We investigated the molecular mechanisms of sodium vanadate (vanadate)-induced nitric oxide (NO) production. Exposure of bovine lung microvascular cells (BLMVEC) to vanadate increased the release of biologically active NO in endothelium/smooth muscle cocultures, as measured by the accumulation of its surrogate marker, cGMP. This release was sensitive to NO synthase (NOS) inhibition and was greater than that observed with ionomycin. Although calcium chelators (BAPTA, EGTA) inhibited basal and ionomycin-induced NO production, they failed to inhibit vanadate-induced NO release. Moreover, in the absence of calcium/calmodulin, cell lysates from vanadate-treated cells exhibited greater NOS activity compared with control cells. Vanadate activates the phosphoinositide3-kinase (PI3-K)/Akt pathway, which is known to increase endothelial NOS (eNOS) activity by direct phosphorylation of Ser-1179. Treatment of BLMVEC with vanadate resulted in phosphorylation of both Akt and endothelial NOS. In addition, wortmannin, a PI3-K inhibitor, blocked both the vanadate-induced phosphorylation of eNOS and the increase in cGMP accumulation. Similarly, adenovirus-mediated gene transfer of an activation deficient form of Akt (AA-Akt) blocked the release of NO brought about by vanadate. To further investigate the mechanism of action of vanadate, eNOS was immunoprecipitated and its association with proteins that alter eNOS activity was tested. Immunoblots demonstrated that the eNOS-caveolin interaction remained unaffected by vanadate, whereas vanadate promoted recruitment of the 90-kDa heat shock protein (hsp90) to eNOS. We conclude that vanadate causes NO release via a mechanism that involves Akt-induced eNOS phosphorylation and increased binding of the activator protein hsp90 to eNOS.