BACKGROUND: The endothelium acts as the sensor of shear stress and as the mediator of flow-induced changes in vessel tone and structure. The purpose of this study was to delineate the signal transduction pathway of flow-induced release of endothelium-derived relaxing factor (EDRF). METHODS AND RESULTS: We used a shear stress apparatus (a modified cone-plate viscometer) to expose cultured endothelial cells to a well-defined laminar fluid flow. Confluent bovine aortic endothelial cells (BAECs) were subjected to varying levels of shear stress, and intracellular cyclic GMP (cGMP) in the BAECs was measured by radioimmunoassay. After 60 seconds of laminar fluid flow, BAEC cGMP increased by 300% from basal levels (from 0.54 to 1.70 pmol/mg protein, P < 0.05). The elevation in intracellular cGMP was proportional to the intensity of shear stress within a physiological range up to 40 dynes/cm2. This increase in cGMP was abrogated by L-N-methyl-arginine (the competitive antagonist of nitric oxide [NO] synthase), indicating that the flow-induced activation of soluble guanylate cyclase was mediated by autocrine NO production. Furthermore, a potassium channel antagonist, tetraethylammonium ion (TEA [3 mmol/L]) and a G(i) or G(o) protein inhibitor, pertussis toxin (100 ng/mL) also blocked the flow-induced increase in cGMP. By contrast, calcium ionophore or atrial natriuretic peptide caused elevations of cGMP that were not affected by TEA or pertussis toxin. CONCLUSIONS: These findings indicate that shear stress elevates endothelial cGMP via an NO-dependent mechanism. The effect of shear stress is mediated by a unique signal transduction pathway that is coupled to a pertussis toxin-sensitive G protein and that requires the activity of an endothelial potassium channel.