We investigated the mechanisms that are responsible for the basal release of endothelium-derived relaxing factor (EDRF), which is likely to be identical with nitric oxide, in the intact coronary circulation. The increase in cGMP content of platelets passing through the coronary bed of the isolated rabbit heart was used as an index of EDRF release. Platelet cGMP content after passage through the heart under control conditions (flow rate of 20 ml/min) amounted to 0.50±0.10 pmol/mg protein. Inhibition of endothelial nitric oxide synthesis by 30 μMNG-nitro-l-arginine (L-NNA) reduced this amount by more than 60%. Increasing flow rate from 20 ml/min to 40 and 60 ml/min led to flow-dependent dilation as reflected by the subsequent drop in perfusion pressure after an initial rise. The flow-dependent dilation was associated with a significant increase in the normalized platelet cGMP content. L-NNA abolished completely both the flow-dependent dilation and the increase in platelet cGMP content. Increasing shear stress by a strong vasoconstriction (1 nM endothelin-1) at constant flow was also accompanied by a 2.5-fold increase in platelet cGMP content. To investigate whether mechanical forces applied to the vascular wall by the myocardial contraction cycle were also a stimulus for EDRF release, cardiac arrest was induced by a continuous infusion of mepivacaine (final concentration, 0.02%). Under these conditions, a decrease in platelet cGMP content comparable to that after nitric oxide synthesis inhibition was observed in the arrested heart. The reappearance of mechanical activity after washout of mepivacaine was associated with the recovery of platelet guanylate cyclase activation, indicating the contribution of the mechanical activity on basal EDRF release. To study further the effects of wall deformation on EDRF release, experiments were performed in isolated saline-perfused rabbit arterial segments. Rhythmic squeezing of the segments led to a significant increase in EDRF release as assessed by assay of guanylate cyclase activity. These data indicate that at least two principles contribute to the basal agonist-independent release of EDRF in the intact coronary bed. Besides the continuously acting wall shear stress imposed by the streaming fluid, the periodic compression of intramyocardial vessels stimulates the formation of EDRF. This EDRF formation may result from the high shear stress imposed on the endothelial lining by the periodic diameter reduction and from the direct deformation of the endothelium.