Renal physiology revisited: amiloride

Diuretics and Electrolyte Disturbances in 1000 Consecutive Geriatric Admissions

Old people are commonly receiving diuretics on admission to hospital. Diuretics are recognized as a risk factor for electrolyte disturbances; controversy exists about the relative risks of different combinations (in particular, co-amilozide [Moduretic]). We recorded the drug history and serum electrolytes in 1000 consecutive admissions to a geriatric hospital, and examined the relative prescribing rates of various diuretics in the community.

Full results were obtained in 929 patients. A history of diuretic prescription was present in 353 (38%) of the patients; the mean serum sodium in this group (95% CI 136.0–137.1 mmol/1) was lower than in the 586 not prescribed diuretics (137.1–137.9 mmol/1). The difference was small but statistically significant (95% CI difference=0.3–1.6 mmol/1; P<0.01).

Hyponatraemia (serum sodium < 130 mmol/l) was not significantly commoner in the 41 patients prescribed co-amilozide than in patients prescribed other diuretics. In general patients prescribed potassium-retaining diuretics had a lower serum sodium than the others.

There was a significant positive correlation between the serum potassium and the log [serum urea) (r=0.26, P < 0.001) and a weak negative correlation existed between sodium and potassium (r= −0.14; P < 0.001). There was an association between the prescription of potassium-retaining diuretics and a higher serum potassium; also an association between the prescription of a loop or thiazide diuretic and a lower serum potassium. These interactions were shown by multiple regression analysis to be independent and additive.

Co-amilozide formed a significantly higher proportion of all diuretics prescribed in the community group than in the inpatient group (18% versus 12%; P < 0.05).

Hyponatraemia was mild and mainly associated with potassium-retaining diuretics in our patients. Our study was unable to confirm or refute any specific dangers of co-amilozide compared with other potassium-retaining diuretic combinations.

A study of plasma sodium levels in elderly people taking amiloride or triamterene in combination with hydrochlorothiazide

This study was performed to compare the effect of one month's treatment with hydrochlorothiazide (25 mg) in combination with either amiloride (2.5 mg) or triamterene (50 mg) on plasma sodium levels in elderly people in institutional care. Fifty residents of NHS nursing or social service residential care established on diuretics for congestive cardiac failure and aged 64 years or over were recruited. Forty-one patients were included in the final data analysis. Patients on hydrochlorothiazide/amiloride had a significantly lower plasma sodium (137 vs 139 mmol/l, 95% confidence interval for difference between medians 0-2 mmol/l) than those on hydrochlorthiazide/triamterene (P = 0.01). In equivalent potassium-retaining doses, amiloride is associated with significantly lower plasma sodium levels than triamterene, when given in combination with hydrochlorothiazide in elderly patients with congestive cardiac failure. This finding adds weight to uncontrolled observations implicating thiazide/amiloride diuretic combinations in causing serious hyponatraemia. This danger, although uncommon, should perhaps influence prescribing habits in an at-risk population.

Characterization of muscarinic receptors in rat kidney

Muscarinic receptors in mammalian kidney seem to be involved in diuresis. In this study we give a detailed characterization of receptors in rat kidney. Specific binding of [3H](-)-quinuclidinylbenzilate ([3H]QNB) to membranes of rat kidney cortex was saturable and of high affinity. A dissociation constant of 0.063 +/- 0.003 nM and a receptor density of 1.46 +/- 0.07 pmol/g wet weight were obtained. The dissociation kinetics could be best described by assuming a mono-exponential function (k-1 = (0.52 +/- 0.1) x 10(-4) s-1). The binding of [3H]QNB reached a maximum in 60 min at 0.6 nM at 37 degrees C. Competition experiments with the enantiomers of benzetimide confirmed the muscarinic nature of the [3H]QNB binding sites. The inhibition constants of pirenzepine (0.23 +/- 0.02 microM), (+-)-hexahydrosiladifenidol (0.040 +/- 0.002 microM), AF-DX 116 (1.45 +/- 0.07 microM), methoctramine (1.67 +/- 0.02 microM) and gallamine (78 +/- 3 microM) classified this receptor as an M3 receptor. Inhibition of [3H]QNB binding by the agonists methylfurtrethonium, arecoline, isoarecoline methiodide, arecaidine propargyl ester and McN-A-343 displayed monophasic inhibition curves. With (+/-)-cis-2-methyl-4-dimethylaminomethyl-1,3- dioxolane methiodide in two out of four experiments a small (11%) population of high affinity agonist sites could be detected. The potassium sparing diuretic amiloride inhibited [3H]QNB binding (36 +/- 3 microM). Although in a way related to the amiloride binding site, the muscarinic receptors in rat kidney are unlikely to be the primary target of diuretic action of this drug.

Amiloride prevents amphotericin B related hypokalaemia in neutropenic patients

Twenty neutropenic patients with various haematological disorders were randomised prospectively to receive either intravenous amphotericin B alone or amphotericin B and oral amiloride 5 mg twice a day for treatment of confirmed or suspected fungal infection. Patients receiving amiloride had a significantly higher plasma potassium (p less than 0.01), a significantly lower urinary potassium loss (p less than 0.01), and required significantly less potassium chloride supplementation to maintain their plasma potassium within the normal range (p less than 0.001). Amiloride was well tolerated, had no clinically important side effects, and provided effective control of plasma potassium in patients treated with amphotericin B.