DOPAMINE D2 RECEPTOR STIMULATION DIFFERENTIALLY AFFECTS VOLTAGE-ACTIVATED CALCIUM CHANNELS IN RAT PITUITARY MELANOTROPIC CELLS

1. Whole-cell voltage clamp recordings were made from 141 rat pituitary melanotropic cells in short-term, serum-free, primary culture. The effects of the dopamine D2 receptor agonist, LY 171555, on sodium, potassium and barium currents were investigated. 2. Application of 1-mu-M-LY 171555 did not affect the inward sodium and outward potassium currents. 3. Application of LY 171555 reversibly inhibited barium currents, with the strongest inhibition on the early inward current. The effect was dose dependent (IC50 = 4 x 10(-8) M), maximal inhibition of the total current was 30% and the LY 171555-induced block (1-mu-M) was reversibly antagonized by (+/-)sulpiride (4-mu-M). 4. Using barium-selective saline solutions, different types of barium current (T, N, and two L components) were identified on the basis of their voltage-dependent kinetics. Their relative amplitudes differed between cells. 5. The T-type current activated at potentials positive to -60 mV, reaching peak amplitude between -20 and -10 mV. At -30 mV, this current was inhibited up to 30% by 1-mu-M-LY 171555. The time constants of activation (10-3 ms) and inactivation (50-20 ms) as well as the voltage dependence of inactivation (potential of half-maximal inactivation (H), -61 mV; slope factor (S), 4.9 mV) were not affected by LY 171555 application. 6. A rapidly inactivating (time constants 100-50 ms), high threshold current component was identified as an N-type current. This current activated at command potentials positive to -30 mV and reached a maximal amplitude at +10 mV. The steady-state inactivation was described by a single Boltzmann equation with H = -65 mV and S = 11.7 mV. Application of 1-mu-M-LY 171555 completely suppressed this current. 7. The slowly inactivating (time constants > 1500 ms), high-threshold, L-type current displayed the same voltage dependence of activation as the N current. The voltage dependence of inactivation was modelled by the sum of two Boltzmann equations (L1:H-1 = -45 mV, S1 = 13.0 mV; L2:H-2 = -11 mV, S2 = 6.0 mV), indicating the existence of two L channel populations. Neither time course, nor voltage dependence of inactivation were influenced by LY 171555. However, LY 171555 induced a slow-down in the time course of activation, which necessitated the use of two time constants to model the activation kinetics. One of these (approximately 2 ms) was also observed under control conditions. To examine whether LY 171555 affects both L current components, T, N and L1 currents were suppressed using a holding potential of -30 mV. The L2 current remaining under these conditions was not affected by LY 171555. The dihydropyridine Bay K 8644 increased the amplitude of the high threshold current and of the LY 171555-suppressed current, thus confirming the effect of LY 171555 on L channels. 8. L currents, elicited with or without a pre-pulse that suppressed the N current, showed a similar, fast time course of activation, indicating that the slow time constant observed during LY 171555 application, does not occur under control conditions. The LY 171555-induced slow-down of activation was not present in test pulses preceded by a pre-pulse. This was explained by the hypothesis that the slow-down of activation of the L current is due to a voltage-dependent relaxation of the L current inhibition. 9. Addition of 100-mu-M-GTP-gamma-S to the pipette saline mimicked the effect of LY 171555 on the L currents. Therefore, the voltage-dependent disinhibition must result from a voltage-dependent effect after the receptor-G-protein interaction. 10. Rat pituitary melanotropic cells possess various calcium channels which are subject to regulation by dopamine D2 receptors. The T current is partially inhibited, possibly due to a reduction in the number of available channels or a reduction in single-channel conductance. The N-type current is totally suppressed, while the two subcomponents of the L current are differentially affected. The L2 current, recorded using a holding potential of -30 mV, is not affected. Component L1, however, is inhibited in a voltage-dependent manner. The slow time constant of activation induced by D2 receptor stimulation may reflect a voltage-dependent conversion of the channel from the inhibited state to a state in which activation can occur. The importance of the various channel types in secretion is discussed.