Screening Methods for Diuretic Agents

SCREENING METHODS FOR DIURETIC AGENTS

Drugs that induce diuresis (enhances urine outflow) are known as diuretics. Many herbal plants like cantaloupe (Cucitmis melo), Dolichos biflorus (virus), radish (Raphanus satlvus), kanguni (Satania italica), Oriental sweet gum (Liq-uidamber orientalis), and kapok tree (Ceibia pentandra) possess diuretic activity. Extracts from these drugs are used in various diseases like hypertension, congestive heart failure, oedema, nephrolithiasis, and urolithiasis. The diuretic activity of these drugs can be evaluated by the following methods.

Diuretic Activity in Rats (Lipschitz Test)

This test is based on the principle that water and sodium excretion in test animals is different as compared to rats treated with a high dose of urea. The ‘Lipschitz value’ is the quotient between excretion by test animals and excre-tion by the urea control.

Adult albino rats weighing 100–200 g are used for the study. Six animals per group are placed in metabolic cages individually provided with a wire mesh bottom and a funnel to collect the urine. Stainless steel sieves are placed in the funnel to retain faeces and allow the urine to pass. The rats are fed with standard diet and water ad libitum. Food and water are withdrawn 17 to 24 h before the experiment. The test compound is administered orally. The other group is treated with urea (1 g/kg) orally. Additionally 5 ml of 0.9% NaCl solution per 100 g of body weight are given by gavage. Urine excretion is recorded after 5 and 24 h. The sodium content of the urine is determined by flame photometer.

Urine volume excreted per 100 g body weight is calcu-lated for each animal, in the group. Results are expressed as the ‘Lipschitz value’, that is, the ratio T/U in which T is the response of the test compound and U that of urea treatment. The value of 1.0 and more are regarded as a positive effect. Potent diuretics having a Lipschitz value of 2.0 and more have been found.

Chronic Renal Failure in Rats

Chronic renal failure is a frequent pathological condition in man. The following assay is used for special pharma-cological studies as well as evaluation of renal toxicity of new chemicals.

Albino rats weighing between 150 and 200 g are used for the study. Rats are anaesthetized by i.m. injection of ketamine (40 mg/kg) and droperidol (0.25 mg/kg). An incision is made in the abdominal wall, and the small bowel and caecum are lifted and placed on saline-soaked sponges. The right kidney is exposed and dissected from the retroperitoneal area, the vascular and ureteric pedicles are ligated with silk sutures, and the kidney removed. The renal artery of the left kidney is dissected into the hilum to expose the three main segmental renal arteries. The kidney is not dissected out of the peritoneum. The anterior caudal branch of the artery is then temporarily ligated to establish the volume of renal tissue supplied. The area of ischemia becomes demarcated within 10–15 s. If this approximates 1 /4 to ⅓ of the kidney, a permanent ligature is placed. The viscera are then carefully replaced in the abdomen and peritoneum and linea alba is closed with a continuous suture. The skin is closed with stainless steel clips.

Blood for serum creatinine is collected by retro orbital puncture under anaesthesia at various time intervals up to 12 months. In association with this, urine is collected every 24 h for the measurement of creatinine, protein, and specific gravity.

Compounds which posses diuretic activity will increase the volume, accompanied by decrease in urine specific gravity which indicates the decrease in concentrating ability of the kidney. Proteinuria is also significantly increased. Terminal uremia occurs after 14–15 months.

Diuretic and Saluretic Activity in Dogs/Cat

Renal physiology of the dog is claimed to be closer to man than that of rats. So dogs have been extensively used to study renal physiology and the action of diuretics. Using catheters, periodic collection of urine can be made with more reliability than in rats.

Dogs or cats are anaesthetized using sodium pentobarbitone 35–50 mg/kg. The lower abdomen is opened. The femoral vein is exposed and cannulated with a suitable venous cannula. The venous cannula is used for the administration of test drugs and saline. The control group receives only water. The standard group receives 1 g/kg urea or 5 mg/kg furosamide per day. The test groups receive different test drugs. The urinary bladder is catheterized through the urethra and connected to a measuring cylinder. The urine is collected, the volume measured and analysed for Na⁺/K^-ions using flame photometry. Chloride ions are estimated using argentometry. The urine volume and electrolyte concentration of test compounds are compared with the control group to determine the diuretic activity.