Impact of angiotensin-converting enzyme inhibition on renal cortical nitrotyrosine content during increased extracellular glucose concentration.

OBJECTIVES: Experiments evaluated the hypothesis that angiotensin-converting enzyme (ACE) inhibition suppresses hyperglycemia-induced nitrotyrosine (NT) production in the renal cortex. DESIGN AND METHODS: Rats were untreated (UNTR, n = 6) or received the ACE inhibitor enalapril (20 mg/kg/day; ENAL, n = 6) for 2 weeks. Renal cortical slices were incubated for 90 min in media containing 5 (normal) or 20 mmol/L (high) glucose. Superoxide anion (O2*-) and nitrate + nitrite (NO(X)) levels were measured in the media. Superoxide dismutase (SOD) activity and NT content were measured in the tissue homogenate. RESULTS: In the UNTR group, high glucose increased O2*- and NO(X) production by the renal cortex (P < 0.05 vs. normal glucose). Likewise, NT content and SOD activity of the renal cortex augmented (P < 0.05 vs. normal glucose). In the ENAL group, O2*- production and NT content were glucose-insensitive, but high glucose exerted an exaggerated impact on NO(X) production and SOD activity (P < 0.01 vs. UNTR in high glucose). CONCLUSION: Accelerated NT content in the renal cortex during high-glucose conditions was prevented by ACE inhibitor treatment. It was suggested that, apart from its anti-hypertensive effect, the mechanism of suppressed NT degradation in the renal cortex by the ACE inhibitor enhances both O2*- degradation per se and antioxidative effects including SOD activation.

Relative contributions of Ca2+ mobilization and influx in renal arteriolar contractile responses to arginine vasopressin.

Experiments addressed the hypothesis that afferent and efferent arterioles differentially rely on Ca2+ influx and/or release from intracellular stores in generating contractile responses to AVP. The effect of Ca2+ store depletion or voltage-gated Ca2+ channel (VGCC) blockade on contractile responsiveness to AVP (0.01-1.0 nM) was assessed in blood-perfused juxtamedullary nephrons from rat kidney. Depletion of intracellular Ca2+ stores by 100 microM cyclopiazonic acid (CPA) or 1 microM thapsigargin treatment increased afferent arteriolar baseline diameter by 14 and 21%, respectively, but did not significantly alter efferent arteriolar diameter. CPA attenuated the contractile response to 1.0 nM AVP by 34 and 55% in afferent and efferent arterioles, respectively (P = 0.013). The impact of thapsigargin on AVP-induced afferent arteriolar contraction (52% inhibition) was also less than its effect on the efferent arteriolar response (88% inhibition; P = 0.046). In experiments probing the role of the Ca2+ influx through VGCCs, 10 microM diltiazem evoked a 34% increase in baseline afferent arteriolar diameter and attenuated the contractile response to 1.0 nM AVP by 45%, without significantly altering efferent arteriolar baseline diameter or responsiveness to AVP. Combined treatment with both diltiazem and thapsigargin prevented AVP-induced contraction of both vascular segments. We conclude that Ca2+ release from the intracellular stores contributes to the contractile response to AVP in both afferent and efferent arterioles but is more prominent in the efferent arteriole. Moreover, the VGCC contribution to AVP-induced renal arteriolar contraction resides primarily in the afferent arteriole.

High glucose augments arginase activity and nitric oxide production in the renal cortex.

To clarify the interaction between arginase and nitric oxide (NO) production in the kidney with normal and high glucose levels, renal cortical slices from male Sprague-Dawley rats were incubated in Hank's solution containing various concentrations of L-norvaline (Nval; an arginase inhibitor), 500 U/mL superoxide dismutase, and either 5 mmol/L (normal) or 20 mmol/L (high) glucose (n = 5 per group). Incubation with Nval increased renal cortical NOX (nitrite + nitrate) production dose-dependently, indicating competition between arginase and NO synthase (NOS) for the substrate (L-arginine). In the basal condition without Nval, high glucose also increased NO(X) production to a rate 3 times that observed during incubation with normal glucose (P < .01). This effect of high glucose was not altered by Nval. Rather, the effects of high glucose and Nval were additive, indicating that the activity of NOS per se is enhanced by high glucose. Direct assay of arginase and NOS activities confirmed stimulation of both enzymes under the high glucose condition (P < .05, P < .01, v normal glucose, respectively). However, high glucose did not change the amount of L-arginine present in renal cortical slices. These data reveal that arginase competes with NOS for L-arginine in the renal cortex, and that high glucose increases the activity of both enzymes without affecting the amount of substrate. These results suggest that increased NOS activity, rather than altered substrate availability, may be the principal factor underlying increased NO synthesis in diabetic kidneys.

Exaggerated impact of ATP-sensitive K(+) channels on afferent arteriolar diameter in diabetes mellitus.

Experiments were performed to determine the involvement of ATP-sensitive K(+) channels (K(ATP) channels) in the renal afferent arteriolar dilation that occurs during the hyperfiltration stage of insulin-dependent diabetes mellitus (IDDM). IDDM was induced in rats by streptozotocin (STZ) injection, and adequate insulin was provided to maintain moderate hyperglycemia. Sham rats received vehicle treatments. Two weeks later, afferent arteriolar function was assessed using the in vitro blood-perfused juxtamedullary nephron technique. Baseline afferent arteriolar lumen diameter was greater in STZ rats (25.9 +/- 1.1 microm) than in sham rats (20.8 +/- 1.0 microm). Glibenclamide (3 to 300 microM) had virtually no effect on afferent arterioles from sham rats; however, this K(ATP) antagonist caused concentration-dependent afferent arteriolar constriction in kidneys from STZ-treated rats, restoring lumen diameter to 20.6 +/- 1.7 microm (P > 0.05 versus sham baseline). In both groups of rats, pinacidil (a cyanoguanidine K(ATP) agonist; 0.3 to 300 microM) evoked concentration-dependent afferent arteriolar dilation, indicating the functional expression of K(ATP) channels; however, lumen diameter was increased by 73% in STZ kidneys but only by 48% in sham kidneys. The gliben-clamide-sensitive afferent arteriolar dilator response to 1 microM PCO-400 (a benzopyran K(ATP) agonist) was also accentuated in STZ kidneys. These observations suggest that increases in both the functional availability and basal activation of K(ATP) channels promote afferent arteriolar vasodilation during the early stage of IDDM, changes that likely contribute to the etiology of diabetic hyperfiltration.