ACTIVATION OF SKINNED MUSCLE-FIBERS FROM THE NORWAY LOBSTER NEPHROPS-NORVEGICUS L. BY MANGANESE IONS

Effects of Mn2+ and Ca2+ on the mechanical properties of glycerinated myofibrillar bundles originating from slow S-1 type muscle fibres of s uperficial flexor muscles of the lobster Nephrops norvegicus were inve stigated. Mn2+ (5-20 mu M) activated the preparations in a dose-depend ent manner. The sensitivity of myofibrillar force generation for Mn2was around 30 times lower than that for Ca2+. The maximal tension prod uced under Mn2+ activation was about 75% of that under Ca2+ activation . At higher free Mn2+ concentrations (>2 mM), the steady-state force d ecreased; it was completely abolished at 30 mM free Mn2+. These high M n2+ solutions were accompanied by changes in MgATP and MnATP concentra tions, and in the ionic strength. Control experiments have shown that none of these parameters seemed fo account fully for the observed forc e depression in high Mn2+ solutions. It is likely that direct effects of Mn2+ such as a change of the myofilament surface charges are respon sible. The maximal unloaded shortening velocity of the myofibrillar pr eparations was shown to be similar under maximal Mn2+ and Ca2+ activat ion. Conversely, the kinetics of stretch-induced delayed force increas e were about two to three times faster under Mn2+ activation. These re sults suggest that certain steps of the cross-bridge cycle depend on t he ion species bound to the regulatory proteins. (C) Chapman & Hall Lt d.