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2 By Hussam A.S. Murad and Khaled A. Mahmoud Department of Pharmacology and Therapeutics Faculty of Medicine, Ain Shams University

3 Several studies have found that hyperuricemia is an independent risk factor for CV diseases after controlling other established risk factors. Also 25%-50% of hypertensive patients are hyperuricemic. Hyperuricemia observed in CV patients is mild in contrast to marked hyperuricemia observed after chemotherapy. Losartan (an AT II receptor blocker) and hydrochlorothiazide (a thiazide diuretic) are commonly used in treatment of hypertension and other CV diseases. Losartan increases urinary uric acid excretion reducing its serum level while hydrochlorothiazide causes hyperuricaemia as an adverse effect. Several studies have found that hyperuricemia is an independent risk factor for CV diseases after controlling other established risk factors. Also 25%-50% of hypertensive patients are hyperuricemic. Hyperuricemia observed in CV patients is mild in contrast to marked hyperuricemia observed after chemotherapy. Losartan (an AT II receptor blocker) and hydrochlorothiazide (a thiazide diuretic) are commonly used in treatment of hypertension and other CV diseases. Losartan increases urinary uric acid excretion reducing its serum level while hydrochlorothiazide causes hyperuricaemia as an adverse effect. The present work was conducted to study the effects of losartan, hydrochlorothiazide and a combination of both on serum uric acid level and blood pressure in a model of mild hyperuricemia and hypertension in rats.

4 (A) Induction of mild hyperuricemia and hypertension in rats: The rats were given a low-salt (0.125% NaCl) and 2% oxonic acid diet for 7 weeks. Oxonic acid is an inhibitor of the hepatic uricase enzyme causing decreased metabolism and elevation of serum uric acid. Before and after the 7 weeks, serum uric acid and blood pressure were measured. Rats with serum uric acid level more than 2.5 mg/dl and blood pressure more than 140 mmHg were considered hyperuricemic and hypertensive. (B) Effects of losartan, hydrochlorothiazide and a combination of both drugs on serum uric acid level and blood pressure in hyperuricemic hypertensive rats : 4 groups ( 6 /group) were used and received the following by gastric gavage for 4 weeks:   Group I (Control group): 0.5 ml D.W.   Group II (Losartan group): Losartan10 mg /kg/ day.   Group III (Hydrochlorothiazide group): Hydrochlorothiazide 3 mg /kg/ day   Group IV (Losartan and Hydrochlorothiazide group): losartan 10 mg /kg/ day and hydrochlorothiazide 3 mg/kg/day. (A) Induction of mild hyperuricemia and hypertension in rats: The rats were given a low-salt (0.125% NaCl) and 2% oxonic acid diet for 7 weeks. Oxonic acid is an inhibitor of the hepatic uricase enzyme causing decreased metabolism and elevation of serum uric acid. Before and after the 7 weeks, serum uric acid and blood pressure were measured. Rats with serum uric acid level more than 2.5 mg/dl and blood pressure more than 140 mmHg were considered hyperuricemic and hypertensive. (B) Effects of losartan, hydrochlorothiazide and a combination of both drugs on serum uric acid level and blood pressure in hyperuricemic hypertensive rats : 4 groups ( 6 /group) were used and received the following by gastric gavage for 4 weeks:   Group I (Control group): 0.5 ml D.W.   Group II (Losartan group): Losartan10 mg /kg/ day.   Group III (Hydrochlorothiazide group): Hydrochlorothiazide 3 mg /kg/ day   Group IV (Losartan and Hydrochlorothiazide group): losartan 10 mg /kg/ day and hydrochlorothiazide 3 mg/kg/day.

5 After 4 weeks, rats were subjected to : (I) Measurement of serum uric acid level : By the enzymatic method. (II) Measurement of the blood pressure : By Harvard indirect tail- cuff method using a pressure transducer and a dual channel chart recorder (Rikadenki, model R-52). By correlation between the point at which the pulse amplitude increased abruptly and the cuff pressure curve, the systolic blood pressure was calculated. (I) Measurement of serum uric acid level : By the enzymatic method. (II) Measurement of the blood pressure : By Harvard indirect tail- cuff method using a pressure transducer and a dual channel chart recorder (Rikadenki, model R-52). By correlation between the point at which the pulse amplitude increased abruptly and the cuff pressure curve, the systolic blood pressure was calculated. mmHg

6 (I)Effects of losartan, hydrochlorothiazide and a combination of both drugs on serum uric acid level in hyperuricemic hypertensive rats : Losartan significantly reduced serum uric acid level while hydrochlorothiazide increased it compared with the control group. The combination of both drugs produced significant reductions compared with the control and hydrochlorothiazide groups while produced a non- significant difference compared with the losartan group. (I)Effects of losartan, hydrochlorothiazide and a combination of both drugs on serum uric acid level in hyperuricemic hypertensive rats : Losartan significantly reduced serum uric acid level while hydrochlorothiazide increased it compared with the control group. The combination of both drugs produced significant reductions compared with the control and hydrochlorothiazide groups while produced a non- significant difference compared with the losartan group.

7 (II)Effects of losartan, hydrochlorothiazide and a combination of both drugs on blood pressure in hyperuricemic hypertensive rats : Losartan, hydrochlorothiazide and the combination of both drugs produced significant reductions compared with the control group. The combination of losartan and hydrochlorothiazide produced significant reductions compared with losartan and hydrochlorothiazide groups. There was a non-significant difference between losartan and hydrochlorothiazide groups. (II)Effects of losartan, hydrochlorothiazide and a combination of both drugs on blood pressure in hyperuricemic hypertensive rats : Losartan, hydrochlorothiazide and the combination of both drugs produced significant reductions compared with the control group. The combination of losartan and hydrochlorothiazide produced significant reductions compared with losartan and hydrochlorothiazide groups. There was a non-significant difference between losartan and hydrochlorothiazide groups.

8 In the atherosclerotic prooxidative environment, the original antioxidant properties of uric acid paradoxically becomes prooxidant, contributing to the oxidation of lipoproteins within atherosclerotic plaques. Consequently, elevations of uric acid more than 4 mg/dl should be considered a "red flag" in patients at risk for CV disease and should be treated to decrease complications. Losartan and its active metabolite inhibit urate uptake by the urate anion transporter (URAT1) reducing urate reabsorption from the proximal tubule in a way similar to that of probenecid. In addition, losartan raises urine pH decreasing the risk of uric acid stone associated with increased uric acid excretion. This interesting property is not shared by other uricosuric drugs. The uricosuric effect of losartan does not vary with sodium diet and is molecule-specific (not shared by other angiotensin II receptor antagonists). In the atherosclerotic prooxidative environment, the original antioxidant properties of uric acid paradoxically becomes prooxidant, contributing to the oxidation of lipoproteins within atherosclerotic plaques. Consequently, elevations of uric acid more than 4 mg/dl should be considered a "red flag" in patients at risk for CV disease and should be treated to decrease complications. Losartan and its active metabolite inhibit urate uptake by the urate anion transporter (URAT1) reducing urate reabsorption from the proximal tubule in a way similar to that of probenecid. In addition, losartan raises urine pH decreasing the risk of uric acid stone associated with increased uric acid excretion. This interesting property is not shared by other uricosuric drugs. The uricosuric effect of losartan does not vary with sodium diet and is molecule-specific (not shared by other angiotensin II receptor antagonists).

9 The uricosuric effect of losartan is clinically important because losartan may obviate the need for specific treatment for hyperuricemia (e.g. allopurinol and uricosuric drugs). Furthermore, because hyperuricaemic patients may already be on several drugs (e.g. due to associated hypertension or hyperlipidemia), compliance may be improved by avoiding additional medication and adverse effects associated with multiple drugs would be decreased. Losartan has a hypouricemic effect that decreases the elevated uric acid level caused by hydrochlorothiazide, in addition to potentiation of its antihypertensive effect. This favors the choice of losartan to be combined with hydrochlorothiazide in treatment of hypertension associated with hyperuricaemia. More studies are recommended to clarify whether use of losartan is only a mere adjuvant to or it may obviate the need for specific treatment for hyperuricemia.

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