Diverse actions of cadmium on the smooth muscle myosin phosphorylation system

The effects of cadmium (Cd) on smooth muscle myosin phosphorylation have been investigated using an in vitro system comprising myosin filaments containing endogenous calmodulin (CM) and myosin light chain kinase (MLCKase). In the absence of calcium (Ca), Cd as well as some other divalent cations caused no activation of phosphorylation. However, when at least one (or possibly two) Ca2+ were bound per CM, the addition of 10 microM to 40 microM Cd2+ resulted in a 2 to 3 fold acceleration of the phosphorylation rate. Higher Cd concentrations caused inhibition of the system independent of Ca2+ concentration through the formation of Cd-ATP complexes. These results explain some previously controversial data on the complex effects of Cd in intact smooth muscles.