DOPAMINE ACTIONS ON CALCIUM CURRENTS, POTASSIUM CURRENTS AND HORMONE-RELEASE IN RAT MELANOTROPHS

1. Intracellular and whole-cell recordings were made from primary cultures of rat intermediate pituitary cells; beta-endorphin secretion was also measured by radioimmunoassay. The effects of dopamine receptor activation on hormone secretion, calcium currents and resting potassium conductance were compared. 2. Spontaneous sodium-dependent action potentials occurred in 82% of cells recorded with intracellular microelectrodes and 64% of cells recorded with whole-cell patch electrodes; the same proportion of cells showed spontaneous calcium-dependent depolarizations in the presence of tetrodotoxin. 3. Calcium currents recorded from holding potentials of -90 or -70 mV showed transient and sustained components, both of which activated at -40 mV and had similar current-voltage relations. Bay K 8644 (1-mu-M) increased both components by about 130% while nifedipine (1-10-mu-M) decreased them by a maximum of 30%. Nickel (500-mu-M) inhibited transient and sustained components by 68 and 50%; cadmium (100-mu-M) abolished the current. omega-Conotoxin (1-mu-M) reversibly inhibited the transient component by 26%. 4. The dopamine D2 receptor agonist, quinpirole (0.1-10-mu-M) inhibited transient and sustained components in all cells by a maximum of 40 and 25% respectively. Quinpirole did not alter the time course of the current. 5. Quinpirole (1-100 nM) hyperpolarized 90% of cells from which intracellular recordings were made and 55% of cells recorded from with whole-cell patch pipettes. Maximum hyperpolarization of 16 +/- 4 mV from a resting potential of -44 +/- 5 mV was observed with 100 nM-quinpirole; concentration producing half-maximal effect was 3 nM. The hyperpolarization resulted from an increase in potassium conductance. 6. Quinpirole (1-100 nM) decreased basal beta-endorphin secretion by 55% and abolished secretion stimulated by Bay K 8644 or isoprenaline; concentrations producing half-maximal inhibitions were 5-10 nM. Tetrodotoxin (1-mu-M), nifedipine (1-mu-M), nickel (500-mu-M) and cadmium (100-mu-M) did not alter basal or stimulated secretion although higher concentrations of cadmium did inhibit stimulated hormone release. 7. Pertussis toxin pre-treatment prevented all actions of quinpirole. 8. Thus, concentrations of quinpirole that abolished stimulated hormone secretion did not alter calcium currents; conversely, concentrations of calcium channel blockers that partially or completely inhibited calcium currents did not alter basal or stimulated secretion. These results may indicate that calcium influx through the voltage-dependent calcium channels measured in these experiments does not contribute significantly to hormone release from melanotrophs. 9. The same concentrations of quinpirole activated a potassium conductance and inhibited hormone release. It is concluded that membrane hyperpolarization may be the primary mechanism by which dopamine receptor activation inhibits hormone release from melanotrophs.