Reconstitution of protein kinase C-induced contractile Ca2+ sensitization in Triton X-100-demembranated rabbit arterial smooth muscle

Triton X-100-demembranated smooth muscle loses Ca 2+ -sensitizing responsiveness to protein kinase C (PKC) activators while intact and α-toxin-permeabilized smooth muscles remain responsive. We attempted to reconstitute the contractile Ca 2+ sensitization by PKC in the demembranated preparations. Western blot analyses showed that the content of the PKC α-isoform (PKCα) was markedly reduced and that the smooth muscle-specific protein phosphatase-1 inhibitor protein CPI-17 was not detectable, while the amount of calponin and actin still remained similar to those of intact strips. Unphosphorylated recombinant CPI-17 alone induced a small but significant contraction at constant Ca 2+ . Isoform-selective PKC inhibitors inhibited unphosphorylated but not pre-thiophosphorylated CPI-17-induced contraction, suggesting that in situ conventional PKC isoform(s) can phosphorylate CPI-17. Exogenously replenishing PKCα alone did not induce potentiation of contraction and only slowly increased myosin light chain (MLC) phosphorylation at submaximal Ca 2+ . PKC in the presence of CPI-17, but not the [T38A]-CPI mutant, markedly induced potentiation of both contraction and MLC phosphorylation. CPI-17 itself was phosphorylated. In in vitro experiments, CPI-17 was a much better substrate for PKCα than calponin, caldesmon, MLC and myosin. Our results indicate that PKC requires CPI-17 phosphorylation at Thr-38 but not calponin for reconstitution of the contractile Ca 2+ sensitization in the demembranated arterial smooth muscle.