DOPAMINE WITHDRAWAL ELICITS PROLONGED CALCIUM RISE TO SUPPORT PROLACTIN REBOUND RELEASE

Dopamine (DA) acts directly on pituitary lactotropes to inhibit the re lease of PRL. Removal of DA elicits a pronounced transient rise in PRL release to values exceeding pre-DA rates (PRL rebound). Electrophysio logical studies have shown that lactotropes exhibit a period of increa sed Ca2+ action potential activity after DA withdrawal, leading to the proposal that enhanced Ca2+ influx during this period may support the rebound secretion of PRL. In the present studies, we investigated the effect of DA application and removal on the cytosolic free calcium co ncentration ([Ca2+](i)) monitored by fura-2 in single rat lactotropes. Unchallenged lactotropes fell into two functionally distinct groups: those with stable [Ca](i) that was not acutely sensitive to extracellu lar Ca2+ and those with spontaneous fluctuations in [Ca2+](i) that wer e dependent upon influx of external Ca2+. There was striking variabili ty in the [Ca2+](i) patterns of the latter group, ranging from irregul ar, low amplitude fluctuations to rhythmic, repetitive oscillations wi th definable rise and decay kinetics. Application of DA resulted in a rapid decrease in [Ca2+](i) concomitant with the cessation of these sp ontaneous [Ca2+](i) fluctuations. After DA removal, these cells resume d oscillatory [Ca2+](i) activities similar to those observed before DA application. In quiescent lactotropes, acute application of DA exerte d no effect on resting [Ca2+](i), but quiescent cells could be activat ed to produce [Ca2+](i) fluctuations by the application and withdrawal of DA. Again, the character of the induced [Ca2+] activity showed sig nificant cell to cell variation. In contrast, the pattern of [Ca2+](i) fluctuations was remarkably characteristic in a given cell in respons e to repeated challenges. A composite [Ca2+](i) profile of 13 cells pa ralleled the PRL secretory rebound after application and removal of DA . The oscillatory rise in [Ca2+](i) is functionally linked to the rebo und release of PRL after DA removal, as both were immediately abolishe d by blockade of Ca2+ influx. These data demonstrate that the rebound secretion of PRL is dependent upon enhanced influx of extracellular Ca 2+ after cells recover from DA-induced hyperpolarization and support t he hypothesis that a population of inactivated Ca2+ channels has been recruited in response to application and withdrawal of DA.