No abstract

TRPV4, a nonselective cation channel of the transient receptor potential (TRP) family, is gated by hypotonicity. Expression of TRPV4 mRNA has been detected in the circumventricular organs of the brain responsible for sensing systemic tonicity and in the kidney distal convoluted tubule (DCT), among other sites. No analysis of TRPV4 expression at the protein level has been undertaken and no systematic analysis of expression of this channel has been reported in the kidney. Via RNAse protection assay and immunoblotting, abundant expression of TRPV4 was detected in the cortex, medulla, and papilla. The expression pattern of TRPV4 was characterized in both rat and mouse kidney, which revealed similar patterns of immunoreactivity. TRPV4 expression was absent from the proximal tubule (PT) and descending thin limb (DTL), whereas the strongest expression was observed in the ascending thin limb (ATL). The thick ascending limb (TAL) was strongly positive as was the DCT and connecting tubule. Importantly, the water-permeant cells of the macula densa were unstained. Moderate TRPV4 expression was noted in all collecting duct portions and in papillary epithelium; intercalated cells (type A) exhibited a particularly strong signal. In all positive segments, TRPV4 expression was concentrated at the basolateral membrane. Therefore, TRPV4 is expressed in only those nephron segments that are constitutively (i.e., ATL, TAL, and DCT) or conditionally (i.e., collecting duct) water impermeant and where generation of a substantial transcellular osmotic gradient could be expected. TRPV4 expression is absent from nephron segments exhibiting constitutive water permeability and unregulated apical aquaporin expression (i.e., PT and DTL). These data, although circumstantial, are consistent with a role for TRPV4 in the response to anisotonicity in the mammalian kidney.

The pathogenesis of renal scarring in chronic cyclosporin nephropathy is unknown. In this study, we evaluated the effects of renin-angiotensin system blockade by enalapril and losartan in a salt-dependent model of cyclosporin-associated chronic tubulointerstitial fibrosis (TIF). Rats kept on normal or low-salt diet were given cyclosporin, cyclosporin+enalapril, cyclosporin+losartan, cyclosporin+enalapril#losartan, or vehicle for 14 and 28 days. Cyclosporin reduced glomerular filtration rate (GFR) in rats fed either diet, but only salt-depleted animals developed significant TIF. Cyclosporin also impaired renal concentrating ability and caused tubular enzymuria. Renin-angiotensin system blockade decreased blood pressure (BP) and promoted afferent arteriolar vasodilatation. Losartan reduced plasma renin activity and prevented cyclosporin-induced increment of cortical alpha 1(I) procollagen mRNA. Renin-angiotensin blockade did not improve GFR and tubular function; however, it strikingly prevented TIF development, even in presence of very low BP. Rats treated with cyclosporin, hydralazine, and furosemide achieved BP values similar to losartan or enalapril groups, but there was no protection against interstitial fibrosis development. These results suggest that cyclosporin-related chronic interstitial injury is mediated by angiotensin II and that the mechanisms promoting the interstitial scarring can be dissociated from glomerular and tubular dysfunction in cyclosporin nephropathy.

Nitric oxide (NO) has been implicated in the pathogenesis of renal hemodynamic changes in diabetes mellitus. However, the contribution of nitric oxide synthase (NOS) isoforms to intrarenal production of NO in diabetes remains unknown. To explore the role of NOS1 in the control of renal hemodynamics in diabetes, we assessed renal responses to inhibition of NOS1 with S-methyl-L-thiocitrulline (SMTC; administered into the abdominal aorta) in moderately hyperglycemic streptozotocin-diabetic rats (D) and their nondiabetic (C) and normoglycemic diabetic counterparts. The contribution of other NOS isoforms was also evaluated by assessing the responses to nonspecific NOS inhibition [N(G)-nitro-L-arginine methyl ester (L-NAME)] in SMTC-treated diabetic rats. The number of NOS1-positive cells in macula densa of D and C kidneys was also evaluated by immunohistochemistry. D rats demonstrated elevated glomerular filtration rate (GFR) compared with C. SMTC (0.05 mg/kg) normalized GFR in D but had no effect in C. SMTC-induced reduction of renal plasma flow (RPF) was similar in C and D. Normoglycemic diabetic rats demonstrated blunted renal hemodynamic responses to NOS1 inhibition compared with hyperglycemic animals. Mean arterial pressure was stable in all groups. L-NAME induced a further decrease in RPF, but not in GFR, in D rats treated with SMTC. Immunohistochemistry revealed increased numbers of NOS1-positive cells in D. These observations suggest that NOS1-derived NO plays a major role in the pathogenesis of renal hemodynamic changes early in the course of diabetes. NOS1 appears to be the most important isoform in the generation of hemodynamically active NO in this condition.

Experimental diabetes is associated with complex changes in renal nitric oxide (NO) bioavailability. We explored the effect of diabetes on renal cortical protein expression of endothelial NO synthase (eNOS) with respect to several determinants of its enzymatic function, such as eNOS expression, membrane localization, phosphorylation, and dimerization, in moderately hyperglycemic streptozotocininduced diabetic rats compared with nondiabetic control rats and diabetic rats with intensive insulin treatment to achieve near-normal metabolic control. We studied renal cortical expression and localization of caveolin-1 (CAV-1), an endogenous modulator of eNOS function. Despite similar whole-cell eNOS expression in all groups, eNOS monomer and dimer in membrane fractions were reduced in moderately hyperglycemic diabetic rats compared with control rats; the opposite trend was apparent in the cytosol. Stimulatory phosphorylation of eNOS (Ser1177) was also reduced in moderately hyperglycemic diabetic rats. eNOS colocalized and interacted with CAV-1 in endothelial cells throughout the renal vascular tree both in control and moderately hyperglycemic diabetic rats. However, the abundance of membrane-localized CAV-1 was decreased in diabetic kidneys. Intensive insulin treatment reversed the effects of diabetes on each of these parameters. In summary, we observed diabetes-mediated alterations in eNOS and CAV-1 expression that are consistent with the view of decreased bioavailability of renal eNOS-derived NO.