BROMOCRIPTINE INHIBITS PROLACTIN SYNTHESIS MORE EFFECTIVELY THAN DOPAMINE

The processes of prolactin (PRL) synthesis, storage and release were s tudied, under the influence of dopamine (DA) and bromocriptine, in ord er to investigate the relationship(s) that exist between these process es. Primary cultured male rat anterior pituitary cells were continuous ly labelled with [H-3]leucine for either 15, 30, 60, 180, or 360 min, with either DA or bromocriptine (experimental), or without a secretago gue (control). The medium was then collected and the cells were fracti onated in order to isolate the granular and cytosolic fraction. The ra tio of [H-3]PRL (newly-synthesized PRL) to total PRL was determined an d this ratio is termed the specific activity of the PRL (PRL-SA). DA i ncreased the media PRL-SA (mPRL-SA) from 15-180 min, compared to contr ol but decreased it from 180-360 min. DA did not significantly affect the cytosolic PRL-SA (cPRL-SA) until 180 min and then increased it fro m 180-360 min, as compared to control. DA did not, however, affect the granular PRL-SA (gPRL-SA) for the entire experimental period. These r esults indicate that DA: (i) induced a preferential release of [H-3]PR L in the short-term (to 180 min), (ii) produced a cytosolic accumulati on of [H-3]PRL, in the long term (after 180 min), and (iii) did not af fect the granular storage of [H-3]PRL. Bromocriptine decreased the mPR L-SA from 15-360 min, compared to control and from 180-360 min, compar ed to DA. Bromocriptine decreased the gPRL-SA from 15-360 min compared to both control and DA. Since bromocriptine decreased the accumulatio n of [H-3]PRL in all fractions, compared to DA and control, we have co ncluded that bromocriptine inhibits PRL synthesis more potently than D A.