Effects of insulin on renal interstitial hydrostatic pressure and natriuretic response to volume expansion in diabetic rats

Diabetes mellitus (DM) is characterized by alterations in fluid balance and blood volume homeostasis. Renal interstitial hydrostatic pressure (RIHP) has been shown to play a critical role in mediating sodium and water excretion under various conditions. The objective of this study was to determine the effects of immediate and delayed initiation of insulin treatment on the restoration of the relationship between RIHP, natriuretic, and diuretic responses to acute saline volume expansion (VE) in diabetic rats. Diabetes was induced by an intraperitoneal injection of streptozotocin (STZ; 65 mg/kg body wt). Four groups of female Sprague-Dawley rats were studied: normal control, group (C), untreated diabetic group (D), immediate insulin-treated diabetic group (DI; treatment with insulin for 2 wk was initiated immediately when diabetes was confirmed, which was 2 days after STZ injection), and delayed insulin-treated diabetic group (DDI; treatment with insulin for 2 wk was initiated 2 wk after STZ injection). RIHP and sodium and water excretions were measured before and during VE (5% body wt/30 min) in the four groups of anesthetized rats. VE significantly increased RIHP, fractional excretion of sodium (FE_Na), and urine flow rate (V) in all groups of rats. Basal RIHP, RIHP response to VE (ΔRIHP), and FE_Na and V responses to VE (ΔFE_Na and ΔV) were significantly lower in the D group compared with the C group of rats. ΔRIHP was significantly higher in both DI and DDI groups compared with D group but was similar to that of the C group of rats. While in the DI group the ΔFE_Na response to VE was restored, ΔFE_Na was significantly increased in DDI compared with D group, but it remained lower than that of the C group. In conclusion, insulin treatment initiated immediately after the onset of diabetes restores basal RIHP and RIHP, natriuretic, and diuretic responses to VE; however, delayed insulin treatment restores the basal RIHP and RIHP response to VE but does not fully restore the natriuretic response to VE.