Induced Contraction in Intact Taenia coli

Vanadate-Induced Contraction in Intact Taenia coli 

Because earlier studies in this and other laboratories showed that inhibitors of tyrosine kinase activity suppressed contraction of smooth muscle (Section II), we suspected that vanadate-induced contraction might be due to its efficacy as an inhibitor of tyrosine phosphatase activity. Our experiments showed that treatment of taenia coli with 1.5 mM vanadate induced a tonic contraction that was associated with enhanced tyrosine phosphorylation of at least three substrates (M 85,000, 116,000, and 205,000 Figs. 2A and 2B). Maximal force attained in response to vanadate was about one-fourth to one-third of the maximal force attained during the phasic contraction elicited by stimu- 

JOSEPH DI SALVO, NIHAL KAPLAN, AND LORI A. SEMENCHUK 

Accordingly, the maintenance of tyrosine phosphorylation during relaxation induced by chelation of extracellular Ca + with EGTA probably reflects persistent inhibition of tyrosine phosphatase activity by the continued presence of vanadate. The tyrosine-phos-phorylated substrates may be poised to promote Ca + entry so that replacement of the medium with fresh Ca2+-PSS allows for rapid entry of Ca2+ and a large spontaneous transient contraction. In contrast, no spontaneous contraction occurred in the experiment with La3+ and vanadate because La3+ blockade of the Ca + channels persists when the medium is replaced with fresh Ca2+-PSS (Fig. 2D). Based on this interpretation of the data, further studies were performed to more directly test the hypothesis that protein tyrosine phosphorylation is an important mechanism for regulating [Ca +Jj in smooth muscle cells. 

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A. Vanadate-Induced Contraction in Intact Taenia coli... 

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