Uric acid and uric acid/creatinine ratio and their correlations with the hemorheological determinants in subjects with subclinical carotid atherosclerosis

1 Introduction

Previously [1], we have evaluated the whole blood viscosity (WBV) at high and low shear rates, plasma viscosity (PV), haematocrit, and mean erythrocyte aggregation in a cohort of 100 subjects with subclinical carotid atherosclerosis (SCA) evaluated using carotid ultrasound. In whole group of these SCA subjects, when compared to a control group, we have observed only a significant increase in BV. However, dividing the SCA subjects according to the cardiovascular risk factors (CRFs) our data indicate a significant increase in WBV, PV, erythrocyte aggregation (EA) and plasma fibrinogen level in the subgroup with 3–5 CRFs. Moreover, dividing the subjects according to the logarithm of the product of triglycerides and fasting plasma glucose level (TyG), surrogate index of insulin resistance, we observed a significant increase in BV and in EA in the group with high TyG levels.

In whole cohort of SCA subjects, we have also observed a relationship between the serum uric acid (SUA) and the hemorheological determinants. In fact, besides the correlation between SUA and BV, we have observed a positive correlation among SUA and haematocrit, PV and EA. Up to now, it has been observed a significant correlation between SUA and BV in healthy mountains population [2], while in subjects with abdominal obesity [3] no relationship was evident between SUA and PV. Other authors [4] have instead described a relationship between SUA and haematocrit in subjects with untreated borderline hypertension and in patients with established hypertension associated with left ventricular hypertrophy. Other researchers [5] have described a positive correlation between SUA and haematocrit in a cohort of subjects (11-to 21 years old) with SUA > 5.5 mg/dL.

As regards, the EA influences the flow dynamics and especially the resistance of blood in vivo; in vivo EA arises at low shear forces or stasis, and it is the principal determinant of low shear rate blood viscosity (LSBV) [6]. This rheological determinant is primarily regulated by the intrinsic cell characteristics of RBCs, the concentration of the macromolecules and/or by the plasma level of some proteins [7]. There are two models for EA [8]: bridging and depletion. In the first model EA occurs when the bridging forces exceed the disaggregating forces. In the second one, EA occurs because of a lower localized protein or polymer near the erythrocyte surface compared to the suspending medium. In both models, disaggregation forces are respectively electrostatic repulsion, membrane force and mechanical shearing. The term “aggregability” indicates the intrinsic tendency of erythrocyte to undergo aggregation, while the aggregation refers to the rate, extent, or strength of EA in any medium.

Among the main parameters that regulate the erythrocyte membrane stability must be consider the membrane electrical properties [9], referable to the sialylated glycoproteins present in the outer leaflet of red cell membrane, that develops a repulsive electric zeta potential between red cells [10]. Generally, the zeta potential is an electrochemical property of cell surface, detected by the net electrical charge of surface molecules. The erythrocyte membrane is negatively charged and is ringed by a fixed layer of cations in his turn surrounded by a cloud-like diffused layer of a combine of cations and anions. Within the diffused layer, the Brownian move of erythrocytes and the flow of the medium produce a shear plane, capable to dived unfixed ions from those associated with the fixed layer, and the potential at this shear pale is the zeta potential [11].

Usually, the techniques employed for the examination of the erythrocyte zeta potential are the laser doppler velocimetry [12], the double optical tweezers [13] and the electrophoretic mobility [14]. This potential has been examined in diabetes mellitus and in diabetic cardiovascular disease [12], in young and old red cells fractionated by Percoll method [15], during the storage period [16], and in hypertensives with and without diabetes mellitus [17].

In the last years, some researchers have demonstrated that in experimental models SUA favours the EA, and this event is related to the decrease of the erythrocyte zeta potential [18, 19]. In the past years, a research group, at different times and with several experimental models, had already described the damaging effects of the monosodium urate on the erythrocyte membrane, quantifying them as percentage of hemolysis [20–22].

Considering all these information, in our cohort of subjects with SCA, divided according to the number of CRFs and to the insulin resistance degree, we have examined the behaviour of SUA and uric acid/creatinine ratio (SUA/Cr ratio) and the correlation among these parameters with the main determinants of the hemorheological profile.

2 Subjects

The investigated group include 100 subjects (43 men and 57 women, median age 66.00 (25.00) years) with SCA. This vascular condition was demonstrated by a carotid ultrasound examination. The common carotid artery, the bifurcation and the internal carotid artery have been examined bilaterally with a linear 7.5 MHz ultrasound probe, using an Esaote MyLab 25 and following standard hospital procedures. The carotid atherosclerotic plaques, unilateral in 43 and bilateral in 57 subjects, were all fibrocalcific type with no implications in terms of the hemodynamic profile. In all the SCA subjects was examined also the ankle-brachial index being less than 0.90 in 3 subjects, and they resulted to be asymptomatic for peripheral arterial disease. The subject group involved in the study had no evidence of clinically significant cardiovascular diseases by history, physical examination, ECG, echocardiogram, or chest x-ray.

3 Methods

Venous blood samples were collected by venous puncture in the morning from the antecubital vein of fasting subjects and immediately transferred to anticoagulated glass tubes for the evaluation of the following parameters:

Serum uric acid using the colorimetric method (mg/dl)

4 Statistical analysis

The statistical difference between the data expressed as medians and interquartile ranges of serum uric acid and serum uric acid/serum creatinine ratio of each subgroup belonging to the whole group of the SCA subjects was analysed according to the Mann-Whitney test and the null hypothesis was p > 0.05. The correlation coefficients between serum uric acid, serum uric acid/serum creatinine ratio and the hemorheological determinants were examined using the Spearman rank correlation coefficient and the null hypothesis was p > 0.05.

5 Results

In the whole group of SCA subjects BMI was 28.08 (18.26) Kg/m², SBP 130.0 (20.0) mmHg, DBP 80.0 (10.0) mmHg, fasting glucose level 94.50 (17.0) mg/dl, total cholesterol 199.5 (53.7) mg/dl, HDL-C 50.50 (15.0) mg/dl, LDL-C 122.2 (40.2) mg/dl and triglycerides 102.0 (62.55) mg/dl.

The SCA subject group got later divided according to the number of cardiovascular risk factors (hypercholesterolemia-73% -, arterial hypertension-66% -, family history of cardiovascular disease -65% -, smoker or ex-smoker –52% -, metabolic syndrome –33% -, obesity –28% -, diabetes mellitus –15% -) into two subgroups. 43 of them had 1 to 2 cardiovascular risk factors (CRF) and 57 had 3 to 5 CRFs. The same group was divided into two subgroups according to their TyG parameter. Such further division was carried out according to their logarithm of the product of triglycerides and fasting plasma glucose level (TyG index); this parameter is considered a marker of insulin resistance [16]. The SCA group was then divided into those with a low and high TyG index according to its median value. All the characteristics of two subgroups are reported in Tables 1 2.

In the SCA subgroup with 3–5 RFs (Table 1), in comparison with SCA subgroup with 1-2 RFs, is present an increase in waist circumference (WC) and in fasting blood glucose level. In SCA subgroup with high TyG value (Table 2), when compared with SCA subgroup with low TyG level, is present an increase in BMI, WC, fasting blood glucose level and triglycerides, and a decrease in HDL-cholesterol.

In Table 3 it is observed that the SUA is significantly increased in SCA subjects with 3–5 CRFs and the same behaviour is evident in the insulin resistance degree. Instead, no significant difference has been observed in the SUA/Cr ratio between SCA subjects divided according to the number of CRFs and insulin resistance degree. In Table 4 are showed the results of the correlations between SUA and the hemorheological determinants in the whole group of subjects and in the division made according to the amount of CRFs and insulin resistance degree. In fact, SUA in the whole group of subjects is related to high and low shear rates whole blood viscosity (LSWBV), PV, fibrinogen, and EA. SUA is only correlated to PV and EA in the subgroup with 3–5 CRFs, and only with PV in the subgroup with high TyG. Considering instead the SUA/Cr ratio (Table 5), we have observed in the whole group of SCA subjects a positive correlation between this ratio and LSWBV. Splitting the SCA subjects for CRFs and degree of insulin resistance emerge new information. In fact, the SUA/Cr ratio correlates with LSWBV, PV and EA in the subgroup with 3–5 CRFs and a similar trend is present in the subgroup with high TyG (Table 5), where this ratio correlates with LSWBV, PV, fibrinogen level and EA.

The data obtained from this preliminary study show clearly how the behaviour of SUA is significantly influenced by the number of CRFs and by insulin resistance degree.

Physiologically, SUA is the final product of dietary and of endogenous purine metabolism, and its concentration depends on the balance between the intake, endogenous synthesis, excretion through kidney (2/3) and gut (1/3), and purine metabolism.

Although several studies have demonstrated an association between SUA increase and cardiovascular disease, it has been also proposed that the link between SUA and this cardiovascular risk may happen also with normal SUA level [23–25].

Our data about the SUA concentration in the subgroups of SCA are based on the fact that our cohort exhibits a cardiovascular or cardiometabolic clustering judged by the percentage of CRFs discovered in each of these subjects. Even if the SCAPIS Pilot [26] has demonstrated that the SUA level results not associated with the carotid atherosclerotic plaques, it has been assessed that the CRFs, and especially their number [27], affect the SUA and vice versa. This assertion results evident considering the subjects with dyslipidaemia [28, 29], the subjects with arterial hypertension [30, 31], the subjects with family history of cardiovascular events [32, 33], the subjects with metabolic syndrome [34, 35] and the subjects with insulin resistance, obesity, prediabetes, and diabetes mellitus [36, 37]. About the behaviour of SUA pointed out in relation to the number of CRFs we must also consider that in smokers its concentration has been found reduced by some authors [38] while others [39] have found a SUA increase in ex-smokers.

In our study, the behaviour of the SUA/Cr ratio is quite different in comparison with SUA; this ratio does not distinguish the SCA subjects divided according to the number of CRFs and to the degree of insulin resistance. This parameter, up to now, has been used in type 2 diabetic patients, in metabolic syndrome, as predictor of incident metabolic syndrome and as a predictor of incident chronic kidney disease in type 2 diabetic patients [40, 41].

The correlation coefficients between SUA and WBV show how in the entire group of SCA subjects the SUA is related with WBV and with each of its determinants. Previously, other authors [42] have described a positive correlation between calculated WBV at shear rate of 208 s^–1 and SUA in type 2 diabetes mellitus with metabolic syndrome, while others [43] have ascertained that in Ecuadorian patients with arterial hypertension the subgroup with calculated high WBV showed a significant increase in SUA. The data obtained examining the correlation coefficients in each subgroup of SCA subjects show that SUA is related with PV and EA in the subgroup with 3–5 CRFs and only to the PV in which with high TyG. Different are the correlation between SUA/Cr ratio in the whole group and in the subgroups of SCA subjects. In the entire group this ratio is related to the LSWBV only, while in the different subgroups besides to confirm this relationship with LSWBV, this ratio results related to the PV and EA in the subgroup with 3–5 CRFs and, with the same hemorheological parameters, in the subgroup with high TyG.

Moreover, a datum from this double division of SCA subjects is the correlation between SUA and SUA/Cr ratio with PV. To explain these correlations, it is helpful to take in consideration that both these parameters are influenced by cardiometabolic clustering and by insulin resistance, and that, among the main components of PV, a key role is played by fibrinogen that results positively related with SUA in the whole group of SCA subjects and with SUA/Cr ratio in the subgroup with high TyG.

Another observation that has been an important aim of this study is the relationship between SUA and SUA/Cr ratio with EA. We have already underlined the role carried out by SUA and monosodium urate on the erythrocyte zeta potential and how the changes of this potential modify the erythrocyte membrane stability inducing an erythrocyte aggregability. The data of this study point out how SUA and SUA/Cr ratio influence the EA in the whole group of subjects and in the different subgroups. In the last years some authors [18, 19] have highlighted this link can be observed also when the value of SUA is normal. At the same time, it is plausible retain that the employment of xantine oxidase inhibitors, but in general the urate-lowering therapy (ULT), may be protective relatively to the cardiovascular involvement, considering that the decrease of the SUA might reflect on the hemorheological profile. Up to now, several authors [44, 45] have investigated about ULT and cardiovascular safety in patients with gout, as well as have described the effects of ULT nearly the oxidative stress and the chronic inflammation that almost always accompany the clinical course of the gout. Considering that the normal values of SUA, and its cut-off, are not definitive, we must wonder about the likelihood that the cardiovascular protective role carried out by ULT in patients with gout as well as in subjects with hyperuricemia may also happen by the bettering of the hemorheological profile.

In conclusion, in SCA subjects the concentration of SUA, but not the SUA/Cr ratio, is influenced by the number of CRFs and by insulin resistance degree. In the whole group and the subgroups of SCA subjects, SUA and SUA/Cr ratio are related besides with WBV, with PV and EA. It is possible that urate-lower therapy reducing SUA might reflect on the hemorheological profile which role in the cardiovascular pathophysiology is well known.