The effects of some divalent metals on cardiac and branchial Ca2+-ATPase in a freshwater fish Saccobranchus fossilis

The heart and gill of a freshwater fish Saccobranchus fossilis have been shown to contain a Ca2+-activated ATPase involved in Ca2+ transport. Enzyme showed optimal activity at 3 mM Ca2+ and 3 mM ATP for gills and at 3 mM Ca2+ and 1 mM ATP for heart. Mg2+ was equally effective in stimulating enzyme activity but was not essential for hydrolysis. Maximum activity was found in heart ventricular muscles as compared to gills. Among all the metals tested Hg2+ was the most toxic (IC50, 0.75 and 0.85 microM for heart and gill, respectively) followed by Pb2+, Mn2+, and Cd2+. The inhibition was concentration dependent and reached almost 100% with each metal at the highest concentration. Stimulation of enzyme activity was observed at lower concentrations of Mn2+ and Cd2+ but not with Pb2+ and Hg2+. Stimulation was more pronounced with Mn2+ than with Cd2+ in both heart and gills. The results indicated that the inhibitory effect of these metals might be through the Ca2+-ATPase which is a manifestation of the calcium pump in various tissues.