A microelectrode for continuous monitoring of redox activity in isolated perfused tubule segments

SGLT2 inhibition and renal urate excretion: role of luminal glucose, GLUT9, and URAT1

Inhibitors of the Na⁺-glucose cotransporter SGLT2 enhance urinary glucose and urate excretion and lower plasma urate levels. The mechanisms remain unclear, but a role for enhanced glucose in the tubular fluid, which may interact with tubular urate transporters, such as the glucose transporter GLUT9 or the urate transporter URAT1, has been proposed. Studies were performed in nondiabetic mice treated with the SGLT2 inhibitor canagliflozin and in gene-targeted mice lacking the urate transporter Glut9 in the tubule or in mice with whole body knockout of Sglt2, Sglt1, or Urat1. Renal urate handling was assessed by analysis of urate in spontaneous plasma and urine samples and normalization to creatinine concentrations or by renal clearance studies with assessment of glomerular filtration rate by FITC-sinistrin. The experiments confirmed the contribution of URAT1 and GLUT9 to renal urate reabsorption, showing a greater contribution of the latter and additive effects. Genetic and pharmacological inhibition of SGLT2 enhanced fractional renal urate excretion (FE-urate), indicating that a direct effect of the SGLT2 inhibitor on urate transporters is not absolutely necessary. Consistent with a proposed role of increased luminal glucose delivery, the absence of Sglt1, which by itself had no effect on FE-urate, enhanced the glycosuric and uricosuric effects of the SGLT2 inhibitor. The SGLT2 inhibitor enhanced renal mRNA expression of Glut9 in wild-type mice, but tubular GLUT9 seemed dispensable for the increase in FE-urate in response to canagliflozin. First evidence is presented that URAT1 is required for the acute uricosuric effect of the SGLT2 inhibitor in mice.

Mechanism of the uricosuric action of E3040, a drug used to treat inflammatory bowel disease II: study using DBA/2N mice

In the initial phase of clinical studies, it was shown that E3040, a new type of anti-inflammatory drug, reduced plasma uric acid levels. The present study describes a comparison of the excretion of uric acid in the proximal tubules of the kidney after administration of E3040 and its conjugates, sulphate and glucuronide, with that of other general uricosuric agents in DBA/2N mice. The aim of this investigation was to elucidate the mechanism for the uricosuric action of E3040. It was found that E3040 increased the excretion rate of uric acid in a dose-dependent manner, and the excretion rates following 10 and 50 mg/kg doses were significantly greater than that of the control group. The paradoxical effect observed with probenecid was not seen in the E3040 dose-response curve for the uric acid excretion rate. Neither E3040-sulphate nor E3040-glucuronide increased the excretion rate of uric acid significantly, even at a high dose, such as 200 mg/kg. In the pyrazinoic acid suppression test, the uric acid excretion rate after concomitant administration of E3040 and pyrazinoic acid was significantly higher than that after administration of pyrazinoic acid alone, and the rate after concomitant administration was 30% of the level after administration of E3040 alone. The change in the excretion rate of uric acid after concomitant administration of E3040 and pyrazinoic acid was similar to that of AA193, a selective inhibitor of the presecretory reabsorption of uric acid. From these results, it appears that E3040 may exert its uricosuric action by reducing the presecretory reabsorption of uric acid rather than increasing its secretion.

A microelectrode for continuous recording of volume fluxes in isolated perfused tubule segments

Manufacture, properties and use of a micro enzyme electrode for continuous monitoring of volume fluxes in the isolated tubule preparation is described. The specific electrode is a galactose-oxidase enzyme electrode, which can be used to detect changes in raffinose concentrations. The electrode's response to raffinose is almost linear over concentrations from 0-12 mmol/l. The electrode equally responds to galactose as to raffinose but is insensitive to other sugars, to pH changes (from 6.0-8.0), CO2 (from 1-10%) and electrolytes tested. Reducing O2 from 100 to 10% and to 1%, leads to a reduction of the reading by 10% and 30%, respectively. The reading is almost doubled when the temperature is increased from 20-40 degrees C. Furthermore, reducing agents such as uric acid and ascorbic acid interfere with the reading. If these substances and raffinose are omitted from the perfusate for isolated perfused proximal mouse tubules, the reading is identical in perfusate and collected fluid, indicating that the tubular epithelium does not produce substances in sufficient amounts to interfere with the electrode reading. After addition of 6 mmol/l raffinose to the perfusate the raffinose concentration in the collected fluid of 0.76 +/- 0.05 mm segments of straight proximal mouse tubules (perfusion rate = 3.4 +/- 0.45 nl/min) is 10.2 +/- 0.3 mmol/l, indicating a volume reabsorption of 1.5 +/- 0.3 nl/min. Peritubular application of acetazolamide reduces the volume reabsorption by 42 +/- 4%.

A microelectrode for continuous monitoring of glucose concentration in isolated perfused tubule segments

The design and the application of a micro-enzyme-electrode for continuous monitoring of glucose concentration in the isolated tubule preparation is described. The principle of the electrode is the amperometric detection of hydrogen peroxide, which is a product of the oxidation of D-glucose by glucose oxidase immobilized at the tip of a micro-electrode. The resulting current causes a voltage deflection across a resistor in series with the electrode that is correlated directly with the glucose concentration. The electrode response to glucose is almost linear over the concentration range from 0 to 12 mmol/l with a slightly diminished slope in the higher range. Other sugars (12 mmol/l raffinose, galactose, fructose, sucrose, mannitol), pH (from 6.5 to 8.0) and pCO2 (from 1 to 10 kPa) do not influence the reading. A reduction of pO2 in the test solution to 1 kPa blunts the reading. Raising the temperature from 20 degrees C to 40 degrees C leads to a pronounced increase of the voltage deflection at a given glucose concentration. Interference is observed with strongly reducing agents such as L-cysteine, ascorbic acid and uric acid. At defined conditions the electrode is well suited to measure continuously glucose concentration in the luminal fluid at the collection site of the isolated perfused tubule of the kidney. Experiments are presented which illustrate the performance of the glucose electrode in this isolated tubule set-up. Peritubular reduction of potassium concentration or the application of ouabain diminish glucose reabsorption.