No abstract

Glomerular basement membrane (GBM) was labeled in vivo by the injection of tracer amounts of tritiated proline into normal and streptozotocin-diabetic rats. Basement membrane biosynthesis and turnover were determined from the specific activities of proline and hydroxyproline in samples purified following osmotic lysis of glomeruli isolated 4 h to 12 days after injection. Peak radiolabeling of normal and diabetic GBM occurred within 24-48 h and 48-72 h, respectively, and, when corrected for differences in the serum proline specific activities, [3H]proline incorporation was greater in diabetic than in normal samples. In contrast to the subsequent time-dependent progressive decline in radiolabeling in basement membranes from normal animals, specific activities of proline and hydroxyproline in diabetic glomerular basement membrane did not change significantly over the same period of observation. Renal cortical mass and glomerular basement membrane collagen content were preserved in diabetic animals despite loss of body weight. The findings are compatible with prolongation of glomerular basement membrane turnover in experimental diabetes, and suggest that diminished degradation contributes to the accumulation of glomerular basement membrane that is characteristic of chronic diabetes.

Glomerular basement membrane (GBM) was labeled in vivo by the injection of tracer amounts of [35S]-sulfate into normal and streptozotocin-diabetic rats. The biosynthesis and turnover of sulfated glycosaminoglycans in the GBM was determined from the specific activity of [35S] after pronase digestion of basement membranes purified from glomeruli isolated 1-7 days after injection. Peak radiolabeling of both normal and diabetic GBM occurred 24 h after injection and, when corrected for differences in serum sulfate specific activities, was less in diabetic than in normal samples. The specific activity of GBM sulfate, expressed as cpm/microgram uronic acid, progressively diminished over the ensuing period of study in both normal and diabetic samples. The rate of decrease in specific activity of [35S]-labeled GBM was not significantly different in diabetic preparations compared with that in normal controls. The findings are compatible with diminished sulfation and/or production but normal turnover of glycosaminoglycans in the renal GBM in experimental diabetes.

Incubation of purified rat glomerular basement membrane (GBM) with [14C]-glucose in vitro resulted in the incorporation of [14C] into acid-precipitable radioactivity in a reaction that was time and temperature dependent. Findings with rat lens capsule basement membrane (LCBM), an anatomically distinct but chemically similar extracellular matrix, incubated for varying times at different temperatures with [14C]-glucose at constant specific activity were similar. Nonenzymatic glycosylation of basement membrane, documented by hydroxymethylfurfuraldehyde generation after incubation with unlabeled glucose, increased in proportion to the ambient glucose concentration over a range of 5--100 mM. Acid-precipitable radioactivity also increased in proportion to [14C]-glucose concentration, although this method overestimated glycosylation about 15-fold at 5--20 mM glucose and 50-fold at 50--100 mM glucose. Coupled with recent in vivo studies, these findings indicate that exposure to increased glucose concentration alters the chemistry of glomerular and other basement membranes. Since accumulation of basement membrane characterizes several of the microangiopathic sequelae of diabetes, the role of increased nonenzymatic glycosylation on the structure, function, and metabolism of basement membrane warrants investigation.

No abstract