Abstract
Introduction
Hypertension is considered to be one of the major public health problems worldwide [1]. It is the most common cardiovascular disease in Black Africans and a major cause of morbidity and mortality among Nigerians [2]. Hypertension is one of the most prevalent non-communicable conditions worldwide; it is responsible for an estimated 45% of deaths due to heart disease and 51% of deaths due to stroke globally [3]. The onset of hypertension is caused by complex interaction between genetic predisposition and environmental factors [4]. Hypertension is associated with the incidence of arthrosclerosis [5]. Previous clinical and epidemiological studies have defined plasma lipoprotein levels such as high density lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol (LDL-C) and increased total cholesterol (TC) as strong predisposing factors of hypertension [6]. High salt intake is one of the most important environmental risk factor in the pathogenesis of hypertension, which directly causes damage to the vital systems such as central nervous system, renal system and vascular system [7]. Previous studies have shown that there are strong relationships between hypertension and cardiovascular disease [8], with approximately two thirds of stroke and one half of coronary heart disease cases worldwide [9].
It was reported that about 95% of cases of hypertension is of primary type [10]. This kind of hypertension has no identifiable aetiology, and traditionally has been viewed as a consequence of interaction between environmental factors such as sodium intake and genetic factors. Other factors include sedentary life style, visceral obesity, stress and potassium deficiency [11]. All are predisposing factors of essential hypertension characterized by increased peripheral vascular resistance to blood flow Epidemiological studies have shown that, incidence of primary hypertension increases with age, and individuals with relatively high blood pressures at younger ages are at increased risk for the development of primary hypertension [12]. It tends to cluster in families and represents a collection of genetically based diseases or syndromes with several resultants inherited biochemical abnormalities [13] and then may increase risk for cardiac, cerebral, and renal damages. Elevation of renin an enzyme secreted by the kidney, is also a risk factor [14].
The other form of hypertension (secondary hypertension), has an identifiable cause and affecting approximately 5–10% of hypertensive patients [15]. It results from the interplay of several pathophysiological mechanism regulating plasma volume, peripheral vascular resistance and cardiac output, all of which may be increased. Secondary causes include diabetes and Cushing’s syndrome (overproduction of cortisol) [16].
According to the World Health Organization (WHO), approximately 80% of the population in some African and Asian countries and 38% in the Americans depend primarily on complementary and alternative medicine for the prevention, protection and treatment of diseases [17].
Camel milk (CM) is gaining increasing recognition due to its beneficial effects in the control and prevention of multiple health problems [18]. Camel’s milk is an excellent source of well balanced nutrients and also exhibits a range of biological activities that influence digestion, metabolic responses to absorbed nutrients, growth and development of specific organs and resistance to diseases. These biological activities are mainly due to the presence of peptides and protein in milk [19]. Camel milk has been suggested in the management of various diseases [20]. Camel milk has medicinal properties including antibacterial and antiviral activity [21], which may be due to higher concentration of lactoferrin, immunoglobulins, lysozyme and vitamin C which are reported to play a crucial role in the determination of these properties [22]. The milk is used therapeutically against oedema, jaundice, problems of the spleen, tuberculosis, asthma, anaemia, piles and diabetes [23]. Some studies suggested that camel milk, besides being a good nutrient source; it is also an excellent source of components that are involved in some biological activities, one of which is defence against free radicals and reactive oxygen species [24]. It is characterised with low cholesterol, low sugar, high minerals (sodium, potassium, iron, copper, zinc and magnesium), high vitamins (A, B2, C and E) and large concentrations of insulin [25]. The reported health benefits of camel milk justified the great public concern and stimulated our interest to investigate its effects on the management of hypertension.
Materials and methods
Chemicals and reagents
All the chemicals and reagents used in this study were of analytical grade. Glucose oxidase kit (product of Randox), Insulin assay kit (product of SPI USA) and lipid profile assay kit (product of randox) were used in this work.
Experimental animals
Wistar albino rats of both sexes weighing between 170–220 g were used for the study. The animals were purchased and allowed to acclimatize to the environment for 7 days. All animals were housed in cages (8 rats/ cage), and fed with pelletized growers’ feed (Vital feed, Jos, Nigeria) and allowed access to water ad libitum throughout the experimental period.
Induction of hypertension in rats
The rats were placed on 8% w/w salt diet [26] for 6 weeks, except the control group, and then treated with camel milk for 4 weeks while sustaining salt-diet administration.
Measurement of blood pressure
The baseline blood pressure was measured by tail-cuff method using non-invasive Ugo Basile, series 58500 blood pressure recorder. The average of three readings was taken for each rat and the blood pressure of the rats were monitored throughout the experimental period. Subsequent measurements were done every week.
Collection of milk sample
The milk was collected by cameleer using hand milking from lactating camel (Camelus dromedarius), near Usmanu Danfodiyo University Second Gate at Kwalkwalawa Village, Wammakko Local Gov’t area of Sokoto State, Nigeria. It was collected in a sterile screw jar and kept in a cool container with ice block until transported to the laboratory where it was kept at temperature of –4°C. The pH of the milk was checked every day before administration, to ensure the freshness of the milk.
Grouping of animals
The animals were randomly divided into 4 groups of 8 rats each.
Group I: normal (control group)
Group II: salt-loaded, untreated
Group III: salt-loaded treated with Camel milk (5mls/kg/day) [25]
Group IV: salt-loaded, orally dosed with 100 mg/kg Metformin+10 mg/kg Nifedipine.
Preparation of serum
Twenty four hours after the last treatment and ten hours fasting, the animals were anaesthetised with chloroform vapour and blood samples were collected through cardiac puncture into labelled tubes. The samples collected were allowed to clot and centrifuged at 4000 g for ten minutes. Sera were separated and used in the biochemical analysis
Biochemical analysis
Insulin was estimated by SPI bio rat insulin enzyme immunoassay kit according to Grassi and Pradelles [27]. The test is based on the competition between unlabelled rat insulin and acetylcholinesterase linked to rat insulin (tracer) for limited specific guinea-pig anti-rat insulin antiserum sites. The complex guinea-pig antiserum-rat insulin binds to the goat anti-guinea-pig antibody that is attached to the well. The plate is then washed and Ellman’s reagent (enzymatic substrate for AChE and chromogen) is added to the wells and the AChE tracer acts on the Ellman’s reagent to form a yellow compound which is determined spectrophotometrically at 405 nm. The intensity of the colour is proportional to the amount of tracer bound to the well and is inversely proportional to the amount of free rat insulin present in the well during the immunological incubation.
Serum glucose was estimated by glucose oxidase method using Randox kit [28].
Insulin resistance index was calculated by Homeostasis Model Assessment- Insulin Resistance (HOMA-IR) as described by Matthews et al. [29]. The equation used is;
Serum total cholesterol (TC) was estimated by enzymatic method using Randox kit [30].
Serum HDL-C was estimated by enzymatic method of Burstein et al. [31] using Randox Kit. Serum Triglyceride was assayed by the method of Tietz [32], using Randox Kit.
Serum LDL-C was calculated using Friedewald formula [33]
Serum VLDL-C was calculated using Friedewald formula [33].
Data analysis
Data were expressed as mean ± standard deviation of 8 rats in each group. All the biochemical parameters were analysed statistically using one way analysis of variance (ANOVA), using Graph pad instat software (version 5 San Diego, USA). Results were considered statistically significant at p < 005. The data of blood pressure profile (mean ± standard error) was presented in figures.
Results
The effect of Camel milk supplementation on systolic blood pressure (SBP) and pulse rate (PR) are presented in Figs. 1 and 2 respectively. The result indicated that six weeks administration of 8% w/w salt diet increased the SBP and PR. The elevated SBP and PR were significantly decreased in the group administered with 100 mg/kg Metformin+10 mg/kg Nifedipine, and the group supplemented with camel milk. On the other hand, the salt-loaded untreated group showed a progressive increase of SBP and PR throughout the experimental period.
The effect of camel milk supplementation on serum glucose, insulin and HOMA-IR (which defined insulin resistance) in rats with hypertension is presented in Table 1. The result showed statistically significant decrease (P < 0.05) in serum glucose, insulin levels and HOMA-IR of the camel milk supplemented group and drug treated group as compared with salt-loaded untreated group. On the other hand, there was no significant difference (P > 0.05) between camel milk supplemented group, drug treated group and control; in all the parameters analysed.
The results of the lipid profile of salt-induced hypertensive rats were presented in Table 2. Statistical analysis of the results revealed significant decrease (P < 0.05) in TC, TAG, LDL-C and VLDL-C of the group supplemented with camel milk as compared with salt-induced untreated group. On the other hand, serum level of HDL-C of Camel milk supplemented group increased significantly (P < 0.05) when compared with salt-induced untreated group. The results also showed strong similarities (P > 0.05) in TC, TAG, HDL-C, LDL-C and VLDL-C between the group supplemented with camel and control group, so also no statistical significant different (P > 0.05) is observed between the group supplemented with Camel milk and the group treated with 100 mg/kg Metformin+10 mg/kg Nifedipine.
Discussion
The present study revealed that the 8% salt-diet used; does not only induce hypertension in Wistar albino rats but also associated with insulin resistance. This finding is consistent with the report of Takehide et al. [34]. It was suggested that salt-induced insulin resistance might be attributable to the overproduction of ROS [35]. Our finding confirmed the reports that salt loading to various strains of rats such as Sprague-Dawley rats [36], Wistar rats [37] and Dahl salt-sensitive rats [38], could result to increased mean arterial blood pressure, inhibition of insulin signalling and induces insulin resistance. Dyslipidaemia, a strong predictor of cardiovascular disease, results from increased production of ROS induced by high salt diet. It causes endothelial damage and the loss of physiological vasomotor activity that results from endothelial damage may manifest as increased blood pressure (BP). Therefore, factors like dyslipidaemia that cause endothelial dysfunction may lead to hypertension. Cross-sectional studies have suggested a link between abnormal lipids and hypertension. A few studies have prospectively examined the relationship between plasma lipids and the future development of hypertension [39]. Several studies have indicated a strong relationship between hypertension, dyslipidaemia, insulin resistance and hyperglycaemia [40], all of which are consequences of over production of reactive oxygen species.
Our study also revealed that the high blood pressure observed in the experimental rats due to the administration of 8% salt-diet, was reversed by the camel milk supplementation. The blood pressure lowering effects of camel milk could be attributed to the presence of ACE-inhibitory peptides precursors in the milk. The amino acid sequence was identified as Ala-Ile-Pro-Pro-Lys-Lys-Asn-Gln-Asp [41]. Angiotensin I-converting enzyme (ACE) is a dipeptidyl carboxypeptidase associated with the regulation of blood pressure. ACE inhibition results in a lowering of blood pressure [41]. Al Haj et al. [42], identified ACE-inhibitory precursor peptides from fermented camel milk, and suggested the presence of Tyr, Arg and Pro at their ultimate C-terminal position, making them a possible candidate for ACE- inhibitory activity. These amino acids within the peptides were previously found to have contributed substantially to ACE-inhibitory potency [43]. Other possible mechanism could be due to the presence of antioxidant vitamins and minerals in the milk thereby exhibiting antioxidant activity either by serving as nucleophillic species or scavenge superoxide anions
Insulin resistance and hyperinsulinaemia are good predictors of metabolic syndrome [44]. Our findings indicated a statistically significant (P < 0.05) decrease in insulin levels and insulin resistance in camel milk supplemented as compared with untreated group. Decreased serum insulin and fasting blood glucose levels observed in camel milk supplemented group, reflected increased peripheral response to insulin, as confirmed by the decreased HOMA-IR when compared to salt-induced untreated group. The high concentration of insulin (40 units/l) and insulin-like protein and the immunoglobulin contents of camel milk [45]. Wangoha et al. [46], identified it as a natural product that not only helps glycemic control but also restore the insulin sensitivity, as evidenced in the camel milk-supplemented animals in the present study.
In addition, salt-loaded rats had elevated levels of plasma total cholesterol (TC), triglycerides (TG), low density lipoprotein-C (LDL-C), very low density lipoprotein-C (LDL-C), and significant decreased (P < 0.05) in high density lipoprotein cholesterol (HDL-C). This could be attributed to increase concentration of sodium in circulation which in turn activates sympathetic nervous system [47] as well as increased signalling through the mineral corticoid receptors (MR) [48]. These may lead to increase production of reactive oxygen species which result to oxidative stress, and finally contribute to aetiopathology of dyslipidaemia [49]. Treatment with camel milk restored the above mentioned changes and improved them towards normal levels in the experimental rats. The obvious amelioration of the hyperlipidemia and dyslipidaemia in the camel milk supplemented group is in agreement with previous reports which stated that fresh and fermented camel milk contains Bifidobacteria, which lower plasma lipids in rats administered with a high-cholesterol diet [50, 51]. The hypolipidemic effect of camel milk could be due to its high content of L-carnitine, which decreases cholesterol absorption [52, 53]. Another possible mechanism is that camel milk may alter the PPAR alpha/SREBP1 ratio, as mentioned in the work conducted by Ziamajidi, et al. [54], which led to enhanced activity of the fat-metabolizing enzymes and hormones, resulting in increased caloric loss or decreased fat storage.
Conclusion
The present study confirmed the health promoting effect of camel milk as evident of normalizing the elevated blood pressure and improvement of insulin sensitivity in salt–induced hypertensive rats.