Dopamine inhibits two characterized voltage-dependent calcium currents in identified rat lactotroph cells.
The effects of dopamine (DA) on voltage-dependent Ca2+ currents were investigated in cultured rat lactotroph cells using the patch clamp recording technique. Each recorded cell was identified by the reverse hemolytic plaque assay. In the whole-cell configuration, two types of Ca2+ currents, L and T, were characterized on the basis of their kinetics, voltage sensitivity, and pharmacology. The L component had a threshold of -25 mV, showed little inactivation during a 150-msec voltage step, and was maximal at +10 mV. Cadmium ions (100 microM) significantly reduced its amplitude (75%). The T component was activated at a membrane potential close to -50 mV, was maximal at -10 mV, and showed a voltage-dependent inactivation between -90 and -30 mV. It was quickly inactivated during a maintained depolarization (time constant, 27 ms at -30 mV) and was strongly reduced (80%) by nickel ions (100 microM). Bath application of DA (10 nM) caused a markedly general depression of inward Ca2+ currents, acting differently on the T- and L-type currents. DA application shifted the voltage-dependence of the L-type current activation toward depolarization values (8 mV) without modifying its time- and voltage-dependent inactivation. In contrast, DA enhanced the inactivation of the T-type current by accelerating its time-dependent inactivation (25% decrease in the time constant of inactivation) and by shifting the voltage-dependence of the T-type current inactivation toward hyperpolarizing values (-63 mV in control vs. -77 mV in the presence of DA). These effects of DA were dose-dependent and involved the activation of a D2 receptor type. They were mimicked by bromocriptine application (10 nM), whereas sulpiride (100 nM) blocked the DA-evoked response. The D1 antagonist SCH 23390 was ineffective up to 100 microM. All of these DA-induced modifications in Ca2+ currents were abolished using a GTP-free pipette solution or after pretreatment of cells with pertussis toxin, suggesting that DA can regulate the function of Ca2+ channels through GTP-binding proteins (G-proteins). Our results show that DA acts simultaneously by reducing both voltage-dependent Ca2+ currents on lactotroph cells. Thus, DA reduces the entry of Ca2+ ions across the surface membrane and thereby influences electrical activity and the cytosolic free Ca2+ concentration involved in both basal and evoked PRL release.
Lledo
P M
PM
INSERM U.176, 33077 Bordeaux, France.
Legendre
P
P
Israel
J M
JM
Vincent
J D
JD
D000818:Animals; D001971:Bromocriptine / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002413:Cations, Divalent; D002478:Cells, Cultured; D004298:Dopamine / Q000494:pharmacology*; D004553:Electric Conductivity; D005260:Female; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials; D009532:Nickel / Q000494:pharmacology; D037342:Pertussis Toxin; D010902:Pituitary Gland / Q000187:drug effects / Q000502:physiology*; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000187:drug effects / Q000502:physiology; D017448:Receptors, Dopamine D2; D013469:Sulpiride / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D015220:Calcium Channels; D002413:Cations, Divalent; D011954:Receptors, Dopamine; D017448:Receptors, Dopamine D2; D010566:Virulence Factors, Bordetella; D002104:Cadmium; D001971:Bromocriptine; D013469:Sulpiride; D009532:Nickel; D006160:Guanosine Triphosphate; D011388:Prolactin; D037342:Pertussis Toxin; D004298:Dopamine