Effect of thyroliberin on the concentration of adenosine 3′:5′-phosphate and on the activity of adenosine 3′:5′-phosphate-dependent protein kinase in prolactin-producing cells in culture

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Abstract
The effects of thyroliberin [thyrotropin releasing hormone] were studied in cultured rat pituitary-tumor cells that synthesize and secrete prolactin (the GH4C1 cell strain). Prolactin and cyclic[c]AMP were measured by radioimmunological methods, and a cAMP-dependent protein kinase was characterized by using histone as substrate. Prolactin release was studied after 5-60 min of treatment, and synthesis after 48 h of treatment with thyroliberin. One-half maximum stimulation of release and synthesis were observed at 0.25 and at 4 nM, respectively. cAMP was temporarily increased in cell suspensions after treatment with thyroliberin, and 1/2 maximum stimulation was observed at 25 nM. Dibutyryl cAMP increased prolactin release and synthesis, 1/2 maximum effects being obtained at 20 .mu.M. A cAMP-dependent protein kinase, which was 1/2 maximally stimulated at 30 nM-cAMP, was demonstrated. An increase in the activity ratio (- cAMP/+ cAMP) of the cAMP-dependent protein kinase was observed after treatment with thyroliberin. Total protein kinase activity in the presence of cAMP was unaltered. The time-course of enzyme activation was similar to that of cAMP formation and corresponded to the time when prolactin release was first observed. Thyroliberin induces cAMP formation, resulting in the activation of a cAMP-dependent protein kinase.