THE EFFECT OF BENDROFLUMETHIAZIDE ON THE RENAL HANDLING OF CALCIUM, MAGNESIUM AND PHOSPHATE IN NORMOCALCAEMIC RENAL STONE FORMERS AND IN HYPERPARATHYROIDISM

The evidence-based use of thiazide diuretics in hypertension and nephrolithiasis

Thiazide-type diuretics are commonly used in the treatment of hypertension and nephrolithiasis. Evidence from randomized clinical trials needs to be considered in decisions about agent choice and dose. In nephrolithiasis, one of the major limitations of the literature is a paucity of data on the dose-response effect of hydrochlorothiazide (HCTZ) on urinary calcium excretion. The best available evidence for prevention of stone recurrence suggests the use of indapamide at 2.5 mg/d, chlorthalidone at 25 to 50 mg daily, or HCTZ 25 mg twice a day or 50 mg daily. In hypertension, chlorthalidone (12.5 to 30 mg daily) may be the best choice when a diuretic is used for initial therapy, with indapamide (1.5 mg daily) being a valuable alternative for older patients. When adding a thiazide to other drug classes, indapamide (2.5 mg daily) has demonstrated value in hypertensive patients who have had a stroke, and HCTZ (12.5 to 25 mg daily) has a safe track record in several patient groups. Although chlorthalidone has not been tested as add-on therapy, the authors believe it is a safe option in such cases.

The effects of thiazide diuretics on calcium metabolism in the aged

OBJECTIVE

To determine the effects of long-term use of the thiazide diuretic chlorthalidone on serum bone-related biochemical variables in older persons.

DESIGN

Cross-sectional comparison.

SETTING

Community-dwelling subjects who had participated in the Systolic Hypertension in the Elderly (SHEP) program.

PARTICIPANTS

Sixty-six Caucasians (36 male and 30 female), age range 70 to 89 years, of whom 23 were taking a thiazide diuretic.

MEASUREMENTS

25-hydroxyvitamin D (25OHD), parathyroid hormone, osteocalcin, calcitonin, and serum bioavailable testosterone.

RESULTS

In both groups, there was a high prevalence of low 25 (OHD) levels (30%). Log 25OHD showed a significant inverse relationship to parathyroid hormone (r = .33, P < 0.05). Thiazide users had lower levels of osteocalcin (P < 0.05) and parathyroid hormone levels (P < 0.05) compared with non-thiazide users. Male thiazide users had decreased bioavailable testosterone levels compared with non-thiazide users (P < 0.05). Serum osteocalcin was significantly related to bioavailable testosterone in men not on thiazide (r = .43, P < 0.05).

CONCLUSIONS

Hypovitaminosis D is a common finding in older individuals with associated elevations in parathyroid hormone. Parathyroid hormone and testosterone concentration (in men) are correlated with serum osteocalcin, a measure of osteoblastic activity. Long-term thiazide use alters these relationships and produces a biochemical profile suggestive of decreased bone formation. Reduced bioavailable testosterone may also play a role in these biochemical changes.

Do diuretics cause magnesium deficiency?

1. Controlled trials, of which there are few, do not substantiate claims that diuretics play a role in causing magnesium deficiency. Consequently, the vast majority of patients taking conventional doses of thiazide diuretics (i.e. bendrofluazide 2.5 mg day-1 or equivalent) do not need magnesium supplements. On balance, potassium-sparing diuretics tend to increase serum and intracellular magnesium content; this should not be taken as evidence of prior magnesium deficiency. It remains theoretically possible that large doses of loop diuretics given more than once daily for long periods could induce negative magnesium balance and magnesium deficiency. However, it has been difficult to run appropriately controlled trials in conditions where such therapy is needed (i.e. heart failure) and until more reliable information becomes available no absolute recommendation can be made. 2. Methods for the measurement of intracellular free magnesium levels are now available and are more relevant to the assessment of magnesium deficiency than total intracellular magnesium content; the complex relationship between intracellular free and total magnesium content remains to be defined. Future work involving the effect of diuretics on intracellular free magnesium measurements should make every attempt to avoid the errors of trial design and multiple publication that litter current and past literature.

Plasma calcium difference between man and vertebrates

1. Literature data about the plasma content of total calcium, ionized calcium and inorganic phosphate in healthy animals and man of different age and sex were collected. 2. It was established that under normal conditions ionized calcium is about 45% of total calcium. 3. The degree of saturation of these blood samples with respect to octocalcium phosphate OCP was calculated. 4. In young animals and man the blood plasma has a higher degree of saturation than in adult animals and man. 5. The blood plasma of healthy animals is supersaturated with respect to OCP during their whole life. 6. However, the blood plasma of healthy human adults is slightly undersaturated with respect to OCP.

Magnesuria induced by thiazides and the influence of triamterene

The effects on magnesium excretion of 4 short-term diuretic treatments (methyclothiazide 2 mg either alone or associated with increasing doses of triamterene) were evaluated in 8 normal volunteers and compared to spontaneous variations during placebo administration. The thiazide exerted a small but significant magnesuric effect, which was prevented only by the lowest dose (25 mg) of triamterene. Larger doses had no protective effect on thiazide-induced magnesuria. Independently of their absolute effects on magnesium excretion, all diuretics impaired the normal ability of the kidneys to compensate fully for the expected changes in magnesium reabsorption induced by extracellular volume contraction.

Biochemical effects in patients with calcium oxalate stone disease during combined treatment with bendroflumethiazide and magnesium oxide

Thirteen patients with calcium oxalate stone disease were treated with 2.5 mg bendroflumethiazide and 200 mg magnesium oxide twice daily for 1 year, and the effects on serum and urine constituents were evaluated. Serum magnesium and potassium decreased, whereas urate increased during treatment. The association of decreased urinary calcium and increased urinary magnesium resulted in a reduced Ca/Mg quotient and apparently a lower risk of forming urine supersaturated with calcium oxalate. Combined treatment with a thiazide and magnesium appears to alter the composition of urine in a way that might be more favourable than treatment with either substance alone.

Metabolic effects of thiazide versus placebo in patients under long-term treatment for recurrent urolithiasis

Thiazide induced metabolic effects reflected in serum and 24 h urine excretions were investigated in general practice in 48 recurrent urinary calcium stone formers on prophylaxis with 50 mg hydrochlorthiazide or placebo (average duration of treatment 3 years). After 10 months of treatment, there was a significantly greater decrease in urinary calcium excretion among patients receiving thiazide than in patients given placebo (p less than 0.05). There was a significant increase in urinary magnesium in the thiazide group (p = 0.02), while there was no significant alteration in urinary uric acid. An increase was observed in the 24 h urine volume of 400 ml in the placebo group in contrast to a decrease in the thiazide group. The effects on serum calcium, potassium and uric acid were in accordance with previous reports. Few and only mild side effects were observed except for one attack of gout. The necessity of having adequate control groups in such studies is emphasized.

Metabolic effects of bendroflumethiazide in patients with recurrent calcium oxalate stone disease

Two groups of patients with urolithiasis were treated with 2.5 mg. (group A) and 5.0 mg. (group B) bendroflumethiazide daily. There were 14 men and 3 women in group A, and 14 men and 2 women in group B in whom metabolic effects were followed during 1 year of treatment. Serum calcium was significantly increased in group B after 1 month but later returned to the pretreatment level. A significant decrease in serum magnesium was recorded in group B after 6 and 10 months. No significant effect on serum calcium or magnesium was observed in group A. Serum potassium decreased in both groups but serum urate remained at the pre-treatment level. An increased alkalinity was noted in both groups. Urinary calcium was decreased significantly only in group B. Although significantly increased excretion of magnesium was observed after 1 and 6 months in group A this was not encountered in group B, and after 12 months urinary magnesium was at the pre-treatment level in both groups. Urinary excretion of oxalate, urate and citrate appeared to be unaffected by the treatment. The inhibition of calcium oxalate crystal growth and urine volume did not change. The calcium/magnesium quotient decreased in both groups as did the calcium times oxalate/ magnesium quotient. The main metabolic effect of bendroflumethiazide, with respect to its stone prophylactic property, appears to be a decrease in the calcium/magnesium quotient and a dose of 5 mg. per day probably is more satisfactory than a 2.5 mg. dose.

Prevention of calcium stones with thiazides

On the basis of almost 15 years of experience with thiazide treatment in 346 patients with calcium stones, we believe that the following conclusions are justified: 1) Stone progression ceases in at least 90% of patients who take hydrochlorothiazide (50 mg, twice daily) on a regular basis. 2) A reduced dose of hydrochlorothiazide, i.e., 25 mg twice daily, appears to be effective in a significant proportion of patients. 3) Thiazides are effective in normocalciuric as well as hypercalciuric patients and in most patients with tubular ectasia (medullary sponge kidney). 4. Side effects necessitate discontinuation of thiazide treatment in approximately 7% of patients. The incidence and severity of side effects is reduced by initiating treatment with a small dose and by increasing the dose progressively until the full maintenance dose is achieved. A trial with a reduced dose is warranted in patients who are unable to tolerate the regular maintenance dose. 5) The therapeutic efficacy of thiazides in stone prevention cannot be accurately predicted by the degree of hypocalciuric response. Stone prevention may cease despite a minimal hypocalciuric response, whereas stone progression may occur when an adequate hypocalciuric response has taken place. 6) In addition to the hypocalciuric action, thiazides reduce urine oxalate excretion and increase urine zinc and (probably) magnesium; these effects probably contribute to the efficacy of this agent in stone prevention.