Troponin I inhibitory peptide suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation

To explore possible mechanisms involving the thin filament-linked regulation of contraction in living smooth muscles, we studied the effects of a synthetic peptide of rabbit cardiac troponin I [residues 136-147] (TnIp), which is a minimal sequence required to inhibit striated muscle acto-tropomyosin-myosin ATPase activity, on the mechanical properties of β-escin skinned preparations of taenia caeci from guinea pig. TnIp reversibly suppressed the Ca²⁺-activated force without significant effects on the Ca²⁺ sensitivity and on the phosphorylation level of myosin regulatory light chain (MLC₂₀). TnIp also reversibly suppressed the Ca²⁺/calmodulin-independent contraction induced by 30mM Mg²⁺. An analogue of TnIp, which lost inhibiting action on acto-tropomyosin-myosin ATPase activity, affected neither Ca²⁺-activated nor 30mM Mg²⁺-induced contraction. These results indicate that TnIp suppresses the force generation in smooth muscle by directly interfering with cross-bridge formation rather than inhibiting the Ca²⁺/calmodulin-dependent thick and thin filament activating processes.