This animal model demonstrates that an increase in lysosomal turnover and hence urine NAG activity, occurs when increased protein is presented to the tubular cells. Urine NAG activity is thus a measure of altered function in the renal tubules and not simply an indicator of damage.

SUMMARY Differential histological staining of various tissues is achieved in epon, araldite or glycol methacrylate sections 0·25–0·5 μ thick, from human, guinea‐pig and rat origin. The staining procedure is as follows: 1% methylene blue in 1% borax 15–30 sec at 70°C; wash in hot water; 2% basic fuchsin in distilled water 1–2 min at room temperature; wash and mount in DPX. Results: with all tissues: nuclei pale violet, cytoplasm blue. Special histological components: collagen red, myelin deep blue, elastin deep violet, axoplasm pale blue. Although of general utility the method is especially useful for peripheral nerve and blood vessels.

We examined glomerular basement membrane anionic site distribution identified by cationic gold in seven patients with insulin-dependent and four patients with non-insulin-dependent diabetes mellitus, presenting a spectrum of clinical and glomerular changes. Anionic sites were investigated by pretreatment of tissue with glycosaminoglycan-degrading enzymes prior to cationic gold staining. The distribution of chondroitin sulphate proteoglycans--a previously unrecognized glomerular basement membrane component--and type IV collagen was examined by immunoelectron microscopy to identify structural changes in the basement membrane. Findings were compared with those of non-diabetic patients showing minor proteinuria and morphologically normal glomerular basement membranes. Two patients, originally diagnosed as having diabetic nephropathy were also examined at 19 weeks and 5 years after renal transplantation. Characteristic redistribution of type IV collagen and chondroitin sulphate proteoglycans was noted in thickened glomerular basement membrane segments (> 400 nm) of diabetic patients and those with renal transplants. Extension of anionic sites deep into the glomerular basement membrane at pH 2.5, together with loss of interna sites at pH 5.8 is unique to diabetic nephropathy. Reduced charge density was apparent in some patients due to thickening of the glomerular basement membrane, although the number of anionic sites per unit length of membrane was actually increased. Thus, charge aberration in diabetic nephropathy is due to displacement rather than loss of anionic sites. Removal of more than 90% of these sites by heparitinase, confirms their association with heparan sulphate proteoglycans. Similar derangement of anionic sites in all patients with diabetic nephropathy irrespective of the degree of proteinuria, suggests that a heparan sulphate proteoglycan-related charge barrier plays a minor role in controlling permeability of the diabetic glomerular basement membrane.

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