Effects of Mn2+ and Ca2+ on the mechanical properties of glycerinated myofibrillar bundles originating from slow S1 type muscle fibres of superficial flexor muscles of the lobster Nephrops norvegicus were investigated. Mn2+ (5-20 microM) activated the preparations in a dose-dependent manner. The sensitivity of myofibrillar force generation for Mn2+ was around 30 times lower than that for Ca2+. The maximal tension produced under Mn2+ activation was about 75% of that under Ca2+ activation. At higher free Mn2+ concentrations (>2 mM), the steady-state force decreased; it was completely abolished at 30 mM free Mn2+. These high Mn2+ solutions were accompanied by changed in MgATP and MnATP concentrations, and in the ionic strength. Control experiments have shown that none of these parameters seemed fo account fully for the observed force depression in high Mn2+ solutions. It is likely that direct effects of Mn2+ such as a change of the myofilament surface charges are responsible. The maximal unloaded shortening velocity of the myofibrillar preparations was shown to be similar under maximal Mn2+ and Ca2+ activation. Conversely, the kinetics of stretch-induced delayed force increase were about two to three times faster under Mn2+ activation. These results suggest that certain steps of the cross-bridge cycle depend on the ion species bound to the regulatory proteins.
Adenosine Triphosphate Animals