Non-genomic effect of l-triiodothyronine on calmodulin-dependent synaptosomal protein phosphorylation in adult rat cerebral cortex

The present study was undertaken to examine calmodulin-dependent effect of thyroid hormones (THs) on synaptosomal protein phosphorylation in mature rat brain. Effect of L-triiodothyronine (L-T3) on in vitro protein phosphorylation was measured in a hypotonic lysate of synaptosomes prepared from adult male rat cerebral cortex, incubated in presence and absence of calcium ion (Ca²⁺) and calmodulin. L-T3 significantly enhanced incorporation of ³²P into synaptosomal proteins as compared to basal level of phosphorylation in the presence of Ca²⁺ and calmodulin. Under these conditions, increase in protein phosphorylation was 47, 74 and 52% for 10 nM, 100 nM and 1 μM L-T3, respectively. Chelation of Ca²⁺ using ethylene glycol-bis (2-aminoethylether)-N, N, N', N'-tetraacetic acid (EGTA) inhibited the effects of Ca²⁺/calmodulin on TH-stimulated protein phosphorylation levels. This study suggests that a high proportion of L-T3-stimulated protein phosphorylation involves Ca²⁺/calmodulin-dependent pathways in adult rat cerebrocortical synaptosomes.