Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats

James Rendt, Gerry S. Oxford

Research output: Contribution to journalArticle

9 Citations (Scopus)

Abstract

Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (I(Ca)) in pituitary cells from normal cycling female rats and in GH4C1 pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 μM) failed to significantly inhibit I(Ca) in either GH4C1 cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 μM) did not reduce I(Ca) in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit I(Ca) in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of I(Ca) by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of I(Ca) by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL secretion in normal female lactotrophs.

Original languageEnglish (US)
Pages (from-to)501-508
Number of pages8
JournalEndocrinology
Volume135
Issue number2
DOIs
StatePublished - Aug 1994
Externally publishedYes

Fingerprint

Lactotrophs
Dopamine D2 Receptors
Quinpirole
Calcium Channels
Calcium
Dopamine
Nystatin
Potassium Channels
Pituitary Neoplasms
Barium
GTP-Binding Proteins
Immunoblotting
Membranes

ASJC Scopus subject areas

  • Endocrinology
  • Endocrinology, Diabetes and Metabolism

Cite this

Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats. / Rendt, James; Oxford, Gerry S.

In: Endocrinology, Vol. 135, No. 2, 08.1994, p. 501-508.

Research output: Contribution to journalArticle

@article{e51a2521a83e4c29b5028438af610584,
title = "Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats",
abstract = "Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (I(Ca)) in pituitary cells from normal cycling female rats and in GH4C1 pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 μM) failed to significantly inhibit I(Ca) in either GH4C1 cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 μM) did not reduce I(Ca) in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit I(Ca) in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of I(Ca) by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of I(Ca) by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL secretion in normal female lactotrophs.",
author = "James Rendt and Oxford, {Gerry S.}",
year = "1994",
month = "8",
doi = "10.1210/en.135.2.501",
language = "English (US)",
volume = "135",
pages = "501--508",
journal = "Endocrinology",
issn = "0013-7227",
publisher = "The Endocrine Society",
number = "2",

}

TY - JOUR

T1 - Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats

AU - Rendt, James

AU - Oxford, Gerry S.

PY - 1994/8

Y1 - 1994/8

N2 - Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (I(Ca)) in pituitary cells from normal cycling female rats and in GH4C1 pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 μM) failed to significantly inhibit I(Ca) in either GH4C1 cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 μM) did not reduce I(Ca) in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit I(Ca) in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of I(Ca) by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of I(Ca) by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL secretion in normal female lactotrophs.

AB - Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (I(Ca)) in pituitary cells from normal cycling female rats and in GH4C1 pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 μM) failed to significantly inhibit I(Ca) in either GH4C1 cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 μM) did not reduce I(Ca) in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit I(Ca) in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of I(Ca) by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of I(Ca) by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL secretion in normal female lactotrophs.

UR - http://www.scopus.com/inward/record.url?scp=0028167819&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0028167819&partnerID=8YFLogxK

U2 - 10.1210/en.135.2.501

DO - 10.1210/en.135.2.501

M3 - Article

C2 - 8033799

AN - SCOPUS:0028167819

VL - 135

SP - 501

EP - 508

JO - Endocrinology

JF - Endocrinology

SN - 0013-7227

IS - 2

ER -