Calmodulin and Ca²⁺‐ and Calmodulin‐Dependent Protein Kinase in Rat Anterior Pituitary Gland

Calmodulin and Ca²⁺‐ and calmodulin‐dependent protein kinase were identified in the rat anterior pituitary gland. The concentration of calmodulin was 1.18 ± 0.11 μg/mg protein (n = 7) in the cytosol fraction. The calmodulin of the anterior pituitary gland co‐migrated with brain calmodulin on sodium dodecyl sulfate polyacrylamide gel electrophoresis. The K^a value of the partially purified enzyme for Ca²⁺ was 3.3 μ.M in the presence of 0.30μ.M calmodulin. Trifluoperazine and chlorpromazine, calmodulin‐interacting agents, inhibited enzyme activity, with Kⁱ values of 1.3 and 2.6 x 10^‐5M, respectively. The enzyme was resolved into two peaks of activity, with sedimentation coefficients of 5.5 S and 16.5 S, by sucrose density gradient centrifugation. At least nine proteins were phosphorylated by the enzyme in a Ca²⁺‐ and calmodulin‐dependent manner. In light of these results, the possibility that calmodulin and the calmodulin‐activatable protein kinase system are involved in the mediation of the Ca²⁺ effect on hormone release from the anterior pituitary gland must be given consideration.