| Abstract: | The dynamics of dopamine (DA) action on PRL release was studied in superfused rat anterior pituitary cell aggregates, cultured for for 5 days either in conventional or in serum-free defined medium. In aggregates cultured in conventional medium 0.1-1 nM DA applied for 20 min provoked a rapid and concentration-dependent inhibition of PRL release, lasting only a few minutes, after which there was a gradual rise in secretion up to near baseline levels. A sustained inhibition was obtained from DA concentrations more than or equal to 10 nM. When DA, used at the latter concentration, was withdrawn from the superfusion medium, a marked rebound secretion of PRL occurred, exceeding basal release for as long as 40-50 min. Rebound secretion was not followed by a compensatory fall in secretion rate. After a 10-min pulse of 10 or 30 nM DA, the amount of PRL released above baseline was considerably higher than the amount of PRL not released during the time DA was present. The latter stimulation of PRL release was not seen after a 40- or 90-min exposure time to DA. However, when DA was given for 40 min in 10 pulses of 4 min (4 min DA on 4 min DA off), a clear-cut stimulation of PRL release followed the termination of the pulses. When the serum used in the culture medium was extracted with dextran-coated charcoal, post-DA rebound secretion of PRL was markedly diminished. The latter secretion pattern partially reappeared when the extracted serum was supplemented with 10 nM dexamethasone. DA had similar effects on PRL release in aggregates cultured in serum-free defined medium. Dexamethasone did not affect DA-inhibition but strongly stimulated post-DA rebound, and this effect was potentiated by T3 present in the defined medium. There was three to four times more PRL secreted in excess of basal release than was inhibited during exposure to DA. The present data suggest a dual action of DA on PRL release: inhibition during tonic exposure to the catecholamine and inhibition-mediated stimulation after pulsatile exposure. |
