Signaling mechanisms for alpha2-adrenergic inhibition of PACAP-induced growth hormone secretion and gene expression grass carp pituitary cells

Am J Physiol Endocrinol Metab. 2007 Jun;292(6):E1750-62. doi: 10.1152/ajpendo.00001.2007. Epub 2007 Feb 20.

Abstract

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a potent growth hormone (GH)-releasing factor in lower vertebrates. However, its functional interactions with other GH regulators have not been fully characterized. In fish models, norepinephrine (NE) inhibits GH release at the pituitary cell level, but its effects on GH synthesis have yet to be determined. We examined adrenergic inhibition of PACAP-induced GH secretion and GH gene expression using grass carp pituitary cells as a cell model. Through activation of pituitary alpha2-adrenoreceptors, NE or the alpha2-agonist clonidine reduced both basal and PACAP-induced GH release and GH mRNA expression. In carp pituitary cells, clonidine also suppressed cAMP production and intracellular Ca2+ levels and blocked PACAP induction of these two second messenger signals. In GH3 cells transfected with a reporter carrying the grass carp GH promoter, PACAP stimulation increased GH promoter activity, and this stimulatory effect could be abolished by NE treatment. In parallel experiments, clonidine reduced GH primary transcript and GH promoter activity without affecting GH mRNA stability, and these inhibitory actions were mimicked by inhibiting adenylate cyclase (AC), blocking protein kinase A (PKA), removing extracellular Ca2+ in the culture medium, or inactivating L-type voltage-sensitive Ca2+ channels (VSCC). Since our recent studies have shown that PACAP can induce GH secretion in carp pituitary cells through cAMP/PKA- and Ca2+/calmodulin-dependent mechanisms, these results, taken together, suggest that alpha2-adrenergic stimulation in the carp pituitary may inhibit PACAP-induced GH release and GH gene transcription by blocking the AC/cAMP/PKA pathway and Ca2+ entry through L-type VSCC.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenylyl Cyclase Inhibitors
  • Adrenergic alpha-Agonists / pharmacology*
  • Animals
  • Calcium / metabolism
  • Calcium Channels, L-Type / drug effects
  • Carps
  • Cell Line
  • Clonidine / pharmacology
  • Cyclic AMP / antagonists & inhibitors
  • Cyclic AMP / biosynthesis
  • Cyclic AMP-Dependent Protein Kinases / antagonists & inhibitors
  • Gene Expression / drug effects*
  • Growth Hormone / antagonists & inhibitors*
  • Growth Hormone / genetics*
  • Growth Hormone / metabolism
  • Intracellular Membranes / metabolism
  • Pituitary Adenylate Cyclase-Activating Polypeptide / pharmacology*
  • Pituitary Gland / cytology
  • Pituitary Gland / metabolism*
  • Promoter Regions, Genetic / drug effects
  • RNA Stability / drug effects
  • RNA, Messenger / metabolism
  • Signal Transduction / physiology*
  • Transcription, Genetic / drug effects
  • Transfection

Substances

  • Adenylyl Cyclase Inhibitors
  • Adrenergic alpha-Agonists
  • Calcium Channels, L-Type
  • Pituitary Adenylate Cyclase-Activating Polypeptide
  • RNA, Messenger
  • Growth Hormone
  • Cyclic AMP
  • Cyclic AMP-Dependent Protein Kinases
  • Clonidine
  • Calcium