The role of myosin phosphorylation in histamine secretion from rat basophil leukemic (RBL) cells has been investigated. In unstimulated cells histamine secretion was about 1% of the total cell content and the stoichiometry of myosin phosphorylation was 1 mol phosphate/mol heavy chain and 0.4 mol phosphate/mol light chain. In the case of the myosin light chains, all of the phosphate was confined to a single site known to be phosphorylated by myosin light chain kinase. Upon stimulation for 10 min with DNP-BSA, the amount of histamine secretion from the cells reached 40-50% of the total content and the stoichiometry of phosphorylation was 1.78 mol phosphate/mol heavy chain and 0.84 mol phosphate/mol light chains. In this case, two distinct populations of light chains were detected: monophosphorylated and diphosphorylated, each of which contained a mixture of sites phosphorylated by myosin light chain kinase and protein kinase C. We are currently investigating the effect of specific inhibitors/activators of these two kinases in order to establish the specific role of myosin phosphorylation in histamine secretion from these cells.
