The effects of the metal ions Mn2+ and Co2+ on muscle contraction in the Norway lobster Nephrops norvegicus (L.)

The effects of the metal ions manganese and cobalt on force production by t he abdominal superficial flexor muscle of the Norway lobster, Nephrops norv egicus, have been studied in response to both neuronal stimulation and elec trical field stimulation applied to an isolated neuromuscular preparation, and by selectively blocking synaptic transmission with Ivermectin. In respo nse to both forms of stimulation, low concentrations of manganese added to the standard N. norvegicus saline increased the contractile force produced by the muscle, whereas higher concentrations of manganese inhibited both re sponses in a dose-dependent manner, until force was completely abolished at concentrations above 2.9 mM manganese. Cobalt ions produced similar effect s, and no significant difference was found between the concentration of the two ions at 50% force inhibition (K-m) or between the two stimulation meth ods (manganese: 1.22 mM; cobalt: 1.29 mM, P = 0.86). This suggests that the y have a similar mode of action, and a postsynaptic site of inhibition. The se K-m values are considerably higher than the concentrations of these ions known to accumulate in the haemolymph of N. norvegicus under eutrophic con ditions, and it therefore seems unlikely that accumulations of manganese or cobalt ions under such conditions would cause any significant inhibition o f muscle contraction force.