Amiloride and calciuria

The glomerulus filters the ionized and complexed forms of calcium, that is approximately 60% of the total plasma concentration. Two-thirds of the filtered load is reabsorbed along the proximal tubule via paracellular pathways composed of tight-junction proteins, claudin-2 and claudin-12, a process accompanying sodium and water reabsorption. The thick ascending limb of Henle’s loop accounts for much of the remainder, also by paracellular pathways involving claudin-16 and claudin-19. The calcium-sensing receptor plays an important role at this site, since its activity increases the expression of claudin-14, a protein that blocks calcium reabsorption. Next, ‘fine tuning’ comes about in the distal convoluted tubule and connecting tubule by a transcellular pathway composed of the transient receptor potential apical calcium channel, TRPV5, the calcium shuttle protein, calbindin-_D28K and the basolateral proteins, namely the plasma membrane calcium ATPase (PMCA1B), and the sodium–calcium exchanger [1]. Further downstream in the collecting duct, TRPC3 may have a role in calcium reabsorption in response to vasopressin [2]. Understanding the molecular pathways conferring renal calcium reabsorption is important to explain how dietary and pharmacological interventions could influence urinary calcium excretion.

Kidney stones are common worldwide and are associated with alterations in urinary solute composition including hypercalciuria. The genetics of renal stone disease has enabled uncovering specific molecular mechanisms mentioned above and genome-wide association studies have uncovered unexpected polygenic factors that should increase our understanding of stone pathogenesis [3]. Also highly relevant is the effect of vitamin D supplementation on urinary calcium excretion. The wonders of vitamin D for the immune system are highly touted, particularly during the coronavirus disease 2019 epidemic [4], and the elderly, who commonly have reduced vitamin D levels, have an increased exposure from prescriptions that may increase the risk for stone formation [5].

Pharmacological calcium stone prevention is a highly desirable goal but has oftentimes proved inadequate. Thiazide diuretics reduce the risk of recurrent renal calculi; however, their efficacy remains contested in spite of research spanning many decades. A recent meta-analysis concluded that thiazide diuretics reduce the incidence of recurrent renal calculi and 24-h urinary calcium level. However, the benefits are marginal, the evidence quality is low and considering potential adverse effects, poor patient compliance and economic burden of long-term medication, their use in preventing recurrent kidney calculi is not generally recommended [6]. Alternative or additional approaches to the hypercalciuria dilemma would be most welcome.

Although a reduction in urinary calcium excretion has been attributed to thiazide diuretics for many years, the mechanism is not straightforward [7]. As mentioned above, calcium and magnesium resorption along the renal tubule depends upon distinct transcellular and paracellular pathways that involve a diverse protein array. The known pharmacological effects of thiazide diuretics directly involve sodium reabsorption, namely the sodium–chloride cotransporter (NCC) and thus, the effects on calcium are indirect and involve establishing a prerequisite electrochemical gradient. Thus, their effects on divalent cation handling are variable. Acutely, thiazides cause a robust decrease in urinary calcium excretion. Increased proximal tubular calcium reabsorption is implicated in parallel to volume contraction. Gitelman syndrome is a model of diminished NCC function. In an experimental setting, Gitelman patients were given isotonic saline, 1 L/h for 3 h [8]. Even with volume expansion, the urinary calcium excretion remained low, implying that hypovolaemia is not the sole cause of hypocalciuria in Gitelman patients. Presumably mechanisms directly related to NCC are involved. Furthermore, NCC expression is calcium dependent and can be modulated by parvalbumin expression [9]. Belge et al. [9] showed that parvalbumin regulates NCC by modulating intracellular calcium. These results could also provide insight into the calcium-sparing action of thiazide diuretics.

Potassium sparing diuretics also alter divalent cation transport. Administration of triamterene to normal subjects initially decreased calcium excretion [10]. Spironolactone also seems to reduce urinary calcium excretion [11]. Costanzo [12] observed a reduced fractional calcium excretion after amiloride administration. By using micropuncture, addition of amiloride to the perfusate augmented reabsorption of calcium from the late portion of the distal convolution, yet both fluid and sodium reabsorption were inhibited. The relevance of these findings to effects in humans given these potassium sparing diuretics long-term is unclear. However, a report in this issue of Nephrology Dialysis Transplantation may shed new light on this issue [13].

The authors report on a study of 48 healthy young men ingesting a relatively high-sodium, low-potassium diet (250/40 mmol/day) for a week, to suppress aldosterone secretion. These subjects were then given amiloride 20 mg orally, while urine was collected and lithium clearance was conducted. Urinary calcium decreased precipitously by about 80% in every subject (mean 7 µmol/min to about 1 µmol/min at 3 h, maintained at 1 µmol/L at 6 h). The effects on urinary sodium and potassium were much less impressive. The lithium clearance data support the interpretation that the effects were proximal in nature. In the previously published data from this study [14], arginine vasopressin (dDAVP) (4-μg intravenous bolus followed by 4 μg over 2 h) was administrated before and after a 7-day administration of 20 mg/day amiloride. Urine and blood samples were collected before and at the end of the dDAVP infusion, to measure sodium, potassium, creatinine and osmolality concentrations. In this paper, the authors have in addition analysed the samples for calcium. The data here are solely from day 7 and reflect an acute effect of amiloride. Amiloride was actually given for 7 days; however, no further data on calcium excretion aside from the acute values are available.

We would imagine that ample epithelial sodium channel (ENaC) data on calcium excretion would be available from the literature. In Liddle’s original report, a meticulous study that included triamterene administration in patients with activating ENaC mutations, sadly no calcium data were reported [15]. Incredibly, we were unable to find information on calciuria in ENaC mouse models. Noblins et al. [16] reported on four patients with Liddle’s syndrome. Two had hypercalciuria and nephrocalcinosis; however, a detailed evaluation with pharmacological intervention was not performed.

Blanchard et al. [14] performed their study not to investigate calcium handling, but instead to inspect the antinatriuretic effect of vasopressin and possible ENaC dependency. They found that the antinatriuretic effect of dDAVP was indeed amiloride sensitive. Bamberg et al. [17] recently investigated the possibility of V2-receptor-dependent calcium reabsorption in the isolated perfused kidney model. We found that V2 stimulation with dDAVP featured stable osmolality and calcium reabsorption, whereas without dDAVP a simultaneous fall in urine osmolality and calcium resorption occurred. In that study, we inhibited ENaC with amiloride and observed the expected increase in potassium reabsorption along with decreased sodium reabsorption. However, calcium handling was not perturbed (Figure 1). We concluded that evidence exists for a stimulatory role of the V2 receptor in renal calcium reabsorption but could not find an effect of amiloride in this experimental setting. The discrepancy from the present study may be related to absence of systemic influences when studying an isolated organ, which illustrates the complexity of the mechanisms that control calcium handling.

FIGURE 1

Fractional reabsorption of sodium (FR_Na), potassium (FR_K) and calcium (FR_Ca) in response to amiloride (5 µmol/L) in isolated perfused kidneys with added desmopressin (DDAVP; 400 ng/L) or control. The left panel represents the time series data of fractions observed during the 120-min experiment period, whereas the right panel displays mean values at baseline (0–60 min) and at amiloride exposure (60–120 min). Data from Bamberg et al. [17] with added calcium values.

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Amiloride is available in France as a 5-mg tablet and is also available in The Netherlands. Combination preparations containing 5 mg amiloride and 50 mg (or 2.5 and 25 mg) hydrochlorothiazide are, however, generally available but we could find no literature on this product and possible efficacy in stone prevention. We conclude that the current limited results are of great interest but require substantial clinical investigation in normal volunteers and patients before general conclusions can be drawn.

CONFLICT OF INTEREST STATEMENT

J.S. is employed at the Swedish Medical Products Agency, SE-751 03 Uppsala, Sweden. The views expressed in this paper are the personal views of the authors and not necessarily the views of any government agencies. Otherwise, there are no disclosures.

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