Calmodulin and In Vitro Regenerating Frog Sciatic Nerves: Release and Extracellular Effects

Although calmodulin (CaM) is commonly considered to be an intracellular protein, it has been suggested lately that it is released and exerts functions extracellularly. In the present investigation this was studied in in vitro regenerating adult frog (Rana temporaria) sciatic nerves. Using a multi‐compartment incubation chamber, the non‐neuronal cells in the outgrowth region of such nerves were radiolabelled with amino acid precursors. Based on immunological criteria, these cells were shown to release CaM. When the nerves were treated with CaM, both the outgrowth of sensory axons and the injury‐induced proliferation of non‐neuronal cells were partially inhibited. The inhibitory effects occurred even when the incubation medium contained as little as 30 pM CaM. Furthermore, treatment with anti‐CaM antibodies resulted in reduced outgrowth, which suggested that during normal conditions extracellular CaM is kept at an optimal concentration. Finally, conditioned medium was found to contain several CaM‐binding proteins. The present findings may reflect an earlier unknown function of extracellular CaM in controlling various growth mechanisms in integrated tissues.