Urokinase-derived peptides regulate vascular smooth muscle contraction in vitro and in vivo

We examined the effect of urokinase (uPA) and its fragments on vascular smo oth muscle cell contraction, Single-chain uPA inhibits phenylepherine (PE) -induced contraction of rat aortic rings, whereas two-chain uPA exerts the opposite effect. Two independent epitopes mediating these opposing activiti es were identified. Angstrom 6, a capped peptide corresponding to amino aci ds 136-143 (KPSSPPEE) of uPA, increased the EC50 of PE induced vascular con traction sevenfold by inhibiting the release of calcium from intracellular stores. Angstrom 6 activity was abolished by deleting the carboxyl-terminal Glu or by mutating the Ser corresponding to position 138 in uPA to Glu. A single-chain uPA variant lacking amino acids 136-143 did not induce vasorel axation, A second epitope within the kringle of uPA potentiated PE-induced vasoconstriction. This epitope was exposed when single-chain uPA was conver ted to a two-chain molecule by plasmin. The isolated uPA kringle augmented vasoconstriction, whereas uPA variant lacking the kringle had no procontrac tile activity. These studies reveal previously undescribed vasoactive domai ns within urokinase and its naturally derived fragments.