Phorbol ester-induced protein phosphorylation and contraction in sphincter smooth muscle of rabbit iris.

Phorbol 12,13-dibutyrate (PDBu) was employed in studies designed to determine the role of C-kinase in muscle contraction in the iris sphincter. PDBu induced MLC phosphorylation and contraction in a dose- and time-dependent manner. Maximum responses exerted by PDBu were about 50-60% of that obtained with CCh, and were totally dependent on the presence of extracellular Ca2+. PDBu had no effect on basal IP3 levels, however it blocked the CCh-stimulated accumulation of IP3. PDBu-induced effects were potentiated by ionomycin, and inhibited by the C-kinase antagonist H-7. These results provide further evidence for the involvement of C-kinase in mediating the sustained phase of the contractile response in the iris sphincter.