Melatonin Inhibits the Activation of Cyclic AMP‐Dependent Protein Kinase in Cultured Pars Tuberalis Cells from Ovine Pituitary

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Abstract
The effect of melatonin upon the activation of the intracellular effector enzyme, cyclic AMP (cAMP)-dependent protein kinase (PKA), was investigated in primary cultures of ovine pars tuberalis cells. Incubation of these cells with forskolin caused a rapid and dose-dependent activation of PKA (ED(50) 10( approximately 6)M). When cells were incubated with forskolin and melatonin simultaneously, the activation of PKA by forskolin was dramatically inhibited. This inhibitory effect of melatonin was dose-dependent (ED(50) 10(-10)M). Furthermore, treatment with melatonin rapidly deactivated PKA in cells prestimulated with forskolin. When pars tuberalis cell extracts were incubated with 8N(3)-[(32)P]cAMP, an analogue of cAMP used for photoaffinity labelling of native PKA, specific binding was observed in three bands with M(r) of 54, 52 and 48 kd, representing the regulatory subunits of PKA II (in phosphorylated and dephosphorylated forms) and PKA I, respectively. These results indicate that melatonin is a potent inhibitory regulator of cAMP-mediated signal transduction in the ovine pars tuberalis, and suggest that the cellular effects of melatonin in this tissue are mediated by the dephosphorylation of specific substrate proteins.