Correlation between mitochondrial enlargement in renal proximal tubules and microalbuminuria in rats with early streptozotocin-induced diabetes.

To clarify the ultrastructural changes in renal proximal tubules causing microalbuminuria in the early stage of diabetic nephropathy, three different groups of rats were prepared: rats with streptozotocin (STZ)-induced diabetes given no treatment (DMut; n = 7), rats with STZ-induced diabetes treated with insulin (DMt; n = 7), and non-diabetic rats injected with citrate buffer (control; n = 7). In each group, the laboratory findings, ATP content of the renal cortex, and the size of proximal tubule cells and their nuclei and mitochondria (MT) were determined. In two weeks after the start of the study, MT in renal proximal tubules showed diffuse enlargement in the DMut group as compared with those in the control group. Renal cortical ATP content, fractional sodium excretion (FENa), urinary excretion of beta 2-microglobulin and albumin were also increased significantly in the DMut group relative to the controls. In the DMt group, most of the examined parameters returned almost to normal. There were positive correlations between each of the following parameters: hyperglycemia and MT enlargement, MT enlargement and increased cortical ATP content, increased cortical ATP content and increased FENa, increased FENa and increased urinary excretion of beta 2-microglobulin and albumin. On the basis of these results, we conclude that mitochondrial enlargement, resulting from disturbed metabolism of ATP, may reduce active transport in renal proximal tubules, which, in turn, may impair reabsorption in the tubules. This would cause urinary excretion of low-molecular-weight proteins and microalbumin in the early stage of diabetic nephropathy.