SULFONAMYL DIURETICS--MECHANISM OF ACTION AND THERAPEUTIC USE

The mechanism whereby sulfonamyl diuretics are effective is through the blockage of the renal tubular reabsorption of chloride. The excretion of sodium, potassium and water is a passive one to maintain ionic equilibrium. Chlorothiazide has been shown to be almost ineffective as a diuretic agent per se. Although it does block a moiety of the renal tubular reabsorption of bicarbonate, the effect is merely a transient one.

FURTHER OBSERVATIONS CONCERNING THE HYPERGLYCEMIC ACTIVITY OF BENZOTHIADIAZINES

Hyperglycemia has been produced by a series of BZD in the normal rat. Diazoxide is the most potent hyperglycemic agent examined in this series and its action is complemented by combination with several other BZD. Examination in pregnant rats showed a complementary activity between diazoxide and trichlormethiazide, which had not been obtained in the normal rat.

Hyperglycemic action is obtained in hypophysectomized, and demedullated rats, thus excluding these endocrine organs as necessary for production of hyperglycemia. The results in adrenalectomized rats are inconclusive.

Histological sections showed evidence of degranulation in the beta cells of the islets of Langerhans in rats treated chronically with trichlormethiazide and diazoxide, an effect which was not shown during an acute experiment.

The hyperglycemic action of these BZD could be overcome by oral tolbutamide.

The hyperglycemic action of hydrochlorothiazide, a potassium losing BZD, as well as of diazoxide, which is not known to cause potassium loss, was overcome by concomitant administration of potassium salts.

Although the etiology of BZD hyperglycemia remains obscure, its production in the normal rat by many substances of similar chemical structure, lends support to the recently made clinical observations concerning the long term diabetogenic action of BZD in man.4

EFFECT OF HYDROCHLOROTHIAZIDE ON SERUM AND URINARY CALCIUM AND URINARY CITRATE

Benzothiadiazines, potent oral diuretics, also inhibit carbonic anhydrase, although to a lesser degree than acetazolamide (Diamox). The latter has little effect on urinary calcium but causes hypocitriuria which predisposes to nephrolithiasis. The effect of benzothiadiazines on urinary calcium/citrate balance is less well known. In this study, the fall of urinary calcium and citrate was roughly 50% and 30%, respectively, in normal and hypertensive subjects during the prolonged administration of hydrochlorothiazide, a member of the benzothiadiazine family. Hence, in contradistinction to acetazolamide, this drug produces hypocalciuria but depresses the urinary citrate less. Consequently, the benzothiadiazines would not be expected to predispose to nephrolithiasis. In addition, hydrochlorothiazide was found to produce a 10-20% fall in creatinine clearance and an initial rise of roughly 0.7 mg./100 ml. in serum calcium concentration. There are indications that the benzothiadiazines affect the overall metabolism of calcium and citrate.