Dopamine inhibits voltage-activated calcium channel currents in rat pars intermedia pituitary cells

Several lines of evidence suggest that dopamine acts as a neurotransmitter that inhibits both hormone secretion and electrical activity in pituitary intermediate cells (melanotrophs). In this study we examined the effects of exogenously applied dopamine on voltage activated calcium currents recorded with the whole-cell mode of the patch-clamp technique from short-term primary cultures of melanotrophs. Two types of calcium currents were distinguished by their voltage dependence and kinetics of inactivation similar to the low voltage-activated currents (LVA; or T-type) and high voltage-activated currents (HVA; N&L-types) of calcium currents. Exogenously applied dopamine (2-20 microM) reversibly reduced both LVA and HVA types of calcium currents. Evidence for these results came from experiments in which LVA and HVA calcium currents were separated by stepping to different membrane potentials from a fixed holding potential (Vh) or by changing Vh. These results suggest that dopamine can regulate the entry of calcium into melanotrophs by acting on at least two different populations of calcium channels thereby affecting hormone secretion and electrical activity.