EFFECTS OF WITHDRAWAL OF DOPAMINE ON TRANSLOCATION OF PROTEIN-KINASE-C ISOZYMES AND PROLACTIN SECRETION IN RAT LACTOTROPH-ENRICHED PITUITARY-CELLS - MODULATION OF SUBSTANCE P-MEDIATED RESPONSES

The present study deals with the effects of withdrawal of dopamine (DA ) on the translocation of protein kinase C (PKC) isozymes and release of prolactin (Prl) in resting- and substance P (SP)-stimulated culture s of enriched rat pituitary lactotrophs. Following a brief tonic input (10 min), DA withdrawal induced a redistribution of PKC alpha- and be ta-immunoreactivity (IR) to the particulate fraction with maximal leve ls, attained after 5 min, remaining translocated for 20 min. DA withdr awal prolonged the effect of SP-induced translocation of PKC alpha- an d beta-IR. Similar effects were detected when the catalytic activity o f PKC in response to DA withdrawal was evaluated. Thus, DA washout red istributed PKC catalytic activity and prolonged the effect of SP on ca talytical PKC translocation to the particulate fraction. Pretreatment of cells with the protein kinase A inhibitor, rp-adenosine-3',5'-cycli c monophosphothionate (rp-cAMP), reduced the amount of PKC alpha- and beta-IR redistributed after DA withdrawal. Furthermore, this treatment also reduced the DA withdrawal effect on SP-mediated translocation of PKC alpha- and beta-IR. Methoxyverapamil, a blocker of voltage-gated Ca2+ channels, completely inhibited the redistribution of PKC isozymes after DA withdrawal, but also reduced the potentiating effect of DA w ithdrawal on SP-induced redistribution of PKC isozyme-IR. In perifused enriched lactotrophs, DA withdrawal induced a release of Prl that las ted 45-55 min and prolonged the effect of SP on Prl secretion. rp-cAMP did not significantly affect Prl release due to DA removal, but the p rolonging effect of DA withdrawal on SP-induced Prl secretion was abol ished. Methoxyverapamil completely abolished the rebound release of Pr l after DA withdrawal, and the potentiating effect of DA removal on SP -mediated Prl release was also diminished. Readdition of DA after DA w ithdrawal was able to suppress the translocation of PKC isozyme-IR and catalytic activity and to reduce the release of Prl to baseline level s. Moreover, readdition of DA reduced the potentiating effects of DA w ithdrawal on the same parameters after SP-stimulation of cells. On the basis of these results it is concluded that in resting cells followin g DA withdrawal prolactin is released and specific PKC isozymes and co ncomitant catalytic activity are translocated to the particulate fract ion in enriched lactotrophs. While cAMP/PKA and influx of Ca2+ seem to work in concert in translocating PKC, influx of Ca2+ is the primary m echanism responsible for the rebound release of Prl after DA withdrawa l. DA withdrawal exerts a potentiating effect on SP-induced PKC transl ocation and Prl release. It is suggested that the biochemical events i nvolved in these processes are cAMP/PKA and Ca2+ influx.