Studies with the optically active isomers of the new diuretic drug ozolinone. I. Differences in stereoselectivity of the renal target structures of ozolinone

Site and mechanism of the action of diuretics

The mechanism of action of diuretics can be established by studying the molecular mechanism of action, the site of action within the nephron, and the relationship between the pharmacokinetics of the diuretic and its effect. The molecular mechanism of action is known for diuretic agents such as acetazolamide (carbonic anhydrase), theophylline (phosphodiesterase), digitalis glucosides (Na-K-ATPase), spironolactone (aldosterone antagonism) and dopamine (specific receptors?). The "receptor" for the clinically most important diuretics, i.e. loop diuretics, thiazides, and other potassium-sparing diuretics is, however, unknown. It appears from recent studies of the ion transport in the diluting segment that there probably is a sodium-chloride co-transport in this segment and that loop diuretics specifically inhibit the active chloride transport. The main site of diuretic action is well established for the different groups of diuretics: carbonic anhydrase inhibitors act on the proximal tubulus, loop diuretics on the diluting segment, thiazides on the cortical diluting segment/distal tubulus, and potassium-sparing agents on distal tubulus/collecting ducts. Moreover, some diuretics have additional tubular sites of action. It is also important to realize that other effects of diuretics, e.g. inhibition of the tubuloglomerular feedback mechanism or renal and extra-renal hemodynamic effects, can modify the tubular diuretic effect. Finally, the renal handling of diuretics is of importance to the diuretic effect by determining the concentration of the drug at the "receptor" sit (s). It is emphasized that knowledge of the different aspects of the mechanisms of action of diuretics is a prerequisite for rational use of diuretics, clinically as well as experimentally.

Stereospecificity of the effects of ozolinone on renal hemodynamics and on segmental tubular sodium reabsorption in conscious rats

The study was performed to elucidate the effects of the two stereoisomers of ozolinone (d,l) on renal hemodynamics and proximal tubular Na reabsorption. Clearance experiments were performed in conscious water-loaded female Wistar rats. The clearances of [3H]inulin, [14C]tetraethylammonium and lithium were used as estimates for glomerular filtration rate, renal plasma flow and delivery of fluid from the proximal tubules, respectively. When the baseline parameters had stabilized, d- or l-ozolinone was injected i.v. in doses of 4, 20 and 100 mg/kg. 1-Ozolinone caused a transient and dose-dependent diuretic-natriuretic response with no evidence of a ceiling. At peak natriuresis, 2.5-5 min after 100 mg/kg of 1-ozolinone, the fractional Na excretion was increased from 0.5 to 25%; this was associated with an increased fractional excretion of lithium from 27 to 60%, and small transient decreases of renal hemodynamic parameters. d-Ozolinone had no significant effects except for a small natriuresis after 100 mg/kg. It is concluded that in water-loaded conscious rats 1-ozolinone is a powerful diuretic which, in contrast to d-ozolinone, increases the delivery of fluid from the proximal tubule as judged from changes in lithium clearance.

Stereoselective inhibition of chloride transport by loop diuretics in pancreatic beta-cells

The effects of loop diuretics on 36Cl- uptake was tested in isolated beta-cell-rich pancreatic islets. Bumetanide reduced the 36Cl- influx and the levorotatory form of ozolinone reduced both the influx and equilibrium content of 36Cl- in the islets, whereas the dextrorotatory form was largely inactive. The data suggest that the beta-cells are equipped with a loop diuretic-sensitive system for 36Cl- uptake and that this system is confined to a sterically well-defined structure.

Ototoxicity of indacrinone is stereospecific

Indacrinone (MK-196) is a loop diuretic which consists of a racemic mixture. The purpose of this study was to evaluate the individual enantiomers in the chinchilla model to determine whether these compounds affect auditory function and whether a difference in ototoxic potency exists. Very little change of endocochlear potential (EP) or compound action potential (CAP) was noted in animals receiving the (+)-enantiomer. On the other hand, chinchillas injected with the (-)-enantiomer were found to have a dose related reduction in both CAP and EP. These findings suggest the possibility that the diuretic receptor in the kidney and the receptor mediating ototoxicity in the cochlea, may have similar steric requirements for interacting with loop diuretics.

Chloride transport inhibition by various types of loop diuretics in fish opercular epithelium

Isolated opercular epithelia of killifish (Fundulus heteroclitus), mounted in an Ussing chamber, were used to study the effects of various diuretics on chloride transport, measured as short-circuit current (SCC). The acidic 'loop' diuretics, ethacrynic acid and azosemide, and the basic 'loop' diuretics, muzolimine and MK 447, reduced SCC and exhibited similar dose-effect curves, with EC50s for SCC of 64, 17, greater than 500 and 224 microM, respectively. The alkaline diuretic tizolemide (HOE 740) and the p-COOH-analogue of sulphanilamide were inactive, suggesting that the chloruretic effects of these agents are of a thiazide type. The method can thus discriminate between the effects of loop and thiazide types of diuretics, but not between those of structurally highly different 'loop' diuretics of an acidic and basic nature. Monomethylation of the SO2NH2 group of bumetanide had no effect on the activity of this agent whereas dimethylation reduced it fourfold. The (-)enantiomers of the 'loop' diuretics indacrinone and ozolinone were four and greater than 100 times more active, respectively, than the (+)forms. These results are in accordance with those obtained for the same drugs in the mammalian kidney, and point to the presence of a highly specific binding site for these diuretics. Attempts were also made to explore the prerequisites for binding of the loop diuretic to the active site. Pretreatment of the opercular epithelium with an alpha-L-fucose-binding lectin did not prevent the inhibitory actions of furosemide and indacrinone. Probenecid and (+)ozolinone, both of which block organic anion transport, did not prevent the effects of bumetanide and (-)ozolinone.

Quantitative evaluation of ototoxic side effects of furosemide, piretanide, bumetanide, azosemide and ozolinone in the cat--a new approach to the problem of ototoxicity

A new method for the quantitative assessment of acute ototoxic side effects of drugs is described. It is suitable for screening purposes. The method is based on the determination of the toxic dose (TD50) which causes a defined hearing loss in 50% of the animals tested. The hearing loss is defined as a complete suppression of the compound action potential (CAP) of the auditory nerve, elicited by clicks 30 dB above threshold. This is approximately equivalent to a clinical hearing loss of 30 dB. The TD50 is used to estimate the therapeutic range. With this approach ototoxic side effects of furosemide, piretanide and bumetanide were compared quantitatively in cats. The TD50 values for CAP suppression were 18.37 mg/kg for furosemide; 4.29 mg/kg for piretanide and 2.21 mg/kg for bumetanide. As equipotent diuretic doses are 2.61 mg/kg for furosemide, 0.26 mg/kg for piretanide and 1.16 mg/kg for bumetanide, it appears that the relative ototoxicity is least for piretanide and highest for bumetanide. Plasma concentrations, determined initially and when recovery of CAP to 50% of control had occurred, indicate that bumetanide may be more slowly eliminated from the cochlear spaces than furosemide and piretanide. In addition azosemide and ozolinone were tested. The TD50 for azosemide was less than 10 mg/kg. With ozolinone where there are two isomers, only the diuretic (-)ozolinone was ototoxic; the TD50 was less than 100 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Enantiomers of benzothiadiazine diuretics by direct chromatographic resolution of the racemic drugs

A number of racemic benzothiadiazine diuretics and two carbonyl analogue drugs were resolved into their optical isomers by liquid chromatography on chiral polyacrylamides 1. Enantiomeric resolution, which was, in some cases, almost complete, depended considerably on the substitution of the heterocyclic moiety of the drug molecules. Synthesis of the new adsorbent lb is described. The enantiomers of the benzothiadiazines penflutizide (2a) and bendroflumethiazide (2b) in high optical purity, as well as enriched (+)-buthiazide (2j) were obtained by repeated chromatography on a semipreparative scale. Chiroptical data, optical purity employing the chromatographic method, and first-order racemization kinetics as a function of pH in aqueous solutions were determined.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

Effect of captopril on intrarenal blood flow

The inhibition of the renin-angiotensin system by captopril was used in pentobarbital-anesthetized dogs to confirm that the vasoconstrictive action of endorenally synthesized angiotensin II predominates on the efferent glomerular arteriole. Ozolinone is a loop diuretic with two isomers. Only (-)- ozolinone is diuretic, whereas both isomers have a renal vasodilatory effect which predominates on the efferent glomerular arteriole. Only the diuretic isomer increases renin release. The renin hypersecretion is simultaneous with recovery from the initial fall in filtration fraction, because of postglomerular vasodilatation. This recovery does not occur with (+)- ozolinone and is inhibited by pretreatment with captopril. This confirms that vasoconstrictive action of angiotensin II predominates on the efferent glomerular arteriole. Such a vasoconstrictive effect might affect blood flow in the vasa recta, which arise from the efferent arterioles of juxtamedullary glomeruli. This action might enable the renin-angiotensin system to participate in the control of renal medullary blood flow and urinary concentration.

Attenuation by d-ozolinone of l-ozolinone-induced diuresis in rats

The effect of the non-diuretic dextrorotatory isomer of ozolinone on l- ozolinone -induced diuresis was studied in anesthetized rats. After intravenous application d- ozolinone attenuated l- ozolinone -induced increase in renal fluid, sodium, potassium and chloride excretion. Microperfusion experiments of the loop of Henle in vivo revealed that no interaction between the two stereoisomers occurred at this main site of tubular action of l- ozolinone . Since both isomers share the same organic acid transport pathway in the proximal tubule it is assumed that d- ozolinone attenuates the l- ozolinone -induced diuresis because it depresses proximal secretion of l- ozolinone and thereby partially prevents transfer of this diuretic to the tubular fluid.

Pharmacodynamics and kinetics of etozolin/ozolinone in hypertensive patients with normal and impaired kidney function

The effect on urinary electrolyte excretion, renin release and plasma norepinephrine of single oral doses of 400 mg etozolin (E) and of 40 mg furosemide (F) were studied in hypertensive patients with normal (n = 6) and impaired kidney function (n = 6). E caused a marked saluresis up to 24 hours, showing its long duration of action. F, however, displayed a brief, brisk peak diuresis, followed by a rebound from the 4th to the 24th hours. The brisk peak diuresis induced by F was associated with pronounced release of renin, almost twice that induced by E. In chronic renal failure the renin release in relation to the magnitude of the diuresis was increased, i.e. the sensitivity of these patients to changes in water homeostasis was increased. E and F stimulated the sympathetic system to roughly the same extent. Patients with essential hypertension had higher plasma levels of norepinephrine than hypertensive patients with chronic renal failure. In addition, hypertensive patients with normal renal function (n = 4) and varying degrees of renal impairment (n = 11) were also given 400 mg daily for 2 weeks. Effects on blood pressure and electrolyte homeostasis were monitored, as well as the plasma kinetics of metabolite I, ozolinone. At the end of the 2 week treatment E had significantly lowered systolic (-12 mm Hg) and diastolic (-9 mm Hg) blood pressure, and had produced a significant loss of body weight, without altering plasma electrolytes or blood chemistry. There was no accumulation of the effective metabolite ozolinone under conditions of severe impairment of kidney function.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of "high ceiling" diuretics on active salt transport in the cortical thick ascending limb of Henle's loop of rabbit kidney. Correlation of chemical structure and inhibitory potency

The group of "high ceiling" diuretics consists of a variety of chemically different potent diuretic and saluretic substances. Appart from a few exemptions direct evidence for an action of these substances in the thick ascending limb of the loop of Henle (TAL) is still lacking. For furosemide, we have reported recently that it inhibits most likely the Na+-2 Cl--K+ cotransport system present in the lumen membrane of the TAL. The present study tests: 1. whether other "high ceiling" diuretics have a similar site and mode of action, and 2. how modifications of the furosemide molecule alter the inhibitory potency. Isolated cortical TAL (cTAL) segments (n = 185) of rabbit kidneys were perfused in vitro. The equivalent short circuit current (Isc = transepithelial PD/transepithelial resistance), as a measure of active salt transport was correlated to the dose of 64 substances. Several diuretics, such as 2-aminomethyl-4-(1,1-dimethyl-ethyl)-6-iodophenol hydrochloride (MK 447), hydrochlorothiazide, muzolimine, etozoline, tizolimide, amiloride, and triamterene were ineffective both from the lumen and basolateral side at concentrations as high as 10(-4) - 10(-3) mol X 1(-1). The phenoxyacetic acids ethacrynic acid, indacrinone (MK 196), and to less an extend tienilic acid were inhibitory active. They differed from furosemide in one or more of the following criteria: delayed onset, incomplete reversibility, stronger action from the bath, different slope of the dose response curve. Similarly, 1-ozolinone acted stronger from the bath. In contrast, the diuretics of the furosemide type and related compounds (bumetanide and piretanide) showed rapid onset and complete reversibility of inhibition. These substances acted stronger from the lumen. The individual positions in the benzyl ring of the diuretics were differently affected by substitutions, leading to parallel shifts in the dose response curves with halfmaximal inhibition at concentrations ranging between 8 X 10(-8) to greater than 10(-4) mol X 1(-1). For these substances the calculated Hill coefficients were close to unity: 0.96 +/- 0.05. We conclude that the so called "high ceiling" or "loop" diuretics consist of at least 3 groups: 1. drugs that do not interfere with the active salt transport in the cTAL segment, 2. drugs that interfere by so far not characterised mechanisms, and 3. drugs of the furosemide type which inhibit the Na+-2 Cl--K+ cotransport system in the lumen membrane of the cTAL segment.

Diuretic agents: actions on a molecular level

The criteria upon which diuretics are classified are based upon their site of action within the nephron. Carbonic anhydrase inhibitors act in the proximal tubule, high-ceiling diuretics in the ascending loop of Henle, the thiazides in the early distal tubule and the potassium-sparing diuretics in the late distal tubule and in the collecting duct. According to the localization of carbonic anhydrase acetazolamide acts on three different sites in the proximal tubule cells. The loop diuretics inhibit the secondary active chloride reabsorption. Experiments on the isolated stripped rabbit colon under the condition of stimulated chloride secretion reveal striking similarities between the receptors for chloride reabsorption in the luminal cell membranes of the ascending loop of Henle and in the serosal cell membranes of the colon. The potassium-sparing diuretics act by blocking sodium channels in the distal parts of the nephron. The lumen negative potential difference decreases and potassium secretion is diminished.

Studies on the renal receptors of loop diuretics

The principal renal action of loop diuretics is to inhibit active NaCl transport in the thick ascending limb of the loop of Henle. However, apart from their specific tubular action, kidney function may be affected in other ways. By employing the enantiomeres of the new loop diuretic ozolinone, non-stereospecific increase in renal blood flow and inhibition of tubular secretion of p-aminohippurate and urate were found. In contrast, electrolyte transport in the loop of Henle was inhibited stereospecifically. Perfusion of single loops of rat kidneys in vivo with sugar-specific lectins suggested that a fucose-containing glycoprotein is involved in electrolyte transport in this tubular segment. Loop diuretics might interact with this glycoprotein, leading to inhibition of electrolyte transport. Evidence is presented to suggest that this protein is the Tamm-Horsfall protein. This fucose-containing glycoprotein is localized in the cell membranes of the thick ascending limb of the loop of Henle. It binds furosemide at concentrations very close to those required for inhibition of electrolyte transport in vivo. Furthermore, there is complete agreement between the sodium concentration necessary for stimulation of active transport in isolated thick ascending limbs of the loop of Henle and that necessary for binding of furosemide to the Tamm-Horsfall glycoprotein.

Molecular actions of diuretics

The criteria upon which diuretics are classified is based upon their site of action within the nephron. Carboanhydrase inhibitors act in the proximal tubule, high-ceiling diuretics in the ascending loop of Henle, the thiazides in the early distal tubule and the potassium-sparing diuretics in the late distal tubule and in the collecting duct. On the molecular level diuretics do not inhibit Na+-K+-ATPase but interfere with the permeability of the tubule membranes or transport systems for certain ions and thus also influence the potential differences in the different parts of the nephron. Since carboanhydrase is located in the proximal tubule cells, not only in the cytosol but also in the brushborders and in the peritubular membranes, acetazolamide and other carboanhydrase inhibitors act on three different sites in these cells. The loop diuretics inhibit the secondary active chloride reabsorption. The receptors in this part of the nephron are stereospecific. Only the levorotatory isomere of ozolinone has active diuretic properties whereas the dextrorotatory isomere does not. Perfusion experiments of the loop of Henle with different lectins give evidence that glycoproteins containing alpha-1-fucose are involved in the reabsorption of Na+ and Cl-. Experiments on the isolated stripped rabbit colon under the condition of chloride secretion reveal striking similarities between the receptors for chloride reabsorption in the luminal cell membranes of the ascending loop of Henle and in the serosal cell membranes of the colon. The potassium-sparing diuretics amiloride and triamterene act by blocking sodium channels in the distal parts of the nephron. Thus the lumen negative potential difference decreases and (passive) potassium secretion is diminished.

Stereospecific inhibition by ozolinone of stimulated chloride secretion in rabbit colon descendens

The effects of the diuretic drug ozolinone on electrogenic Cl- secretion by rabbit colonic mucosa were investigated in vitro. Electrical properties and unidirectional Cl- fluxes were measured in stripped preparations mounted in Ussing-type chambers. After abolition of electrogenic Na+ absorption by amiloride (10(-4) mol/l) on the mucosal side electrogenic Cl- secretion was induced by addition of PGE1 (10(-6) mol/l, serosal side) and theophylline (10(-2) mol/l, both sides). Under these conditions, the monitored short-circuit current (Isc) equals the amount of Cl- secreted as evidenced by determination of unidirectional Cl- fluxes. After establishing a stable Cl- secretion its sensitivity to the enantiomers of the diuretic was studied. Only levorotatory (-)-ozolinone, but not the dextrorotatory (+)form, inhibited Cl- secretion on serosal application. This effect was fully accounted for by a reduction in the serosal-to-mucosal Cl- fluxes (JClsm). It was readily reversible and concentration-dependent with a Ki value of 6 x 10(-4) mol/l, but absent when the drug was added to the mucosal side. The results are in agreement with the hypothesis that loop diuretics inhibit a coupled NaCl entry mechanism across the baso-lateral membrane into colonic epithelial cells. This mechanism is though to account for Cl- influx into the cells.

Studies with the optically active isomers of the new diuretic drug ozolinone. II. Inhibition by d-ozolinone of furosemide-induced diuresis

The effect of the non-diuretic dextrorotatory isomer of ozolinone on furosemide-induced diuresis was studied by means of clearance and micropuncture techniques in rats. After intravenous injection, d-ozolinone antagonized the furosemide-induced increase in renal fluid and electrolyte excretion in a dose-related manner. Microperfusion experiments of Henle's loop in vivo revealed that d-ozolinone did not interfere with the action of furosemide at this tubular site. However, d-ozolinone markedly decreased the furosemide to inulin clearance ratio, presumably as a consequence of inhibition of furosemide secretion into the proximal tubules. It is assumed that, in consequence of a high affinity for the proximal organic acid transport system, d-ozolinone depresses proximal tubular furosemide secretion and prevents transfer of this diuretic to the tubular fluid. Thus, under the influence of d-ozolinone, furosemide cannot reach the loop of Henle in sufficient amounts and its diuretic effect is blocked.