Renal uptake of dimercaptosuccinic acid and glomerular filtration rate in chronic nephropathy at angiotensin converting enzyme inhibition

Renal uptake of 99mTc-dimercaptosuccinic acid is dependent on normal proximal tubule receptor-mediated endocytosis

(99m)Tc-labeled dimercaptosuccinic acid ((99m)Tc-DMSA) accumulates in the kidney cortex and is widely used for imaging of the renal parenchyma. Despite its extensive clinical use, the mechanism for renal targeting of the tracer is unresolved. Megalin and cubilin are cooperating receptors essential to the proximal tubule endocytic uptake of proteins from the glomerular ultrafiltrate. We have used megalin/cubilin-deficient mice produced by gene knockout to determine whether receptor-mediated endocytosis is responsible for the renal uptake of (99m)Tc-DMSA.

METHODS

Control or megalin/cubilin-deficient mice were injected intravenously with 0.5 MBq of (99m)Tc-DMSA or (99m)Tc-mercaptoacetyltriglycine (MAG3). Whole-body scintigrams and the activity in plasma, urine, and the kidneys were examined 6 h after injection. The size and identity of (99m)Tc-DMSA-bound proteins in urine were analyzed by fractionation by centrifugation and separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by autoradiography and mass spectrometry.

RESULTS

No renal accumulation of (99m)Tc-DMSA was identified in scintigrams of megalin/cubilin-deficient mice. The renal accumulated activity of the tracer was reduced to 11.4% (± 2.5%, n = 7) of the normal uptake in control mice, correlating with a reduction in renal megalin/cubilin expression in knockout mice to about 10% of normal. The reduced renal uptake in megalin/cubilin-deficient mice was accompanied by an increase in the urinary excretion of (99m)Tc-DMSA. Size separation of the urine by ultracentrifugation and sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that in megalin/cubilin-deficient mice an increased amount of (99m)Tc-DMSA was excreted in an approximately 27-kDa form, which by mass spectrometry was identified as the plasma protein α1-microglobulin, an established megalin/cubilin ligand.

CONCLUSION

(99m)Tc-DMSA is filtered bound to α1-microglobulin and accumulates in the kidneys by megalin/cubilin-mediated endocytosis of the (99m)Tc-DMSA protein complex. Renal accumulation of (99m)Tc-DMSA is thus critically dependent on megalin/cubilin receptor function and therefore is a marker of proximal tubule endocytic activity.

Contradictory supranormal nuclear renographic differential renal function: fact or artifact?

We previously reported contradictory supranormal nuclear renographic differential renal function in cases of unilateral hydroureteronephrosis, in which the kidneys with hydroureteronephrosis paradoxically had a greater differential function than the contralateral normal mate, based on diethylenetriaminepentaacetic acid (DTPA) nuclear renography. To evaluate whether DTPA supranormal differential function represented true hyperfunction, patients with DTPA supranormal differential function were evaluated with dimercaptosuccinic acid (DMSA) nuclear renography and the results were compared. A total of 16 patients with unilateral hydronephrosis was identified to have DTPA differential function of 53% or more. They were younger than age 1 year and had never undergone any urological surgery. In all 16 patients the DMSA differential function (mean 51.1%, range 42 to 57%) was lower than their own corresponding DTPA differential function (mean 58.3%, range 53 to 66%, p < 0.0001). In addition, the DMSA differential function was not significantly different from the intuitively anticipated mean of 50% (p = 0.48). The DTPA supranormal differential function identified in our patients was not corroborated by the DMSA differential function. With recent evidence that DMSA differential function may be a better predictor of outcome following relief of unilateral ureteral obstruction consideration should be given to using DMSA as a potentially more relevant method for differential function measurement in the setting of unilateral hydronephrosis. Conversely, until the potential deficiencies of DTPA are fully understood caution should be exercised in the interpretation of DTPA differential function in the setting of hydronephrosis.