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Figure 2 Vasorelaxation response of preconstricted aortic rings to increasing concentrations of ACH, PRA and SIM. Responses to ACH and PRA were obliterated following removal of the endothelial cells. Responses to SIM were seen only with the open lactone ring form (OR) of the compound (responses to SIM in the closed lactone ring preparation (CR) were no different from those produced by the DMSO vehicle used to solubilize the compound). (*) Indicates difference from responses to all other agents or pretreatment. p < 0.05.