Sphingomyelinase induces endothelium-dependent vasorelaxation through two mechanisms without cytosolic Ca²⁺ elevation in endothelial cells in situ

Abstract

Sphingomyelinase (SMase) is a key enzyme which produces lipid mediator, ceramide. SMase induced endothelium-dependent vasorelaxation, which was inhibited not only by LNMMA but also by high K⁺-depolarization. Fluorometric measurement of NO and [Ca²⁺]i revealed that SMase produces NO without [Ca²⁺]i elevation in endothelial cells in situ, in which only eNOS, but not iNOS, was expressed. Confocal images of immunostaining showed SMase induced dissociation of eNOS from plasma membrane caveolae and translocation to cytosol in endothelial cells in situ. Both microelectrical and fluorometric measurements of the membrane potential demonstrated that SMase induced membrane hyperpolarization in smooth muscle cells in situ of arterial strips with endothelium, but not in smooth muscle cells without endothelium and endothelial cells per se. SMase and bradykinin increased ceramide level of endothelial cells in situ. These results suggest that SMase induces endothelium-dependent vasorelaxation, as mediated by both endothelial NO production and EDH of smooth muscle, and is accompanied with ceramide production but not [Ca²⁺]i elevation. [Jpn J Physiol 54 Suppl:S77 (2004)]