REDUCED PROXIMAL TUBULE ANGIOTENSIN-II RECEPTOR EXPRESSION IN STREPTOZOTOCIN-INDUCED DIABETES-MELLITUS

Diabetes mellitus is characterized by alterations in the intrarenal renin-angiotensin system, including decreases in glomerular angiotensin II (Ang II) receptor density. Since Ang II regulates proximal tubule transport function, the present studies examined whether diabetes altered expression of proximal tubule receptors. In basolateral membranes from 14 day streptozotocin-induced diabetic rats, specific binding of I-125 Ang II was decreased to 53 +/- 8% of control (3.2 +/- 0.5 vs. 1.5 +/- 0.2 fmol/mg protein; N = 7; P < 0.02). Similarly, in proximal tubule brush border membranes from diabetic animals, specific binding was decreased to 63 +/- 11% of control (1.1 +/- 0.2 vs. 0.6 +/- 0.1 fmol/mg protein; N = 9; P < 0.05). Concomitant insulin treatment reversed the decrease in specific binding of I-125 Ang II to basolateral membranes (109 +/- 26% of control; N = 3) and to brush border membranes (85 +/- 17% of control; N = 6). In order to determine if changes in expression of type-1 Ang II receptors (AT(1)R) accompanied the changes in binding, quantitative polymerase chain reaction of AT(1)R mRNA was performed and expressed as the ratio of the amplified AT(1)R to that of an Msc1/Msc1 internal deletion mutant and normalized to that of beta-actin. In total RNA from proximal tubule suspensions of diabetic animals, AT(1)R mRNA expression decreased by 38% (21 +/- 3 vs. 13 +/- 2 cpm AT(1)R/cpm deletion mutant/cpm beta actin/10(6); N = 4; P < 0.0025). Insulin treatment reverted AT(1)R mRNA expression to control levels (22 +/- 3 cpm AT(1)R/cpm deletion mutant/cpm beta actin/10(6); P < 0.001 compared to the untreated group). Since both AT(1a)R and AT(1b)R exist in rat kidney, restriction digests of the PCR-amplified products were performed with Hae III, which indicated the presence of both subtypes in rat proximal tubule. Expression of both subtypes decreased in diabetic rats. Therefore, in rats with untreated streptozotocin-induced diabetes, both proximal tubule Ang II binding and AT(1)R mRNA levels decrease. Insulin treatment reverses these abnormalities. These findings suggest that, similar to glomeruli, the diabetic milieu leads to decreased expression of proximal Ang II receptors. This defect in receptor expression may contribute to abnormalities in volume regulation and acid/base balance in diabetes.