Effects of hypokalemia and hypomagnesemia on zidovudine (AZT) and didanosine (ddI) nephrotoxicity in rats.

BACKGROUND: Zidovudine (AZT) and didanosine (ddI) are antiretroviral drugs widely used in AIDS patients. Hypokalemia and hypomagnesemia are frequently encountered in AIDS patients using AZT and/or ddI. OBJECTIVE: To verify the effects of AZT and ddI on rat renal function submitted to normal diet, low potassium diet and magnesium-free diet. METHODS: Glomerular filtration rate and renal hemodynamic were measured in Wistar rats submitted to a normal or a potassium-depleted diet. The animals were given AZT, ddI for 15 days. Six groups of rats were studied: normal diet, normal diet + AZT, normal diet + ddI, low K diet, low K diet + AZT and low K diet + ddI. Three additional groups of rats submitted to magnesium depletion for 15 days were also studied: magnesium-free diet, magnesium-free diet + AZT and magnesium-free diet + ddI. RESULTS: AZT and didanosine did not modify renal function of rats on a normal diet. However, in hypokalemic rats, both drugs produced a decrease in glomerular filtration rate and in renal blood flow consequent to renal vasoconstriction and associated with alterations in tubular function (characterized by an increased fractional excretion of sodium). Hypomagnesemia induced a decrease in glomerular filtration rate and in renal blood flow only in AZT-treated rats. CONCLUSION: Our data suggest that hypokalemia predisposes to AZT and ddI nephrotoxicity, while hypomagnesemia predisposes only to AZT nephrotoxicity. Thus, chronic AZT and ddI administration may produce acute renal failure in AIDS patients with hypokalemia and/or hypomagnesemia. Serum K and Mg levels should be carefully monitored in these patients.

Thiazide induces water absorption in the inner medullary collecting duct of normal and Brattleboro rats.

The reduction of urinary volume after the use of thiazide in the treatment of diabetes insipidus (DI) is known as the "paradoxical effect." Since enhanced proximal solute and water reabsorption only partially account for the reduction in urinary volume, an additional diuretic effect on nephron terminal segments was postulated. Thus the aim of our work was to investigate the effect of hydrochlorothiazide (HCTZ) on water transport in the inner medullary collecting duct (IMCD) of normal and Brattleboro rats. Osmotic water permeability (P(f)) and diffusional water permeability (P(dw)) were studied at 37 degrees C and pH 7.4 by the in vitro microperfusion technique. In the absence of antidiuretic hormone (ADH), HCTZ (10(-6) M) added to the perfused fluid enhanced P(f) from 6.36 +/- 0. 56 to 19.08 +/- 1.70 micro(m)/s (P < 0.01) and P(dw) from 38.01 +/- 4.52 to 52.26 +/- 4.38 x10(-5) cm/s (P < 0.01) in normal rats and also stimulated P(f) in Brattleboro rats from 3.53 +/- 1.41 to 11.16 +/- 1.13 micro(m)/s (P < 0.01). Prostaglandin E(2) (PGE(2)) (10(-5) M) added to the bath fluid inhibited HCTZ-stimulated P(f) (in micro(m)/s) as follows: control, 16.93 +/- 2.64; HCTZ, 29.65 +/- 5.67; HCTZ+PGE(2), 10.46 +/- 1.84 (P < 0.01); recovery, 16.77 +/- 4.07. These data indicate that thiazides enhance water absorption in IMCD from normal rats (in the absence of ADH) and from Brattleboro rats and that the HCTZ-stimulated P(f) was partially blocked by PGE(2). Thus we may conclude that the effect of thiazide in the treatment of DI occurs not only in the Na(+)-Cl(-) cotransport in the distal tubule but also in the IMCD.

Influence of parathyroidectomy and calcium on rat renal function.

Parathyroid hormone (PTH) has multiple effects on water and electrolyte transport along the nephron. However, the influences of PTH and calcium on the urinary concentration ability are not fully understood. In this study, clearance and microperfusion studies were performed in thyroparathyroidectomized (TPTX) rats either supplemented (TPTX+Ca(2+)) or not with calcium added to the ingested food as CaCl(2) (1.6 g/100 g). Acid-base data and renal functional parameters were measured in TPTX and TPTX+Ca(2+) rats. Additional studies were performed in the isolated inner medullary collecting tubules of intact and TPTX rats to evaluate the osmotic permeability of this segment in the presence of 10(-6) M PTH added to the bath. In these experiments the possible influence of PTH on antidiuretic hormone induced changes of the osmotic permeability in TPTX and TPTX+Ca(2+) rats was also investigated. In the TPTX+Ca(+) group, the glomerular filtration rate increased significantly when compared to the TPTX group (6.04 +/- 0.42 vs. 4.88 +/- 0.20 ml.min(-1).kg(-1); p < 0.05), but the U/P inulin ratio remained lower than control values (30.8 +/- 1.48 vs. 54.0 +/- 3.5; p < 0.05), which suggests that normal levels of PTH are necessary to maintain the concentrating ability. In a group of TPTX rats, an acute infusion of PTH (0.5 microg.min(-1).kg(-1)) significantly decreased the urinary flow and increased the renal plasma flow, results that agree with the vasomodulator action of this hormone on the renal vasculature. A significant increase in the fractional K(+) excretion observed in the TPTX+Ca(2+) group as compared with both control and TPTX, groups suggests that the excreted load of Ca(2+) may interfere with tubular K(+) handling in the absence of PTH. PTH (10(-6) M) added to the bath of the isolated inner medullary collecting tubules did not change the osmotic permeability, of intact, TPTX, and TPTX+Ca(2+) rats. Furthermore, it did not modify the antidiuretic hormone induced changes in the osmotic permeability. These results suggest that this segment of the nephron is PTH insensitive as far as water and ion transport are concerned.

Evidence for a physiological role of angiotensin-(1-7) in the control of hydroelectrolyte balance.

In this study we evaluated the possibility that angiotensin-(1-7) [Ang-(1-7)] acts as an endogenous osmoregulatory peptide by determining the effect of acute administration of its selective antagonist [D-Ala7]Ang-(1-7) (A-779) on renal function parameters in rats. In addition, we investigated the physiological mechanisms involved in the antidiuretic effect of Ang-(1-7). The antidiuretic effect of Ang-(1-7) (40 pmol/0.05 mL per 100 g BW) in water-loaded rats was completely blocked by A-779 (vehicle-treated, 3.34 +/- 0.43 mL/h; Ang-(1-7), 1.48 +/- 0.23; A-779, 2.72 +/- 0.35; Ang-(1-7) plus A-779, 3.26 +/- 0.49). In contrast, the antidiuretic effect of Ang-(1-7) was not significantly changed by a vasopressin V2 receptor antagonist in a dose that completely blocked the antidiuresis produced by an equipotent dose of vasopressin. In addition, Ang-(1-7) administration did not significantly change vasopressin plasma levels in water-loaded rats. The antidiuretic effect of Ang-(1-7) in water-loaded rats was associated with a reduction of creatinine clearance (0.68 +/- 0.04 versus 1.38 +/- 0.32 mL/min in vehicle-treated rats, P <.05) and an increase in urine osmolality (266.8 +/- 32.7 versus 182.8 +/- 14 mOsm/kg in vehicle-treated rats, P <.05). An effect of Ang-(1-7) in tubular water transport was demonstrated in vitro by a fourfold increase in the hydraulic conductivity of inner medullary collecting ducts in the presence of 1 nmol/L Ang-(1-7). Subcutaneous administration of A-779 (2.3 to 9.2 nmol/100 g) produced a significant increase in urine volume (4.6 nmol/100 g, 0.45 +/- 0.12 mL/h; vehicle-treated rats, 0.16 +/- 0.03 mL/h; P <.05) comparable to that of acute administration of a vasopressin V2 receptor antagonist. The diuretic effect of A-779 was associated with an increase in creatinine clearance and decrease in urine osmolality. In contrast, no significant effects on urine volume were observed after systemic administration of angiotensin subtype 1 or 2 receptor antagonists (DuP 753 and CGP 42112A, respectively). These findings suggest that endogenous Ang-(1-7), acting on specific receptors, participates in the control of hydroelectrolyte balance by influencing especially water excretion.

Effect of insulin on water and urea transport in the inner medullary collecting duct.

The effect of insulin on water and urea transport was examined in normal isolated rat inner medullary collecting duct (IMCD). Hydraulic conductivity (Lp, x 10(-6) cm.atm-1.s-1), diffusional water permeability (Pdw, x 10(-5) cm/s) and [14C]urea permeability (x 10(-5) cm/s) were studied at 37 degrees C and pH 7.4. Insulin (6 x 10(-8) M; 200 microU/ml) added to the bath fluid enhanced Lp from 0.40 +/- 0.10 to 1.21 +/- 1.40 (P < 0.01) and Pdw from 42.40 +/- 3.40 to 58.50 +/- 5.00 (P < 0.02) and also stimulated Lp in a dose-dependent manner. In the presence of antidiuretic hormone (ADH)-stimulated Pdw (10 microU/ml), insulin increased Pdw even more. Prostaglandin E2 (10(-5) M) added to the bath reversibly increased insulin-induced Lp. Forskolin (10(-4) M) blocked the action of insulin. Colchicine (10(-4) M) and V1-receptor antagonist (10(-4) M) inhibited the development but not the maintenance of insulin-stimulated Pdw. Vanadate (2.5 x 10(-6) M) enhanced Pdw. Polymyxin B (10(-5) M) inhibited the insulin-stimulated Pdw, whereas in a glucose-free medium insulin did not enhance Pdw. Urea transport was not affected by insulin. These data suggest that insulin may enhance water transport, probably by stimulating glucose transporters, which would serve as a water channel. We cannot rule out the possibility that insulin may be eliciting existing ADH-like mechanisms of water transport, beyond the microtubule step, to establish water transport.

Phosphate transport in isolated rat inner medullary collecting duct.

Phosphate transport by the inner medullary collecting duct of normal rats was studied using an in vitro microperfusion technique. Net (Jnet), lumen-to-bath (Jlb) and bath-to-lumen (Jbl) phosphate fluxes were measured using 32PO4 as tracer, in the absence of net water absorption. A net absorption of phosphate (22.3 +/- 3.3 pmol cm-2 s-1) was observed by direct determination, and was similar to the difference between the Jlb and Jbl (57.7 +/- 8.2 and 32.2 +/- 1.5 pmol cm-2 s-1 respectively). The addition of amiloride (10 microM) to the perfusate did not change the Jlb of phosphate but blocked the efflux of sodium. Also, the withdrawal of sodium from the bath and perfusion solution did not change the Jlb of phosphate. In parallel, the addition of ouabain (10 mM) to the bath fluid decreased the Jlb of sodium more (37%) than the Jlb of phosphate (12%) and did not change the Jbl of phosphate. The addition of arsenate (10 microM) to the perfusate both in the presence and in the absence of sodium caused a decrease in Jlb, but Jbl remained unchanged, and parathyroid hormone (10 U) added to the bath did not change the Jlb. The increase in pH of the bath and perfusion fluid was associated with an increase in the Jlb of phosphate, and the decrease in pH was similarly followed by a decrease in phosphate efflux. The Jbl did not change with the pH alterations. These data demonstrate that a net phosphate absorption takes place in rat inner medullary collecting duct perfused in vitro and that this transport appears to be independent of sodium absorption and the action of parathyroid hormone. Moreover, a decrease in luminal and bath pH induces a decrease in phosphate efflux.

Renal involvement in leptospirosis: a pathophysiologic study.

The kidney involvement in leptospirosis appears to be a special form of acute renal failure due to a higher frequency of polyuric forms and the presence of hypokalemia with an elevated urinary fractional excretion of potassium. Using a clearance technique, we detected higher fractional urinary potassium excretion in leptospirotic guinea pigs (26.5 +/- 4.7%) than in normal animals (14.1 +/- 2.8%, p < 0.05). After blocking distal NaCl reabsorption with furosemide, it was observed that in leptospirotic animals both fractional sodium excretion (40.0 +/- 7.4%) and fractional potassium excretion (136.3 +/- 32.7%) were higher than in normal animals (20.4 +/- 3.8%, p < 0.05, and 43.6 +/- 9.0%, p < 0.05, respectively). Microperfusion studies showed that the normal and leptospirotic medullary thick ascending limb had both identical transepithelial potential difference (+3.7 +/- 0.4 vs. 3.9 +/- 0.2 mV) and relative sodium-to-chloride permeability. The same technique showed that the osmotic water permeability (Posm; 0.9 +/- 0.4 x 10(-5) cm/s.atm) and diffusional permeability (34.7 +/- 6.6 x 10(-5) cm/s) observed in the leptospirotic inner medullary collecting duct (IMCD) in the presence of vasopressin were unchanged, as was also the case for urea permeability (3.74 +/- 0.7 x 10(-5) cm/s). These data show that acute renal failure in leptospirosis is characterized by tubular changes leading to potassium secretion probably due to a decrease in proximal sodium reabsorption. Furthermore, the inability to concentrate urine evidenced by the low P(o)sm present in leptospirotic animals is due, at least in part, to IMCD resistance to vasopressin.

Calcium transport across rat inner medullary collecting duct perfused in vitro.

Ca2+ transport by the inner medullary collecting duct (IMCD) of normal rats was studied using the "in vitro" microperfusion technique. Net (Jnet), lumen-to-bath (Jl----b), and bath-to-lumen (Jb----l) fluxes of Ca2+ were measured in the absence of net water absorption using 45Ca as a tracer. In the absence of an electrochemical gradient, an important net absorption of Ca2+ (11.1 +/- 1.6 pmol.cm-2.s-1), similar to the difference between the Jl----b and Jb----l, was observed by direct determination at low (5-6 nl/min) and high (12-17 nl/min) perfusion rates. The Jl----b of Ca2+ was reduced by the addition to the bath fluid of ouabain (10(-3) M) and verapamil (10(-4) M), by the presence of amiloride (10(-5) and 10(-3) M) and verapamil (10(-4) M) in the luminal fluid, or by perfusion with a Na+-free solution. Neither the presence of verapamil (10(-4) M) and ouabain (10(-3) M) in the bath nor the withdrawal of Na+ from bath and perfusion solution was able to modify the Jb----l of Ca2+. Incrementing Ca2+ bath concentration increased proportionally the Jb----l of Ca2+. Therefore Ca2+ outflux is in part dependent on Na+-K+-adenosinetriphosphatase luminal membrane Na+ transport and in part inhibited by verapamil. However, Ca2+ influx is independent of Na+ transport, is not blocked by verapamil, but is increased by Ca2+ transtubular gradient, indicating the presence of a passive diffusion mechanism.