Circulatory and excretory organs are morphologically and functionally closely related [1-5]. Combined heart and kidney dysfunction has often been noted by veterinary practitioners as a fundamentally new supra-nosological concept which is referred to as cardiorenal syndrome [6-9]. It should be mentioned that the term “cardiorenal syndrome” is defined as a secondary kidney disorder occurring in the setting of the underlying heart failure [9]. Renocardial syndrome with cardiac dysfunction occurring against the background of the chronic kidney disease has also been described in the literature [6]. At the current stage of veterinary science development, many aspects of clinical manifestation, pathophysiology, methods of early diagnosis and highly effective treatment of cardiorenal syndrome in animals remain understudied. Generally, clinical symptoms appear in the late stages of pathology development [3]. Therefore, working out effective ways of predicting the cardiorenal syndrome development in pedigree animals is of particular relevance.

Hypertrophic cardiomyopathy (HCM) in domestic cats is frequently reported by veterinary practitioners. The risk of cardiorenal complications may increase in the setting of decompensated left ventricular heart failure in cats with HCM [10-12]. It should be stated that the basic pathobiochemical factors underlying the development and progression of cardiorenal syndrome in cats with HCM are not described in the scientific literature. Obviously, there is a high clinical demand for conducting large-scale studies on improvement of diagnostic and therapeutic methods, as well as giving a pathogenetic rationale for the use of nephroprotectors and cardioprotectors in cats with HCM.

The aim of the work was to study the serum biochemical profile in cats with cardiorenal syndrome occurring in the setting of HCM.

The study was conducted in the Department of Veterinary Medicine of the Peoples’ Friendship University of Russia named after Patrice Lumumba and on the basis of veterinary clinics in Moscow and the Moscow Oblast. Physiologically healthy (n = 24, control group) cats with HCM without cardiorenal complications (n = 24, group I), and animals with HCM complicated by cardiorenal syndrome (n = 25, group II) were used in the experiment.

The HCM diagnosis was verified comprehensively. Arterial tonometry was performed using petMAP graphic II device (Cardio Command, USA) according to the standard method [13]. Echocardiography and dopplerography were performed using Mindray DP-60 equipment (China) [14]. Electrocardiographic tests were performed using MIDAS-EK1T-04 device (Russia) [15]. A PCE-90Vet haemanalyser (High Technology Inc., USA) [16] and a SpotChem EZ SP-4430 biochemical analyzer (ARKRAY Factory Inc., Japan) [17] were also used. The level of azotemia (serum creatinine ≥ 200 μmol/L) was considered a reliable criterion for the presence of cardiorenal syndrome.

Blood was collected from the forearm saphenous vein of cats on an empty stomach in the morning hours and placed into vacuum tubes containing blood coagulation activator. Sodium, potassium, ionised calcium, phosphorus, magnesium, glucose, urea, creatinine, C-reactive protein, total protein, albumin, as well as serum activity of aspartate transaminase, alanine transaminase, creatine phosphokinase, lactate dehydrogenase in feline sera were determined according to generally accepted methods. The concentrations of ketone bodies in blood were estimated using a FreeStyle Optium Xceed ketometer (Abbott Diabetes Care Ltd, UK) [18]. The level of symmetric dimethylarginine and cystatin C in serum was tested with enzyme-linked immunosorbent assay. The functional condition of cardiomyocyte membranes was assessed by serum concentration of troponin I [11]. The intensity of lipid peroxidation processes and antioxidant system in blood serum of cats with cardiorenal syndrome was assessed using test kits of Randox Laboratories Ltd. (UK) according to the manufacturer’s instructions on a spectrophotometer UN2CO-WFT2100 (China).

Mann – Whitney and Kruskal – Wallis methods were used to statistically analyze the obtained numerical data in STATISTICA 7.0 [10]. The median (Me) and interquartile range (IQ) values were determined.

The Kruskal – Wallis test performed for cats with feline cardiorenal syndrome showed high values of the H criterion and a high level of statistical significance in terms of the following biochemical parameters typical of such a phenomenon as cytolysis: serum activity of asparagine and alanine aminotransferases, lactate dehydrogenase, creatine phosphokinase, troponin I concentration (Table 1).

Thus, there was a statistically significant increase in serum activity of alanine aminotransferase (1.17-fold; p < 0.001), asparagine aminotransferase (1.95-fold; p < 0.001), lactate dehydrogenase (1.51-fold; p < 0.001), creatine phosphokinase (1.61-fold; p < 0.001) and troponin I concentration (4.00-fold; p < 0.001) in cats with non-complicated HCM forms (group I) as compared with healthy cats. In the sera of cats with HCM complicated by cardiorenal syndrome (group II), as compared with healthy cats, there was a statistically significant increase in activity of alanine aminotransferase (1.23-fold; p < 0.001), asparagine aminotransferase (2.83-fold; p < 0.001), lactate dehydrogenase (2.69-fold; p < 0.001), creatine phosphokinase (2.02-fold; p < 0.001) and troponin I concentration (5.20-fold; p < 0.001). It should also be added that a statistically significant increase in serum activity of aspartate aminotransferase (1.45-fold; p < 0.001), lactate dehydrogenase (1.79-fold; p < 0.001), creatine phosphokinase (1.26-fold; p < 0.01) and serum concentrations of troponin I (1.30-fold; p < 0.01) was found in cats with HCM complicated by cardiorenal syndrome, as compared with diseased cats without such a complication.

Statistically significant changes in serum concentrations of urea, creatinine, symmetric dimethylarginine and cystatin C in cats during the development of cardiorenal syndrome were observed using Kruskal – Wallis test (Table 2).

It has been revealed that a statistically significant increase in serum concentrations of urea (1.63 times; p < 0.001), creatinine (1.27 times; p < 0.001), symmetric dimethylarginine (1.40 times; p < 0.001) was registered in animals with uncomplicated forms of HCM (group I) as compared with healthy ones. There was a statistically significant increase in serum concentrations of urea (2.94-fold; p < 0.001), creatinine (2.72-fold; p < 0.001), symmetric dimethylarginine (2.60-fold; p < 0.001) and cystatin C (1.90-fold; p < 0.001) in cats with HCM complicated by cardiorenal syndrome (group II), as compared with healthy animals. A statistically significant increase in serum concentrations of urea (1.80-fold; p < 0.001), creatinine (2.15-fold; p < 0.001), symmetric dimethylarginine (1.86-fold; p < 0.001) and cystatin C (1.81-fold; p < 0.001) was established in cats with HCM complicated by cardiorenal syndrome, as compared with the diseased cats without such a complication.

Statistically significant changes in the concentrations of ketone bodies, cholesterol, total protein and albumin in the sera of cats with cardiorenal syndrome were detected using the Kruskal – Wallis test.

The data in Table 3 show that a statistically significant increase in serum glucose concentrations (1.13-fold; p < 0.05) was registered in cats with uncomplicated forms of HCM (group I) as compared with healthy cats. A statistically significant increase in the serum concentrations of ketone bodies (8.50-fold; p < 0.001), cholesterol (1.67-fold; p < 0.001), and a decrease in the concentrations of total protein (1.06-fold; p < 0.05) and albumin (1.10-fold; p < 0.05) were noted in the sera of cats with HCM complicated by cardiorenal syndrome (group II), as compared with healthy cats. A statistically significant increase in serum concentrations of ketone bodies (7.08-fold; p < 0.001), cholesterol (1.62-fold; p < 0.001) was found in cats with HCM complicated by cardiorenal syndrome, as compared with diseased cats without such a complication.

Statistically significant changes in pathobiochemical parameters indicating oxidative stress were observed in cats with cardiorenal syndrome, using the Kruskal – Wallis method.

The numerical data presented in Table 4 show that a statistically significant increase in serum concentrations of malondialdehyde (1.63-fold; p < 0.001), diene conjugates (1.49-fold; p < 0.001), and a decrease in activity of superoxide dismutase (1.53-fold; p < 0.001), catalase (2.33-fold; p < 0.001), glutathione peroxidase (1.23-fold; p < 0.001) were observed in animals with uncomplicated forms of HCM (group I), as compared with healthy cats. A statistically significant increase in the serum concentrations of malondialdehyde (1.79-fold; p < 0.001), ceruloplasmin (2.50-fold; p < 0.001), diene conjugates (1.85-fold; p < 0.001) and simultaneous decrease of activity of superoxide dismutase (1.63-fold; p < 0.001), catalase (4.67-fold; p < 0.001), glutathione peroxidase (1.71-fold; p < 0.001) were observed in sera of cats with HCM complicated with cardiorenal syndrome (group II), as compared with healthy animals. A statistically significant increase in the serum concentrations of malondialdehyde (1.10-fold; p < 0.001), ceruloplasmin (2.14-fold; p < 0.001), diene conjugates (1.25-fold; p < 0.001), a decrease in activity of catalase (2.00-fold; p < 0.001), glutathione peroxidase (1.35-fold; p < 0.01), an increase in activity of glutathione reductase (1.36-fold; p < 0.01) were established in cats with complicated cardiorenal syndrome, as compared with diseased cats without such a complication.

The Kruskal – Wallis test revealed statistically significant changes in biochemical parameters of electrolyte metabolism in cats with cardiorenal syndrome (Table 5).

Hyponatremia, hyperkalemia and hyperphosphatemia occurred in cats with uncomplicated forms of HCM as compared to healthy animals. A trend for development of hyponatremia, hyperkalemia, hyperphosphatemia and hypomagnesemia was observed in sera of cats with HCM complicated by cardiorenal syndrome, as compared to healthy ones. It was established that insignificant hypercalcaemia, as well as significant hyperphosphatemia and hypomagnesemia were found in cats with HCM complicated by cardiorenal syndrome, as compared with diseased cats without such a complication.

The test results showing changes in C-reactive protein concentration in cats during the development of cardiorenal syndrome are presented in the figure.

Reliable changes in C-reactive protein concentration in sera of cats in different experimental groups were established (H = 50.50; p < 0.001; Kruskal – Wallis test). A statistically significant increase in C-reactive protein concentration in sera (1.15-fold; p < 0.001) was registered in animals with uncomplicated forms of HCM as compared to healthy animals. A statistically significant increase in C-reactive protein serum concentration (1.55-fold; p < 0.001) was registered in sera of cats with HCM complicated by cardiorenal syndrome as compared with healthy ones. It was also revealed that there is a statistically significant increase in C-reactive protein serum concentration in cats with HCM complicated by cardiorenal syndrome, as compared with diseased cats without such a complication.

Our study showed that the serum aminotransferase and lactate dehydrogenase activity is increased in cats with HCM. A significant increase in serum troponin concentration in HCM animals is indicative of damage to the cardiomyocyte cell membranes. Similar changes were previously described in the literature [1][10][12].

Azotemia developed in cats with cardiorenal syndrome occurring in the setting of HCM. It is obvious that this pathological process initially develops as prerenal azotemia, against the background of chronic circulatory insufficiency and, as a consequence, poor renal perfusion. However, as the pathology progresses, there is likely to be additional damage and death of nephrons, constituting a renal component in the pathogenesis of azotemia. Renal excretory dysfunction aggravates neuroendocrine shifts in the organism of diseased cats, which is also manifested by electrolyte metabolism disturbance (hyperkalemia, hyponatremia, hypomagnesemia, hyperphosphatemia). Obviously, the pathophysiological mechanism for a marked increase in serum concentration of inorganic phosphorus in animals with cardiorenal syndrome is secondary hyperparathyroidism, which often complicates the course of renal failure [18]. Uraemic toxins accumulating in the body during cardiorenal syndrome have an additional damaging effect on both cardiomyocytes and nephrons, which leads to further progression of the pathological condition.

The detection of ketosis in cats with cardiorenal syndrome was unexpected. It is obvious that there are profound changes in metabolism of animals with cardiorenal syndrome, that are manifested in adipose tissue mobilisation, increased protein and lipid catabolism, energy deficit. In such cases ketogenic amino acids and triglycerides may be the source of synthesis of ketone bodies. This phenomenon requires further large-scale studies. In our case, hypercholesterolemia, moderate hypoproteinemia and hypoalbuminemia were detected in diseased cats.

Systemic inflammation is the key pathogenetic link in the formation of cardiorenal syndrome in cats with HCM. Proinflammatory cytokines, which are produced during the development of the inflammatory response, have an additional damaging effect on cardiomyocytes and nephrons. High C-reactive protein serum concentrations can be considered as a marker of inflammation in cats with cardiorenal syndrome.

It is obvious that activation of the neurohumoral system against the background of circulatory failure and increased myocardial tissue oxygen demand can initiate cardiac cell apoptosis and fibrosis in cardiorenal syndrome. In this direction, further studies are required for morphological verification of the above pathological processes. Metabolic disorder in cardiomyocytes causes the development of oxidative stress, which increases the alterative effect on the myocardium.

Hypertrophic cardiomyopathy results in chronic circulatory failure in cats and may be complicated by cardiorenal syndrome. The activity of aspartate aminotransferase, lactate dehydrogenase, creatine phosphokinase, glutathione reductase significantly increases, and the concentrations of cardiac troponin, urea, creatinine, symmetric dimethylarginine, cystatin C, ketone bodies, cholesterol, malondialdehyde, ceruloplasmin, diene conjugates, potassium, calcium, phosphorus, C-reactive protein increase, the activity of superoxide dismutase, catalase and glutathione peroxidase, as well as the concentrations of total protein and albumin, sodium and magnesium decrease in the sera of cats with cardiorenal syndrome. Cardiorenal complications in cats with feline hypertrophic cardiomyopathy are characterised by the following biochemical syndromes: azotemia, cardiomyocyte cytolysis, electrolyte imbalance, systemic inflammatory reaction, oxidative stress.