POSSIBLE INVOLVEMENT OF A SMALL G-PROTEIN SENSITIVE TO EXOENZYME C3 OF CLOSTRIDIUM-BOTULINUM IN THE REGULATION OF MYOFILAMENT CA2+ SENSITIVITY IN BETA-ESCIN SKINNED SMOOTH-MUSCLE OF GUINEA-PIG ILEUM

The effects of exoenzyme C3 of Clostridium botulinum on Ca2+- and drug-induced tension developments were investigated in beta-escin skinned smooth muscle of guinea pig ileum to test the involvement of a small G-protein in the regulation of myofilament Ca2+ sensitivity. C3 is known to ADP-ribosylate the rho p21 family of small G-proteins. Treatment with C3 (0.35 mu g/ml, for 30 min) shifted the pCa-tension curve rightward along the Ca2+ concentration axis, indicating a decrease in Ca2+ sensitivity of the contractile elements. The inhibitory effect of C3 was not preserved after treatment with GDP beta S (1 mM), an antagonist of GTP for the binding to G-proteins. Stimulation of muscarinic receptors with carbachol (CCh, 100 mu M) shifted the pCa-tension curve leftward, indicating Ca2+ sensitization of tension development. The Ca2+-sensitizing effect of CCh was not observed after C3 treatment. When GTP gamma S (10 mu M), an activator of G-proteins, was applied at a plateau of tension development produced by a moderate concentration of Ca2+ further increase in tension was elicited and the effect of GTP gamma S was inhibited by C3 treatment. The results suggest the possible involvement of a rho p21-like small G-protein in the regulation of Ca2+ sensitivity of smooth muscle myofilaments.