The main role of aldosterone is to maintain body sodium homeostasis by promoting sodium reabsorption in the collecting ducts of the kidney. In the cardiovascular system aldosterone may be harmful in a number of disease states by inducing fibrosis and vascular dysfunction. Novel results from several laboratories suggest that aldosterone also has beneficial effects in the cardiovascular system by stimulating the production of nitric oxide from the endothelium. We tested the effect of aldosterone on vascular function in microdissected, perfused rabbit renal afferent arterioles. Aldosterone per se had no effect on vascular internal diameter in concentrations from 10-10-10-5 mol/L, but after exposure to aldosterone the ability of 100 mM KCl to induce vascular contraction was abolished. This inhibitory effect of aldosterone was observed from less than 1 picomole/L. The inhibitory effect was significant after 5 min and maximal after 20 min, it was fully reversible, and it was abolished by inhibition of mineralocorticoid receptor with spironolactone (10-7 mol/L), but not by inhibition of the glucocorticoid receptor with mifepristone. The effect of aldosterone on vasoconstriction was not blocked by inhibition of transcription by Actinomycin D (10-6 mol/L). Expression of mineralocorticoid receptors and 11-beta-hydroxy-steroid-dehydrogenase 2 was demonstrated by RT-PCR and immunohistochemistry on rat preglomerular renal vasculature and cultures of rat renal vascular smooth muscle cells. Aldosterone did not affect depolarisation-mediated increases in calcium concentration in non-perfused rabbit afferent arterioles as measured by fluorescence-microscopy with Fura-2. Inhibition of phosphatidylinositol (PI)-3 kinase with LY 294002 (3 x 10-6 mol/L) restored sensitivity to K+ in the presence of aldosterone and the catalytic PI-3 kinase subunit p110 ? was demonstrated in afferent arterioles by immunohistochemistry. Inhibition of nitric oxide formation by L-NAME (10-4 mol/L) or inhibition of the NO-target in vascular smooth muscle, the soluble guanylyl cyclase, with ODQ also restored K+-induced vasoreactivity in the presence of aldosterone. Inhibition of heat shock protein 90 with geldanamycin blocked the effect of aldosterone. As a further indication of involvement of the NO pathway, the effect of the NO-donor sodium nitroprusside was similar to that of aldosterone Liu et al. (2003) made similar observations using a different vascular preparation from a different species, and with different agonists. They used rat aortic rings which were mounted in a static setup and preconstricted with phenylephrine. Incubation of rings for 2 minutes with aldosterone led to a concentration-dependent vasodilation, which was significant at 10-12 M aldosterone and maximal at 10-11 M. and which was blocked by spironolactone and LY 290002 . These authours also reported a doubling of PI-3 kinase activity in bovine aortic endothelial cells after exposure to aldosterone, as well as an increase in NO-formation as measured by fluorescence microscopy with DAF fluorophore. We conclude from these results that aldosterone inhibits vasoconstriction by a rapid non-genomic effect. The effect is mediated by the classical mineralocorticoid receptor, and it involves heat shock protein 90, phosphatidylinositol (PI)-3 kinase, endothelial nitric oxide synthase, and liberation of nitric oxide. Based on the results we suggest that healthy individuals with a functioning endothelium the detrimental effects of aldosterone on cardiovascular function are balanced by activation of the potentially beneficial effect of NO. On the other hand, in situations with endothelial dysfunction, such as congestive heart failure and hypertension, the negative effects of aldosterone are unopposed and inhibition of aldosterone may be warranted.