Endothelin-1 stimulates NO production and inhibits cAMP accumulation in rat inner medullary collecting duct through independent pathways

Endothelin(A) (ET(A)) and ET(B) receptor-mediated regulation of nitric oxide synthase 1 (NOS1) and NOS3 isoforms in the renal inner medulla

AIM

Our laboratory and others have shown that endothelin (ET)-1 directly stimulates nitric oxide (NO) production in inner medullary collecting duct (IMCD) cells. The goal of this study was to determine which NO synthase (NOS) isoforms in IMCD are sensitive to ET-1, and the role of ET(A) and ET(B) receptor activation in vivo and in vitro.

METHODS

NOS enzymatic activity and NOS isoform protein expression were examined in cultured IMCD-3 cells and isolated renal inner medulla. ET(B) receptor-deficient homozygous rats (sl/sl) have elevated levels of circulating ET-1 and lack a functional ET(B) signalling pathway in kidneys, and furthermore provides a unique model to study ET(A) receptor signalling in the renal inner medulla in vivo.

RESULTS

Incubation of IMCD-3 cells with exogenous ET-1 (50 nm) resulted in ET(A)-dependent increased NOS1 protein expression in IMCD-3 cells with no effect on NOS2 or NOS3 expression. ET(B) receptor antagonism has no effect on NOS expression in IMCD-3 cells. Consistent with in vitro results, cytosolic NOS1 protein expression was significantly greater in the renal inner medulla of sl/sl rats compared with heterozygous (sl/+) controls, with no alteration in NOS3 expression. In contrast to protein expression data, NOS1- and NOS3-specific enzymatic activities decreased in the cytosolic fraction from the renal inner medulla of sl/sl compared with sl/+.

CONCLUSION

These results provide evidence that both ET(A) and ET(B) receptors regulate NOS isoform activity in the renal inner medulla and specifically support the hypothesis that ET(A) receptor activation increases NOS1 expression.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Altered collecting duct adenylyl cyclase content in collecting duct endothelin-1 knockout mice

Background

Endothelin-1 (ET-1) inhibition of vasopressin (AVP)-stimulated water reabsorption by the inner medullary collecting duct (IMCD) is associated with reduced cAMP accumulation. To determine the effect of ET-1 deficiency, AVP-stimulated cAMP responsiveness was assessed in IMCD from mice with collecting duct-specific deletion of ET-1 (CD ET-1 KO) and from control animals.

Methods

Cyclic AMP production, adenylyl cyclase (AC) mRNA, and AC protein were measured in acutely isolated IMCD.

Results

CD ET-1 KO IMCD had enhanced AVP-stimulated cAMP accumulation. Inhibition of calcium-stimulated AC using BAPTA did not prevent enhanced AVP responsiveness in CD ET-1 KO IMCD. Factors known to be modified by ET-1, including nitric oxide, cyclooxygenase metabolites, and superoxide did not affect the increased AVP responsiveness of CD ET-1 KO IMCD. Differential V2 receptor or G-protein activity was not involved since CD ET-1 KO IMCD had increased cAMP accumulation in response to forskolin and/or cholera toxin. CD ET-1 KO did not affect mRNA or protein levels of AC3, one of the major known collecting duct AC isoforms. However, the other known major collecting duct AC isoform (AC5/6) did have increased protein levels in CD ET-1 KO IMCD, although AC5 (weak signal) and 6 mRNA levels were unchanged.

Conclusion

ET-1 deficiency increases IMCD AC5/6 content, an effect that may synergize with acute ET-1 inhibition of AVP-stimulated cAMP accumulation.

Effect of Endothelin on Sodium/Hydrogen Exchanger Activity of Human Monocytes and Atherosclerosis-Related Functions

The objective of this article is to investigate the influence of endothelin-1 (ET-1) on human monocyte Na(+)/H(+) exchanger (NHE) activity and on the atherosclerosis-related monocyte functions. ET-1 caused an increase in pHi and in (22)Na influx of monocytes. A reversal of ET-1 effect on pHi was observed in the presence of the NHE1 inhibitor, cariporide. In addition, the activation of NHE1 by ET-1 was mediated via protein kinase C (PKC), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K), and NADPH oxidase. Also, a link between ET-1 and nitric oxide (NO) was observed. Furthermore, after ET-1 treatment, an increase of the adhesive capacity, the migration ability on laminin and CD36 expression of monocytes, was observed; using cariporide this increase was abolished. Our results showed that ET-1 induces a signaling pathway with the involvement of PKC, MAPK, PI3K, and NADPH oxidase where NHE1 plays a key role. ET-1 also plays a significant role in atherosclerosis-related functions of human monocytes, via NHE1 activation.

Collecting duct-specific knockout of the endothelin B receptor causes hypertension and sodium retention

Collecting duct (CD)-derived endothelin-1 (ET-1) inhibits renal Na reabsorption and its deficiency increases blood pressure (BP). The role of CD endothelin B (ETB) receptors in mediating these effects is unknown. CD-specific knockout of the ETB receptor was achieved using an aquaporin-2 promoter-Cre recombinase transgene and the loxP-flanked ETB receptor gene (CD ETB KO). Systolic BP in mice with CD-specific knockout of the ETB receptor, ETA receptor (CD ETA KO) and ET-1 (CD ET-1 KO), and their respective controls were compared during normal- and high-salt diet. On a normal-sodium diet, CD ETB KO mice had elevated BP, which increased further during high salt feeding. However, the degree of hypertension in CD ETB KO mice and the further increase in BP during salt feeding were lower than that of CD ET-1 KO mice, whereas CD ETA KO mice were normotensive. CD ETB KO mice had impaired sodium excretion following acute sodium loading. Aldosterone and plasma renin activity were decreased in CD ETB KO mice on normal- and high-sodium diets, while plasma and urinary ET-1 levels did not differ from controls. In conclusion, the CD ETB receptor partially mediates the antihypertensive and natriuretic effects of ET-1. CD ETA and ETB receptors do not fully account for the antihypertensive and natriuretic effects of CD-derived ET-1, suggesting paracrine effects of this peptide.