Possible Involvement of a Small G-Protein Sensitive to Exoenzyme C3 of Clostridium botulinum in the Regulation of Myofilament Ca²⁺ Sensitivity in β-Escin Skinned Smooth Muscle of Guinea Pig Ileum

Abstract

The effects of exoenzyme C3 of Clostridium botulinum on Ca²⁺ -and drug-induced tension developments were investigated in β-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 μg/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 GDPβS (1 mM), an antagonist of GTP for the binding to G-proteins. Stimulation of muscarinic receptors with carbachol (CCh, 100 μM) shifted the pCa-tension curve leftward, indicating Ca²⁺ sensitization of tension development. The Ca^2--sensitizing effect of CCh was not observed after C3 treatment. When GTPγS (10 μM), 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 GTPγS 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.