Dopamine Modulates Exocytosis Independent of Ca²⁺Entry in Melanotropic Cells

Dopamine is a known inhibitor of pituitary melanotropic cells. It reduces Ca2+influx by hyperpolarizing the cell membrane and by modulating high- and low-voltage-activated (HVA and LVA) Ca2+channels. As a result, dopamine reduces the hormonal output of the cell. However, it is unknown how dopamine affects each of the four different HVA Ca2+ channel types individually. Moreover, it is unknown whether dopamine interacts with exocytosis independent of Ca2+ channels. Here we show that dopamine differentially modulates the HVA Ca2+channels and that it affects the stimulus-secretion coupling through a direct effect on the exocytotic machinery. Sustained L- and P-type Ba2+ currents are reduced in amplitude and inactivating N- and Q-type currents acquire different activation and inactivation kinetics in the presence of dopamine. The Q-type current shows slow activation, which is a hallmark for direct G-protein modulation. We used membrane capacitance measurements to monitor exocytosis. Surprisingly, we find that the amount of exocytosis per step depolarization is not diminished by dopamine despite the reduction in Ca2+ current. To test whether dopamine affects the release machinery downstream of Ca2+ entry, we stimulated exocytosis by dialyzing cells with buffered high-Ca2+ solutions. Dopamine increased the amount and the rate of exocytosis. In the first 90 s, the rate of secretion was increased two- to threefold, but it was normalized again at 180 s, suggesting that predominantly vesicles that fuse early in the exocytotic phase are modulated by dopamine. Thus while Ca2+ channels are inhibited by dopamine, the exocytotic machinery downstream of Ca2+ influx is sensitized. As a result, release is more effectively stimulated by Ca2+ influx during dopamine inhibition.