EFFECTS OF MAGNESIUM PYROPHOSPHATE ON MECHANICAL-PROPERTIES OF SKINNED SMOOTH-MUSCLE FROM THE GUINEA-PIG TAENIA-COLI

Effects of the non-hydrolyzable nucleotide analogue magnesium pyrophosphate (MgPP(i)) on cross-bridge properties were investigated in skinned smooth muscle of the guinea pig Taenia coli. A "high" rigor state was obtained by removing MgATP at the plateau of an active contraction. Rigor force decayed slowly towards an apparent plateau of approximately 25-35% of maximal active force. MgPP(i) markedly increased the rate of force decay. The initial rate of the force decay depended on [MgPP(i)] and could be described by the Michaelis-Menten equation with a dissociation constant of 1.6 mM. The decay was irreversible amounting to approximately 50% of the rigor force. Stiffness decreased by 20%, suggesting that the major part of the cross-bridges were still attached. The results can be interpreted as "slippage" of PP(i)-cross-bridges to positions of lower strain. The initial rate ot MgPP(i)-induced force decay decreased with decreasing ionic strength in the range 45-150 mM and was approximately 25% lower in thiophosphorylated fibers. MgADP inhibited the MgPP(i)-induced force decay with an apparent K(i) of 2-mu-M. The apparent K(m) of MgATP for the maximal shortening velocity in thiophosphorylated fibers was 32-mu-M. This low K(m) of MgATP suggests that steps other than MgATP-induced detachment are responsible for the low shortening velocity in smooth muscle. No effects were observed of 4 mM MgPP(i) on the force-velocity relation, suggesting that cross-bridges with bound MgPP(i) do not constitute an internal load or that binding of MgPP(i) is weaker in negatively strained cross-bridges during shortening,