The role of adenosine 3',5'-monophosphate in dopaminergic inhibition of prolactin release in anterior pituitary cells

The relationship between cAMP stimulation and dopaminergic inhibition of PRL release was studied in primary cultures of rat anterior pituitary cells. Bromocriptine, a dopaminergic agonist, and cholera enterotoxin, isobutylmethylxanthine, theophylline, and 8-bromo-cAMP, agents which mimic cAMP action or cause increases in cAMP, were used. Short term PRL release (that which occurs within 1 or 2 h) was stimulated 1.5- to 2-fold by all of the cAMP agents. Bromocriptine (5 nM) decreased basal release and completely abolished any short term stimulation above basal caused by cholera enterotoxin and 8-bromo-cAMP. Isobutylmethylxanthine and theophylline did cause some stimulation of PRL release above basal, but the magnitude of the increase was half that in the absence of bromocriptine. The short term release stimulated by 8-bromo-cAMP was dependent on extracellular calcium. PRL accumulation in the medium for 1-3 days was increased by all of the cAMP agents. A long term increase caused by these agents was also observed in the presence of bromocriptine. The magnitude of the increase in release above basal was the same with and without bromocriptine, but the total PRL in the medium was always less in the presence of bromocriptine, since basal release was reduced. These results show that the short term inhibition of PRL release by bromocriptine cannot be completely reversed by agents which increase cAMP.