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 beta-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 (MLGO). TnIp also reversibly suppressed the Ca2+/calmodulin-independent contraction induced by 30 mM Mg2+. An analogue of TnIp, which lost inhibiting action on acto-tropomyosin-myosin ATPase activity, affected neither Ca2+-activated nor 30 mM 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. (C) 2003 Elsevier Science (USA). All rights reserved.