frequently used in biomedical research. Several different rat strains are available from academic animal houses and commercial suppliers. The selection of the optimal rat model is often done habitually or is based on cost or availability. Whether differences in blood fluidity exist between laboratory rat strains has not been previously evaluated in a coherent study. Most importantly, whether such differences can significantly influence the outcomes of studies with cardiovascular or fluid mechanics components has not yet been established.

The phenotype of rats generally differs across breeds. If kept in an isolated animal house for a period of years, the characteristics of a rat strain from a specific source will change over time. If a strain is reproduced in different facilities over an extended time span, hemorheological parameters may vary for that reason. Changing the breeding modalities from outbred to the easier workable inbred type may further modify the animal’s phenotype. The first aim of this study was to investigate whether any differences in hemorheological parameters are present within commonly used wild-type laboratory rat strains (Study 1). This question is best addressed when the phenotypes of the strains are as diverse as possible. Rat strains which had been isolated for many generations in our animal facility were therefore included. The (originally outbred) Long–Evans strain, for example, has been inbred in our animal house for more than 30 years. These strains had been kept conventionally and although their phenotypical features had been maintained, diversifications in biochemistry between strains may have occurred over that time. Hematology and blood chemistry are provided in the supplementary file. Animals purchased from a commercial vendor at regular intervals were also included in order to avoid a change of phenotype during reproduction in our animal house. These animals were kept in an SPF enclosure.

As HCT is the major determinant of blood viscosity, the two strains identified as having the maximal physiological HCT difference were selected for Study 1. The ‘in-house’ Long–Evans rat strain was found to have the lowest HCT among the five available strains and the Lewis rat strain to have the highest. The hemorheological parameters of these two strains were compared in order to determine whether differences exist between healthy rate strains.

RBC deformability and aggregability were found to differ between these two strains. The differences in RBC aggregability were small and may not be clinically relevant. The differences in RBC deformability, however, may have some clinical importance. Lewis rat strain RBCs required a greater shear stress for their initial deformation than those of the Long–Evans rat strain. Once they reached half of their ‘maximum’ deformation, however, they showed a greater extended symmetry. This diversity was unexpected. RBC transit time in capillaries is affected by cell deformability in addition to cell size. As such, it is recommended that RBC deformability be tested in studies where the microvascular blood flow is analyzed in rats. RBC deformability may also affect RBC alignment in the flow field and in the formation of the marginal cell-free layer in blood vessels.¹⁰ The cell-free layer width was shown to be associated with the endothelial nitric oxide synthase (eNOS) expression via the endothelial shear stress stimulus¹² and the distribution of RBC in subsequent vessels.⁶

The differences in RBC deformability and aggregability were found to have no influence on apparent WBV, as indicated by the flow curves: HCT standardized blood samples were compared. Similar WBV values were observed in standardized HCT samples for the two healthy strains. If differences in the flow curves of healthy rat strains are obtained, the possibility should therefore be considered of this finding being due only to the effect of the HCT.

Plasma TP concentration is the major determinant of PV. Variations of TP during an experiment using rats would therefore modify the animal’s PV to different degrees. Variability in the influence of TP on the values of PV was found in Lewis versus Long–Evans rat strains. This could be important for the expression of endothelial vasoactive factors, given that PV contributes significantly to endothelial shear stress.²³

In Study 2 it was hypothesized that the hemorheological parameters of rat strains which typically develop specific diseases within a time span after birth, may be different from those that have a normal life expectancy. This is hypothesized to be the case even when animals are clinically healthy at the time of examination. For this reason the results of Study 1 (all 65 rats) were pooled together and compared with the values of two mutant strains, the ZDF rat and the Dahl/SS rat. Both of these mutant strains were provided by the vendor. The mutant strain populations included 11–16-week-old animals of both sexes, which were not yet showing clinical signs of their disease. The plasma concentrations of TP, FIBR and CHOL in these mutant rat strains were already enhanced, although the animals were clinically unremarkable. These parameters are known to influence RBC aggregation and membrane deformation. Significant increases in WBV, PV and RBC aggregation were observed in the ZDF and Dahl/SS strains relative to the commonly used strains. The aggregation index M1 in the ZDF rat strain reached unusually high values for the rat species.^1,2,24 Values such as these may occasionally be observed in aged male OFA rats weighing more than 700 g (personal observation). Since the HCT was standardized, the high WBV values in the flow curves of the ZDF rats are likely to be the result of this augmented RBC aggregability, at least at the low shear rates.

The high RBC aggregation of the mutant rats was the most noteworthy finding in Study 2. RBC aggregation is associated with blood flow. Blood flow velocity and the vessel radius determine the vascular shear rate. This in turn regulates rouleau formation. RBC aggregability itself was postulated to be associated with the width of the marginal cell-free layer in the flowing blood.⁹ A low viscosity of the fluid layers that are nearest to the endothelial cells would thereby reduce the endothelial wall stress.^7,12 When a reduced endothelial wall stress is transmitted to the endothelial cell, in the presence of a regular endothelial cell phenotype, this leads to a modulation of the vessel diameter.^14,25–27 The cell-free layer width – which shows spatial variations at bifurcations⁶ – was also shown to influence the dissipation of RBC into subsequent vessels. This effect may occur through plasma skimming of daughter vessels. Findings from a number of authors support this hypothesis: (i) RBC aggregation was found to modulate the intramyocardial HCT gradient in guinea pig hearts;⁸ (ii) RBC aggregation was found to influence the flow resistance in isolated striated muscles;^28–30 (iii) RBC aggregability was shown to affect the vasomotor tone via eNOS expression;^25,26 (iv) the blood flow in a given vascular segment was found to affect the HCT in the subsequent vessels;⁶ and (v) the preferred perfusion of the faster branch at arteriolar bifurcations was found to affect the tissue HCT in striated muscle of obese Zucker rats.³¹ Based on these findings it is proposed that the dissipation of RBC in the peripheral vasculature will be altered by the enhanced RBC aggregation in our mutant rat strains. It is proposed that this effect will be pronounced when the animals develop clinical signs of disease. It is further hypothesized that both RBC ‘feeding’ into subsequent vessels and nitric oxide production in endothelial cells may then be reduced.

Sex-related, as well as gender-related, differences in laboratory values are an important issue in laboratory animal research. This study was not designed to investigate sex differences among the hemorheological variables in the rat strains, and indeed the sample size may be too low to detect significant differences. It is important to note that the results of this study are not modified if the factor of ‘sex’ is included in the analysis. No significant hemorheological differences between sexes in the mutant and commonly used rat strains were observed. Recent studies have demonstrated that sex has an influence on hemorheological parameters in an outbred rat strain³² as well as in humans.^33–35 The results from mutant rats in the current study support these previous findings as they provide a typical tendency, although they lack consistency. The influence of sex and gender on the hemorheological values of laboratory animals is an important issue for future investigation.

In conclusion, differences in blood fluidity were observed among the different laboratory rat strains. RBC membrane flexibility can be diverse in healthy wild-type animals, which are commonly used in biomedical research. This should therefore be measured if the phenotype of the strain is going to be described. The varying degree to which plasma protein influences PV may be of interest for the experimental surgeon/anesthetist or during clinical applications. In rat strains, which develop a disease or abnormal pathology after a certain period, the hemorheological variables may differ profoundly from those of other wild-type strains. This is the case even if they are clinically healthy at the time of examination. Generic values cannot be applied to these rats; and each strain needs to be tested separately. It is possible that this effect may be pronounced in genetically-modified strains. The results from this study show that RBC deformability and RBC aggregability may significantly influence outcomes in cardiovascular studies.^4,36 It is therefore recommended that these parameters should be measured during an animal’s phenotyping.

Footnotes

Acknowledgements

The authors thank Mag. Bernhard Roeder for technical assistance and Dr Elisabeth Williams for editing the English manuscript. We also thank the animal keepers of the Decentralized Biomedical Facilities for their valuable help.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Declaration of conflicting interests

The authors declare that there is no conflict of interest.

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