Permutation Trial of Diuretics

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.

The hypocalciuric effect of thiazides: subcellular localization of the action

The acute administration of thiazides results in a decrease in the urinary Ca2+/Na+ ratio, whereas chronic administration of these diuretics decreases calciuria. In both situations, Ca2+ transport is enhanced in the early part of the distal tubule. The purpose of our study was to determine whether the hypocalciuric action of thiazides was due to a change in the active transport of Ca2+ through the basolateral membrane of the nephron or to an effect (direct or indirect) on the permeability of the distal tubule luminal membrane to calcium. In order to detect intrinsic differences between membranes of the proximal and distal tubules, the effect of the diuretic was examined in proximal and distal tubule preparations, and in basolateral and luminal membranes from the two segments separately. Preincubation of microdissected distal tubules in hypotonic solution containing 500 microM hydrochlorothiazide (HCTZ) did not influence the Ca2(+)-dependent ATP hydrolysis (Ca2+ = 1 microM) nor the Mg2(+)-dependent ATP hydrolysis (Mg2+ = 100 microM). Similarly 100 microM HCTZ did not change the Ca2+ ATPase activity in intact proximal and distal tubule suspensions, at Ca2+ concentrations ranging from 0.05 microM to 1 microM. ATP-dependent Ca2+ transport was present in basolateral membrane vesicles from proximal and distal tubule suspensions. Preincubation of the membranes with 100 microM HCTZ did not influence this transport. A Na+/Ca2+ exchanger, present in the basolateral membranes from the distal tubule, was also insensitive to HCTZ. In contrast, preincubation of luminal membranes from the distal tubules (but not proximal tubules) with 500 microM HCTZ significantly increased the Ca2+ uptake by these membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of volume contraction in the hypocalciuric action of chlorothiazide

Thiazide-induced hypocalciuria usually requires two to four days of drug treatment for its full expression. The present experiments were designed to test the possibility that an immediate hypocalciuria could be produced in subjects whose extracellular volumes were already contracted. In previously untreated subjects, chlorothiazide (2 g/day) did not produce hypocalciuria on the first day of treatment. Hypocalciuria occurred on the second day and was somewhat enhanced on the third and fourth days of treatment. In subjects pretreated with furosemide (40 mg two or three times a day for two days), chlorothiazide produced hypocalciuria on the first day of its use. Under the influence of chlorothiazide, there was a strong linear correlation between the changes in the excretion of calcium and sodium. The results are consistent with the idea that chlorothiazide enhances the ratio of calcium to sodium reabsorptions and that this tendency for absolute hypocalciuria can be overridden by a large natriuretic response to the drug. Volume contraction, which minimizes the natriretic response to diuretics, allows a more immediate and pronounced hypocalciuric response to thiazides.

On the hypocalciuric action of chlorothiazide

Clearance experiments were performed in female mongrel dogs, either intact or thyro-parathy-roidectomized (T-PTX), under pentobarbital anesthesia, to examine the unusual hypocalciuric property of thiazide diuretics. The relationship between calcium clearance (C(Ca)) and sodium clearance (C(Na)) was determined in normal dogs, C(Ca) = 0.79 C(Na); constant infusion of chlorothiazide (CTZ) to provide drug concentrations in plasma of approximately 40 mug/ml modified this relationship; C(Ca) = 0.30 C(Na) (P < 0.001). The magnitude of the dissociating effect of CTZ on the urinary Ca/Na relationship was found to be most highly correlated with urinary drug concentration. Infusion of CTZ (1 mg/min) into one renal artery caused a unilateral decrease (25%) in C(Ca)/GFR while producing a unilateral increase (80%) in C(Na)/GFR. The same dose of CTZ in T-PTX dogs produced an increase in C(Na)/GFR without causing a change in C(Ca)/GFR. The defective response in T-PTX dogs could be ascribed to poor tubular secretion of the drug; when urinary drug concentrations were elevated in T-PTX dogs to the levels found in intact dogs (by infusing more drug), C(Ca)/GFR fell to an equivalent extent. T-PTX dogs showed substantially lower renal extraction of CTZ (42%) than intact dogs (57%); PTH administration to T-PTX dogs increased extraction toward normal (49%). The defective secretion of CTZ could not be attributed to either a decreased tubular maximum or a decreased renal blood flow.

Micropuncture study of diuretic effects on sodium and calcium reabsorption in the dog nephron

A close relationship has been observed between the clearance rates of sodium and calcium under a variety of diuretic conditions. The thiazide diuretics act differently in dissociating the renal tubular reabsorption of sodium and calcium. This phenomenon has been further investigated using recollection micropuncture and clearance techniques in a group of 14 dogs subjected to three consecutive experimental phases: expansion to 3% of body weight (BWt) with Ringer's solution, chlorothiazide infusion at 20 mg/kg/h, and furosemide in a prime of 10 mg/kg/ and a 10 mg/kg/h infusion. Diuretic losses were balanced with infusion of equal volumes of Ringer's solution throughout the experiment. Chlorothiazide increased the fractional excretion (FE) of sodium almost threefold while FE(Ca) was not significantly altered. Furosemide increased FE(Na) and FE(Ca) to an approximately equal, and more marked, degree. This dissociation of sodium and calcium reabsorption after chlorothiazide was also evident in the superficial distal tubule, where (tubule fluid/plasma sodium) (TF/P(Na)) increased from 0.32 to 0.49 (P < 0.01) and TF/(ultrafiltrate)UF(Ca) was unchanged (0.35-0.31). Furosemide markedly reduced the transtubular concentration gradient for both sodium (0.86) and calcium (0.94). TF/P(Inul in) decreased progressively from 3.79 to 2.78 to 2.33 in three phases. In the late proximal tubule, chlorothiazide induced a fall of TF/P(Inul in) from 1.57 to 1.44 (P < 0.01), but the ratio TF/UF(Ca): TF/P(Na) was unchanged. Furosemide had no significant proximal effect. It is concluded that acute administration of chlorothiazide reduces sodium reabsorption in the distal hephron, presumably the cortical diluting segment, without affecting calcium reabsorption.