Dopamine D2-receptor activation differentially inhibits N- and R-type Ca2+ channels in Xenopus melanotrope cells.
Dopamine inhibits pituitary melanotrope cells of the amphibian Xenopus laevis through activation of a dopamine (D2) receptor that couples to a Gi protein. Activated Gi protein subunits are known to affect voltage-operated Ca2+ currents (ICa). In the present study we investigated which Ca2+ currents are regulated by D2-receptor activation and which Gi protein subunits are involved. Whole-cell voltage-clamp patch-clamp experiments from holding potentials (HPs) of -80 and -30 mV show that 28.6 and 36.9%, respectively, of the total ICa was inhibited by apomorphin, a D2-receptor agonist. The inhibited current had fast activation and inactivation kinetics. From an HP of -80 mV, inhibition of N-type Ca2+ currents with omega-conotoxin GVIA and R-type current by SNX-482 reduced the efficacy of the apomorphin-induced inhibition. From an HP of -30 mV this reduction for omega-conotoxin GVIA was still observed. Blocking L-type current by nifedipine or P/Q-type current by omega-agatoxin IVA did not affect apomorphin-induced inhibition at either HP. Our results imply that D2-receptor activation inhibits both N- and R-type Ca2+ currents. Using a strong depolarizing pre-pulse partially reversed the inhibition of the total current by apomorphin. About 50% of this inhibition was achieved through interaction of Gbeta/gamma proteins, and this part of the inhibited ICa had fast activating and inactivating kinetics. However, the other part of the current inhibited by D2-receptor activation may proceed through Galpha-PKA phosphorylation.
Zhang
Hongyan
H
Department of Cellular Animal Physiology, Nijmegen Institute for Neurosciences, Radboud University Nijmegen, Nijmegen, The Netherlands. hongyan@sci.kun.nl
Jenks
Bruce G
BG
Ciccarelli
Alessandro
A
Roubos
Eric W
EW
Scheenen
Wim J J M
WJ
D000818:Animals; D001058:Apomorphine / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D018491:Dopamine Agonists / Q000494:pharmacology; D004594:Electrophysiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D010902:Pituitary Gland / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D017448:Receptors, Dopamine D2 / Q000187:drug effects / Q000378:metabolism*; D014982:Xenopus laevis
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D018491:Dopamine Agonists; D017448:Receptors, Dopamine D2; D001058:Apomorphine