Gentamicin causes endocytosis of Na/Pi cotransporter protein (NaPi-2).

BACKGROUND: Renal toxicity is a major side-effect of aminoglycoside antibiotics and is characterized by an early impairment in proximal tubular function. In a previous study, we have shown that gentamicin administration to the rat causes an early impairment in sodium gradient-dependent phosphate (Na/Pi) cotransport activity. The purpose of our current study was to determine the molecular mechanisms of the impairment in Na/Pi cotransport activity, specifically the role of the proximal tubular type II Na/Pi cotransporter. METHODS: Rats were treated for one, two, and three days with two daily injections of 30 mg/kg body weight gentamicin or the vehicle. RESULTS: Gentamicin caused a progressive decrease in superficial cortical apical brush-border membrane (SC-BBM) Na/Pi cotransporter activity (856 +/- 93 in control vs. 545 +/- 87 pmol/mg BBM protein in 3-day gentamicin, P < 0.01). Western blot analysis showed a parallel and progressive decrease in SC-BBM Na/Pi cotransporter protein abundance, a 50% decrease after one day of treatment, a 63% decrease after two days of treatment, and an 83% decrease after three days treatment with gentamicin. In contrast, gentamicin treatment had no effect on Na/Pi cotransport activity or Na/Pi cotransporter protein abundance in BBM isolated from the juxtamedullary cortex (JMC-BBM). Immunofluorescence microscopy showed a major decrease in the expression of Na/Pi cotransporter protein in the apical membrane of the proximal convoluted tubule, with progressive intracellular accumulation of Na/Pi protein. Colocalization studies showed that in gentamicin-treated rats, Na/Pi protein was colocalized in the early endosomes and especially in the lysosomes. Northern blot analysis of cortical RNA interestingly showed no reduction in Na/Pi cotransporter mRNA abundance even after three days of gentamicin treatment. CONCLUSION: We conclude that gentamicin inhibits Na/Pi cotransport activity by causing a decrease in the expression of the type II Na/Pi cotransport protein at the level of the proximal tubular apical BBM and that inhibition of Na/Pi cotransport activity is most likely mediated by post-transcriptional mechanisms.

Dexamethasone modulates rat renal brush border membrane phosphate transporter mRNA and protein abundance and glycosphingolipid composition.

Glucocorticoids are important regulators of renal phosphate transport. This study investigates the role of alterations in renal brush border membrane (BBM) sodium gradient-dependent phosphate transport (Na-Pi cotransporter) mRNA and protein abundance in the dexamethasone induced inhibition of Na-Pi cotransport in the rat. Dexamethasone administration for 4 d caused a 1.5-fold increase in the Vmax of Na-Pi cotransport (1785 +/- 119 vs. 2759 +/- 375 pmol/5 s per mg BBM protein in control, P < 0.01), which was paralleled by a 2.5-fold decrease in the abundance of Na-Pi mRNA and Na-Pi protein. There was also a 1.7-fold increase in BBM glucosylceramide content (528 +/- 63 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02). To determine whether the alteration in glucosylceramide content per se played a functional role in the decrease in Na-Pi cotransport, control rats were treated with the glucosylceramide synthase inhibitor, D-threo-1-phenyl-2-decanoyl-amino-3-morpholino-1-propanol (PDMP). The resultant 1.5-fold decrease in BBM glucosylceramide content (199 +/- 19 vs. 312 +/- 41 ng/mg BBM protein in control, P < 0.02) was associated with a 1.4-fold increase in Na-Pi cotransport activity (1422 +/- 73 vs. 1048 +/- 85 pmol/5 s per mg BBM protein in control, P < 0.01), and a 1.5-fold increase in BBM Na-Pi protein abundance. Thus, dexamethasone-induced inhibition of Na-Pi cotransport is associated with a decrease in BBM Na-Pi cotransporter abundance, and an increase in glucosylceramide. Since primary alteration in BBM glucosylceramide content per se directly and selectively modulates BBM Na-Pi cotransport activity and Na-Pi protein abundance, we propose that the increase in BBM glucosylceramide content plays an important role in mediating the inhibitory effect of dexamethasone on Na-Pi cotransport activity.

Heterogeneity in gentamicin clearance between high-efficiency hemodialyzers.

Gentamicin is frequently prescribed for treatment of gram-negative infections in patients on dialysis. The amino-glycoside is renally excreted; therefore, dosage modifications are required in patients on hemodialysis. The introduction of short-duration dialysis using high-efficiency hemodialyzers requires that the dialysance of gentamicin be re-examined. We recently demonstrated that high-efficiency dialyzers have a superior gentamicin clearance as compared with conventional dialyzers. In this study we examined the dialysance of gentamicin in two high-efficiency dialyzers using a randomized, controlled, cross-over design. Despite a comparable amount of dialysis (Kt/V urea), rate of dialysis (K/V urea), and ultrafiltration, there were significant differences noted in gentamicin removal between the high-efficiency cuprammonium rayon and polysulfone dialyzers. Furthermore, the amount of dialysis (Kt/V urea) could predict the fractional gentamicin clearance for the TAF 175L dialyzer (Terumo Corp, Somerset, NJ) and may be useful in the initial planning of gentamicin therapy. In addition, since gentamicin is a middle molecule (molecular weight, 500 to 2,000 d), the study suggests a superior middle molecule clearance of cuprammonium rayon dialyzer compared with a high-efficiency polysulfone dialyzer. The substantial heterogeneity in gentamicin clearance, even between dialyzers of the same class, emphasizes the importance of monitoring drug levels in hemodialysis patients receiving this drug.

Southwestern Internal Medicine Conference: bone disease in kidney failure: diagnosis and management.

Recent technologic and therapeutic advances have improved the life of the patient with end-stage renal disease. High efficiency and high-flux hemodialyzer membranes have shortened the time required to dialyze, and recombinant erythropoietin has all but eliminated anemia as a major cause of morbidity, but the problem of renal osteodystrophy remains. The following discussion examines the spectrum of bone and joint disease in the patient with end-stage renal disease. The diagnostic and therapeutic strategies currently being tried in the management of these disorders are discussed.

A comparison of netilmicin and tobramycin therapy in patients with renal impairment.

We evaluated the toxicity and efficacy of netilmicin and tobramycin in 89 older adults with serious bacterial infections and pre-existing renal impairment in a prospective, blinded, randomized trial. Complete resolution or improvement of infection occurred at 34/36 (94%) evaluable sites in netilmicin-treated patients and at 26/31 (84%) evaluable sites in tobramycin-treated patients. 10/44 (23%) netilmicin- and 7/45 (16%) tobramycin-treated patients experienced nephrotoxicity during treatment. The mean serum creatinine level improved significantly at the end of therapy compared to pre-treatment in those patients who did not experience nephrotoxicity in both treatment groups. 5/19 (26%) netilmicin-treated patients and 2/18 (11%) tobramycin-treated patients assessable for ototoxicity experienced decrements in auditory thresholds. Ototoxic netilmicin-treated patients had higher serum netilmicin levels than did non-ototoxic patients. Patients who experienced ototoxicity were not more likely to have experienced nephrotoxicity. The rates of toxicity were not statistically different and were similar to those seen in studies of patients with normal pre-treatment renal function.

Renal cell carcinoma.

Renal cell carcinoma accounts for 3% of all adult cancers and has many unusual features in its presentation, diagnosis, and management. It develops in a significant number of patients with acquired renal cystic disease, a disorder found almost exclusively in chronic hemodialysis patients. Abnormalities of chromosome 3 are frequently found in sporadic and familial forms. Radical nephrectomy remains the only potentially curative therapy for this tumor.

Role of potassium in the pathogenesis of acute renal failure.

Potassium depletion can potentiate several experimental models of acute renal failure. It causes renal vasoconstriction, probably under the influence of vasoconstrictor prostaglandins and angiotensin II, and causes a reduction in vasodilatory prostaglandins. Aminoglycoside nephrotoxicity in experimental animals and in man causes a reduction in serum potassium and in animals it enhances the functional and histological damage produced by aminoglycosides. Chronic potassium loading protects against mercuric chloride, uranyl nitrate, and gentamicin models of acute renal failure. In the gentamicin model, protection is associated with a stimulation of renal cortical Na-K-ATPase activity and a reduction in the level of gentamicin accumulated in cortical tissue. In the clinical setting, potassium deficiency should be avoided in patients at risk for acute renal failure. However, potassium loading should also be avoided, since a falling glomerular filtration rate in the presence of a potassium load could result in potentially serious hyperkalemia.

Protective effect of KCl loading in gentamicin nephrotoxicity.

Aminoglycoside nephrotoxicity in experimental animals can be reduced by calcium loading, inducing diabetes, and giving thyroid hormone, while a potassium deficient diet enhances aminoglycoside nephrotoxicity. This study investigated whether potassium loading protects against gentamicin nephrotoxicity in the rat. In part I, group GK ate a diet containing 3.5% potassium and drank 0.2 mol/L KCl. Pair-fed rats eating a standard diet, group G, ate a 1% potassium diet and drank water. After 10 days, each group received gentamicin subcutaneously, 60 mg/kg twice daily for 8 days. The control groups, K and C, received the high or normal potassium diet, respectively. To control for a protective effect from a high solute load, the effect of equimolar NaCl loading was studied in group GNa and Na. At the end of the 8 days of gentamicin, inulin clearance was significantly higher in GK compared with G(0.6 +/- 0.1 v 0.3 +/- 0.1 mL/min per 100 g body weight [BW], P less than 0.05), but group GNa (0.4 +/- 0.1 mL/min per 100 g BW) was not different from group G. Morphological studies demonstrated that potassium-loaded rats (group GK) had significantly less proximal tubular necrosis compared with rats on a standard potassium diet, group G. Sodium loading did not protect against cellular necrosis. Part II studied renal function, cortical Na,K-adenosine triphosphatase (ATPase) and gentamicin accumulation after 2 days of gentamicin to determine the early functional and biochemical effects of potassium loading before overt renal functional impairment.(ABSTRACT TRUNCATED AT 250 WORDS)

Early selective effects of gentamicin on renal brush-border membrane Na-Pi cotransport and Na-H exchange.

Gentamicin nephrotoxicity is associated with impairments in proximal tubular function. This study determined whether gentamicin administration to the rat, before a reduction in glomerular filtration rate (GFR), causes early and selective alterations in renal cortical brush-border membrane (BBM) enzyme and transport activity, lipid composition, and fluidity. Three days of gentamicin administration caused significant decreases in the Vmax of alkaline phosphatase, the Vmax of sodium gradient-dependent phosphate transport (Na-Pi cotransport), and the Vmax of pH gradient-dependent sodium transport (Na-H exchange). Gentamicin did not affect BBM-bound maltase or leucine aminopeptidase activities and sodium gradient-dependent glucose or proline transport activities. Gentamicin also caused a significant decrease in BBM sphingomyelin, significant increases in BBM phosphatidylcholine and phosphatidylinositol, a significant decrease in the phospholipid fatty acid saturation index, and a significant increase in BBM fluidity, i.e., decrease in the fluorescence anisotropy of diphenylhexatriene. These BBM functional and compositional effects of gentamicin were independent of endogenous parathyroid hormone activity. We conclude that gentamicin causes early and specific alterations in BBM enzyme and transport activity and also lipid composition, which may play an important role in the progression of renal cell injury.

Renal failure following radiologic procedures.

Radiologic procedures that employ intravascular contrast material with or without angiography may lead to renal failure. In procedures that use intravenous contrast alone, the mechanism of renal injury is not precisely known, but direct toxicity to renal tubular cells is likely to be a major factor. Ionic and nonionic contrast agents are both capable of causing this adverse reaction. Renal failure occurring during angiography may also be secondary to the effects of radiocontrast, but the additional possibility that micro cholesterol emboli have been dislodged from atheroma located on the intima of large vessels must be considered. The acute or subacute development of renal failure in the presence of skin changes (livido reticularis), hypertension, multiple organ failure or dysfunction, and a fatal outcome favors the later diagnosis.

Psychogenic polydipsia with hyponatremia: report of eleven cases.

Psychogenic polydipsia is an uncommon clinical disorder characterized by excessive water-drinking in the absence of a physiologic stimulus to drink. The excessive water-drinking is well tolerated unless hyponatremia supervenes. This report describes 11 patients with psychogenic polydipsia and hyponatremia (ten men and one woman) who were collectively hospitalized a total of 70 times for treatment of complications of this disorder. This group differs from the classical patient with psychogenic polydipsia, ie, a hospitalized schizophrenic, in that none was institutionalized and there was a high incidence of chronic alcoholism (10), intractable hiccups (7), self-induced vomiting (6), and laboratory evidence for rhabdomyolysis (5).

Protection by thyroxine in nephrotoxic acute renal failure.

Thyroxine (T4) protects against ischemic and nephrotoxic experimental acute renal failure (ARF). This study examined functional, biochemical, and morphological markers of uranyl nitrate (UN)-induced renal injury in the rat to determine the cellular site at which T4 exerts its protective effect. In experimental group UNT4, 1-thyroxine (10 micrograms/100 g body wt) was given for 10 days prior to and for 4 days following a single subcutaneous injection of UN (0.5 mg/kg body wt). Group UN received only UN, and group CT4 received only T4 for 14 days. Five days after UN administration, plasma creatinine rose from base line in group UNT4 (0.52 +/- 0.30 to 0.84 +/- 0.08 mg/dl, P less than 0.025) and group UN (0.52 +/- 0.03 to 1.64 +/- 0.13 mg/dl, P less than 0.001) but not in group CT4 (0.47 +/- 0.02 to 0.48 +/- 0.04 mg/dl, NS). However, plasma creatinine in group UNT4 was significantly lower than group UN (0.84 +/- 0.08 vs. 1.64 +/- 0.13 mg/dl, P less than 0.001). T4 administration stimulated the basolateral membrane-bound enzyme Na-K-ATPase in the renal cortex homogenate in group UNT4 (13.9 +/- 0.5 micron X mg protein-1 X h-1, P less than 0.005) and group CT4 (16.3 +/- 0.6 micron X mg protein-1 X h-1, P less than 0.001) when compared with controls (11.7 +/- 0.5 micron X mg protein-1 X h-1). Na-K-ATPase activity fell in group UN to 10.0 +/- 0.6 micron X mg protein-1 X h-1 (P less than 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

Drug therapy in the management of acute renal failure.

Pharmacologic intervention aimed at altering the natural history of acute renal failure is a routine practice without scientific support of efficacy. Oliguria has become a separate disease entity with an apparent disregard for the underlying condition that caused it. Volume expansion is clearly beneficial in preventing many volume depleted patients from progressing to acute renal failure. While mannitol and furosemide have been used to "convert" oliguric acute renal failure to the more easy to manage non-oliguric acute renal failure, published reports suggest that responders were not as ill as non-responders. The use of dopamine to increase urine flow in patients with established acute renal failure is the current fashion, but there is little evidence that this drug raises the glomerular filtration rate or shortens the course of acute renal failure. These pharmacologic therapies increase the complexity and cost of care with little tangible evidence of benefit to the patient or the physician caring for the patient.

Morphology of norepinephrine-induced acute renal failure in the dog.

The 40-minute infusion of norepinephrine (NE) into the renal artery of dogs produces a reversible ischemic model of acute renal failure. While the physiology of this model has been extensively studied, no complete description of the pathology exists. This study uses light microscopy and transmission electron microscopy to describe and quantitate the structural and ultrastructural changes which occur in the kidneys of dogs 1, 3, and 24 hours after the intrarenal infusion of 0.75 mg/kg/minute of NE. One hour after a 40-minute NE infusion the majority of convoluted and straight proximal tubules showed apical blebs, loss of brush border, microvillar whorl formation, and mitochondrial condensation and high-amplitude swelling with flocculent densities. Necrotic cells were occasionally seen at 1 hour. The injury was progressive after 3 hours and by 24 hours animals had either complete or partial patchy necrosis of all regions of the proximal tubule. The percentages of injured and necrotic proximal tubules in outer, mid-, and inner cortical regions are presented. We conclude that the extent and pattern of injury seen after NE infusion differs significantly from the renal artery clamping model of ischemia.

Protective effect of thyroxine but not parathyroidectomy on gentamicin nephrotoxicity.

Gentamicin nephrotoxicity increases renal cortex calcium and sodium and decreases renal cortex Na-K-ATPase activity. Human acute renal failure is accompanied by an increase in parathyroid hormone (PTH), a hormone that stimulates calcium uptake by tissues, and by a decrease in thyroid hormone, a hormone that increases renal cortex Na-K-ATPase activity. This study evaluated the role of extracellular calcium, PTH, and thyroxine in the pathogenesis of gentamicin nephrotoxicity. Chronically parathyroidectomized hypocalcemic rats (PTXG) given gentamicin (30 mg/kg s.c. twice daily for 8 days) were not protected from renal failure when compared with intact rats given gentamicin (NG), serum creatinine being 4.4 +/- 1.0 and 3.7 +/- 0.7 mg/dl, respectively, compared with normals (N), 1.2 +/- 0.1 mg/dl. Rats given thyroxine (10 micrograms/100 g body wt for 10 days) before and during gentamicin (PTXT4G) had a serum creatinine not significantly different from normals, 2.1 +/- 0.4 mg/dl. Plasma T4 was reduced in PTXG, NG, and PTXT4G compared with N, but the value for PTXT4G was significantly higher than for either PTXG or NG. Renal cortex Na-K-ATPase activity (mumol Pi X mg prot-1 X h-1) was lower in PTXG (2.3 +/- 0.2) and NG (2.4 +/- 0.5) compared with N (3.7 +/- 0.1), but activity was not reduced in PTXT4G (3.2 +/- 0.2) Thyroxine was protective also against gentamicin nephrotoxicity in intact rats. Clearance and excretion studies indicated that this protection did not result from an increase in glomerular filtration rate, filtered load of calcium, or urinary calcium excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of potassium depletion on tubular morphology in gentamicin-induced acute renal failure in dogs.

Potassium deprivation has recently been reported to potentiate the degree of functional impairment in a gentamicin-induced model of acute renal failure. The present study investigated the effects of two different states of potassium homeostasis on the development of cellular injury in the early stage of gentamicin nephrotoxicity in dogs. Gentamicin (15 mg. per kg. intramuscularly twice daily) was administered for 4 and 7 days to potassium-depleted or potassium-supplemented animals. The results show that potassium supplementation markedly lessens the severity of pathologic alterations induced by gentamicin. In both groups of animals, the S1 and S2 segments of the proximal tubule were the most consistently damaged regions of of the nephron. Potassium-supplemented dogs had a significantly higher number of normal proximal tubule cells than did the animals deprived of potassium and viewed 7 days after gentamicin treatment (77.3 versus 36.9 per cent; p less than 0.025). The degree of total injury to the proximal tubule was significantly higher in potassium-depleted animals than in those supplemented with potassium (59.9 versus 21.9 per cent; p less than 0.05). Only those dogs depleted of potassium prior to the administration of gentamicin had a markedly elevated plasma creatinine level of proximal tubular injury and functional impairment (r = 0.81; p less than 0.005). Potassium supplementation appears to lessen the extent of structural alterations seen in this model of gentamicin-induced acute renal failure in dogs.