Does a low-salt diet exert a protective effect on endothelial function in normal rats?

Sodium restriction is often used as an adjunct in the treatment of conditions characterized by endothelial dysfunction, such as hypertension and heart or kidney disease. However, the effect of sodium restriction on endothelial function is not known. Therefore, male Wistar rats were studied after a fixed salt diet had been maintained (low-salt group: 0.05% NaCl, n = 10; normal-salt group: 0.3% NaCl, n = 10) for 6 weeks. Blood pressure and sodium excretion values were measured once a week. Subsequently the rats were killed, the aorta was removed, and rings were cut. Endothelium-independent (sodium nitrite (SN)) and endothelium-dependent (acetylcholine (ACh)) vasodilator responses were assessed in the presence of indomethacin (a cyclo-oxygenase inhibitor) and in the presence or absence of NG-monomethyl-L-arginine (L-NMMA; a competitive inhibitor of nitric oxide (NO) synthase). Endothelium-independent vasodilatation was not different for the two salt groups. Endothelium-dependent vasodilatation, on the other hand, was different. The response to ACh was almost completely abolished by L-NMMA in the normal-salt group, whereas vasodilatation was partially preserved during L-NMMA in the low-salt group. Accordingly, the L-NMMA-sensitive contribution to ACh-dependent vasodilatation was smaller in the low-salt group. Thus, salt restriction induced a non-NO and non-prostaglandin-dependent vasodilating pathway. By exclusion this could be endothelium-derived hyperpolarizing factor, a pathway of vasculoprotective potential. Accordingly, the relative contributions of the different vasoactive endothelial pathways were affected by salt intake. Further research will be needed to clarify the nature and importance of this non-NO, non-prostaglandin-dependent pathway in the clinical setting as well.