D2 dopamine receptor activation of potassium channels in identified rat lactotrophs: whole-cell and single-channel recording.
Dopamine (DA) is the major physiological regulator of prolactin secretion from the anterior pituitary, exerting a tonic inhibitory control that is mediated by D2 DA receptors. D2 receptors in both the anterior pituitary and CNS are thought to produce some of their inhibitory effects via a coupling to potassium (K+) channels to increase K+ conductance. Utilizing the reverse hemolytic plaque assay and patch-clamp techniques, we characterize the actions of DA on membrane potential and associated DA-activated whole-cell current, as well as the single K+ channels that underlie the response in primary rat lactotrophs. We demonstrate that DA (5 nM to 1 microM) or D2-selective agonists (RU24213 and quinpirole) evoke a hyperpolarization of membrane potential that was blocked by D2 antagonists and associated with an increased K+ conductance. Whole-cell current responses to ramp voltage commands revealed a DA-activated current whose reversal potential was near the calculated Nernst potential for K+, varied as a function of K+ concentration, exhibited some inward rectification, and was Ca2+ independent. The current was insensitive to tetraethylammonium (TEA; 10 mM), partially blocked by 4-aminopyridine (4-AP; 5 mM), and almost completely inhibited by quinine (100 microM). Cell-attached recordings in the presence of DA or a D2 agonist revealed the opening of a K+ channel that was not present in the absence of DA or when a D2 receptor antagonist was included with DA. Analysis of the single-channel current showed the current-voltage relationship to be linear at negative patch potentials and yielded a unitary conductance of 40.2 pS in the presence of 150 mM KCl. The channels were not blocked by TEA (10 mM), were slightly suppressed by 4-AP (5 mM), and were almost completely inhibited by quinine (100 microM). These experiments establish that in primary rat lactotrophs, DA acts at D2 receptors to activate the opening of single K+ channels, which results in an increase in K+ conductance and associated membrane hyperpolarization. This is the first characterization of single DA-activated K+ channels in an endocrine cell.
Einhorn
L C
LC
University of North Carolina, Chapel Hill 27599.
Gregerson
K A
KA
Oxford
G S
GS
D000818:Animals; D004298:Dopamine / Q000494:pharmacology; D015259:Dopamine Agents / Q000494:pharmacology; D004553:Electric Conductivity; D004594:Electrophysiology; D005260:Female; D008564:Membrane Potentials; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D011188:Potassium / Q000037:antagonists & inhibitors; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D011388:Prolactin / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011954:Receptors, Dopamine / Q000502:physiology*; D017448:Receptors, Dopamine D2
D015259:Dopamine Agents; D015221:Potassium Channels; D011954:Receptors, Dopamine; D017448:Receptors, Dopamine D2; D011388:Prolactin; D011188:Potassium; D004298:Dopamine