1. The renal handling of purified human immunoglobulin light chain has been studied with an isolated perfused rat kidney preparation. 2. Human immunoglobulin light chain was freely filtered and largely reabsorbed. Fractional reabsorption was characteristic for each of four light chains and varied between 56% and 86%. No renal tubular maximum for human light chain was obtained. 3. Light chains at concentrations up to 10 times those seen in human myeloma were without effect on glomerular filtration rate or sodium and potassium reabsorption in experiments lasting up to 2 h. 4. Filtered and reabsorbed light chain returned ultimately to the perfusion medium, indicating a unique property of the tubular handling of this protein. None of the inhibitors tested (ouabain, frusemide, acetazolamide, probenicid) influenced light chain reabsorption. 5. The results are taken to indicate that light chain reaches the site of the transport enzyme, Na+,K+-dependent ATPase, at concentrations which vary with the nature of the light chain. This may provide a mechanism for renal damage in patients with myeloma, after prolonged exposure.
The short Synacthen test: with 1 J1.g or 250J1.g ACTH?With interest we have read the article of Wang et al,' and the letter of Price and Ross-about the use and interpretation of the short Synacthen test (SST). We think, however, that there are other issues not raised in these papers. The SST is well validated for the evaluation of the pituitary-adrenal reserve.v' In addition to the route of administration and the time of sampling, the dosage of ACTH should be discussed. It has been shown recently that the dose of 250 Ilg ACTH is an unphysiologically high stimulus that may overestimate adrenocortical reserve.>"We have compared the effects of the classical 250 Ilg ACTH 1-24 dose versus the sensitive 1 Ilg ACTH 1 24 dose stimulation on the plasma cortisol and ACTH levels in healthy control subjects (n = 8) and in patients suspected of having adrenocortical insufficiency (n = 18). Plasma cortisol, and ACTH I _ 39 (endogenous) levels were measured with luminoimmunoassays on an Immulite analyser (Diagnostic Products Corp, Los Angeles, CA, USA).
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