Chronic stimulation of D 2 dopamine receptors specifically inhibits calcium but not potassium currents in rat lactotrophs

Abstract

The present study examines the effect of chronic dopamine treatment, known to inhibit prolactin release from anterior pituitary, on two Ca 2+ and K + currents in cultured rat lactotrophs. K + and Ca + currents were recorded using the whole-cell mode of the patch-clamp technique. The two types of voltage-dependent Ca 2+ currents are called SD and FD (slowly deactivating and fast deactivating current component, respectively) and the two types of voltage-dependent K + currents,I AandI K. All current types were isolated by tail current analysis. The amplitude of both normalized calcium components depended on the length of the culture(n = 48) while normalized amplitudes of both potassium currents remained constant(n = 9). Incubation of cells during 72 h with 50 μM of Actinomycin D, an inhibitor of mRNA synthesis, suggested that this increase in Ca 2+ currents involved the synthesis of proteins. Long-lasting D 2 receptor stimulation (8 days; 10 nM RU 24213) prevented this selective effect through activation of a pertussis toxin-sensitive G protein. We also examined whether cyclic adenosine-3′,5′-cyclic-monophosphate (cyclic AMP) or Ca 2+/phospholipid-dependent protein kinase (protein kinase C) could affect this development of channel activity. The increase of SD Ca 2+ current was enhanced during 3 days of treatment (between day 1 and day 4) with a membrane-permeant analogue of cyclic AMP, 8-bromo-adenosine-3′,5′-cyclic-monophosphate (8-Br-cyclic AMP; 1 mM): a36.6 ± 3.9% increase in SD component amplitude in control cells(n = 18), compared to55.6 ± 7.7% (n = 9) in 8-Br-cyclic AMP-treated cells. Desensitization of protein kinase C by prolonged exposure to phorbol esters increased SD Ca 2+ current amplitude by only11.4 ± 3.5% during a similar period(n = 11). Moreover, down-regulation of protein kinase C considerably reduced the long-term effects of RU 24213, whereas chronic elevation of cyclic AMP did not. These data reveal that chronic treatment of dopamine can prevent Ca 2+ current development activity via a pertussis toxin-sensitive G protein(s) pathway but cyclic AMP independent process. A reduction of protein kinase C activity could mediate, in part, this long-lasting effect of dopamine. These findings may represent an aspect of the mechanism by which chronic dopamine tonically inhibits prolactin release.