Sustained muscle contraction induced by agonists, growth factors, and Ca2+ mediated by distinct PKC isozymes

The role of protein kinase C (PKC) in sustained contraction was examined in intestinal circular and longitudinal muscle cells. Initial contraction induced by agonists (CCK-8 and neuromedin C) was abolished by 1) inhibitors of Ca2+ mobilization (neomycin and dimethyleicosadienoic acid), 2) calmidazolium, and 3) myosin light chain (MLC) kinase (MLCK) inhibitor KT-5926. In contrast, sustained contraction was not affected by these inhibitors but was abolished by 1) the PKC inhibitors chelerythrine and calphostin C, 2) PKC-ε antibody, and 3) a pseudosubstrate PKC-ε inhibitor. GDPβS abolished both initial and sustained contraction, whereas a Gαq/11 antibody inhibited only initial contraction, implying that sustained contraction was dependent on activation of a distinct G protein. Sustained contraction induced by epidermal growth factor was inhibited by calphostin C, PKC-α,β,γ antibody, and a pseudosubstrate PKC-α inhibitor. Ca2+ (0.4 μM) induced an initial contraction in permeabilized muscle cells that was blocked by calmodulin and MLCK inhibitors and a sustained contraction that was blocked by calphostin C and a PKC-α,β,γ antibody. Thus initial contraction induced by Ca2+, agonists, and growth factors is mediated by MLCK, whereas sustained contraction is mediated by specific Ca2+-dependent and -independent PKC isozymes. G protein-coupled receptors are linked to PKC activation via distinct G proteins.