INCREASED MICROALBUMINURIA IN DIABETIC RATS IS INDEPENDENT OF ANGIOTENSIN-II OR GLOMERULAR PROTEOGLYCAN SYNTHESIS

Increased microalbuminuria is seen early in rats with both streptozotocin-induced and genetic (Bio-Breeding) diabetes. This study examines the roles of angiotensin II-dependent mechanism(s) and sulfation of glomerular proteoglycans in this phenomenon, as both processes have been implicated by several lines of circumstantial evidence. Anionic sites in the glomerular basement membrane, attributed to the presence of heparan sulfate, were quantitated by polyethylenimine staining at 15, 21, and 70 days of diabetes in rats treated with streptozotocin, with or without insulin, and at 70 days in the Bio-Breeding rats. All diabetic rats developed increased microalbuminuria: control, 0.08 +/- 0.03 mug/mL glomerular filtration rate, xBAR +/- SD; streptozotocin without insulin at 15 days, 0.92 +/- 0.06 mug/mL (p < 0.05); streptozotocin with insulin at 21 days, 0.61 +/- 0.37 mug/mL (p < 0.05 vs. control). At 70 days, both the Bio-Breeding and the streptozotocin rats sustained their microalbuminuria to the same degree (p < 0.05 vs. control). Enalapril (250 mg/L) in the drinking water of diabetic animals did not reduce the microalbuminuria. Although the polyethylenimine-stained heparan sulfate sites decreased significantly in the streptozotocin rats, they remained unchanged in the Bio-Breeding rats. To determine the cause of reduced heparan sulfate staining, the in vitro synthesis and degree of sulfation of proteoglycans by glomeruli isolated from control and streptozotocin diabetic rat kidneys were compared. The amount of heparan sulfate synthesis and degree of sulfation were unchanged in diabetic glomeruli, although lower incorporation into the extracellular matrix and greater secretion into the medium were noted. These effects were partially normalized by insulin therapy. Neither glomerular cell response to angiotensin II nor altered synthesis of heparan sulfate can account for increased microalbuminuria in the diabetic rat.