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.
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