Dopamine actions on calcium currents, potassium currents and hormone release in rat melanotrophs

Abstract

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 microM) increased both components by about 130% while nifedipine (1-10 microM) decreased them by a maximum of 30%. Nickel (500 microM) inhibited transient and sustained components by 68 and 50%; cadmium (100 microM) abolished the current. omega-Conotoxin (1 microM) reversibly inhibited the transient component by 26%. 4. The dopamine D2 receptor agonist, quinpirole (0.1-10 microM) 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 microM), nifedipine (1 microM), nickel (500 microM) and cadmium (100 microM) 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.(ABSTRACT TRUNCATED AT 400 WORDS)