Abstract P328: Angiotensin II Inhibits the High Salt-induced Production of Sphigosine-1-phosphate in the Renal Medulla

Abstract

We have previously shown that sphigosine-1-phosphate (S1P) produces natriuretic effects via activation of S1P receptor 1 in the renal medulla and that this natriuretic effect may be through inhibition of epithelial sodium channel. The present study examined the expression of the enzymes that produce S1P in the renal medullary tissue and tested the hypothesis that angiotensin II (ANG II) reduces the expression of S1P-producing enzyme and thereby the levels of S1P in the renal medulla. Male adult C56BL/6 mice, 10-12 weeks old, were treated with a low salt diet (LS, 0.4%), high salt diet (HS, 4% NaCl) or HS + ANG II (600ng/kg/min, S.C.) for 10 days. A high salt diet increased the level of S1P, whereas ANG II significantly inhibited the HS-induced increase of S1P levels in the renal medullary tissue. The levels of S1P were 6.6 ± 0.34, 11.4 ± 1.33 and 3.5 ± 0.49 pmol/mg of tissue in LS, HS and HS + ANG II group, respectively. There were no difference in the levels of sphingosine kinase 1 (SPHK1), the enzyme that produces S1P by phosphorylating sphingosine, among the different groups of mice by Western blot analysis. However, a high salt diet increased the protein levels of acid ceramidase (ACDase), an upstream enzyme that produces sphingosine, the substrate for SPHK1. This HS-induced increase in ACDase was inhibited by ANG II. The relative protein levels of ACDase were 1.0 ± 0.07, 1.4 ± 0.07 and 0.17 ± 0.11 in LS, HS and HS + ANG II group, respectively. These results demonstrated that a high salt diet increased the levels of S1P in the renal medulla, probably by increasing the level of one of the S1P-producing enzymes ACDase, and that ANG II reduced the levels of ACDase and S1P in the renal medulla. Given the diuretic effect of S1P, ANG II-induced reduction of S1P production in the renal medulla may be a mechanism contributing to the sodium retention and hypertension associated with excessive ANG II. (Support: NIH grant HL89563)