Pacap stimulation of gonadotropin-II secretion in goldfish pituitary cells: mechanisms of action and interaction with gonadotropin releasing hormone signalling.
Pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been shown to be a hypophysiotropic factor in the goldfish. In this study, we examined the mechanisms of PACAP action on goldfish maturational gonadotropin (GTH-II) release using primary cultures of pituitary cells. The GTH-II response to mammalian PACAP1-38 (mPACAP) was inhibited by a PACAP receptor antagonist suggesting a receptor-mediated action. Addition of either an adenylate cyclase inhibitor or a protein kinase A (PKA) inhibitor reduced the mPACAP-induced GTH-II release. In addition, when GTH-II release was already stimulated by either forskolin or 8-bromo-cAMP (8Br-cAMP), mPACAP did not further increase GTH-II secretion. These results strongly implicated the involvement of an adenylate cyclase/cAMP/PKA pathway in PACAP-stimulated GTH-II release. Although mPACAP induced a rise in intracellular Ca2+ level in identified gonadotropes, results with voltage-sensitive Ca2+ channel inhibitors indicated that the GTH-II responses to mPACAP, forskolin and 8Br-cAMP did not depend upon Ca2+ entry through these channels. Two protein kinase C (PKC) inhibitors did not affect mPACAP-elicited GTH-II release, and mPACAP further increased GTH-II secretion in the presence of PKC activators. These results indicate that PKC-dependent elements are not essential for the stimulatory action of mPACAP in gonadotropes. Interestingly, while GTH-II responses to a stimulatory concentration of mPACAP were additive to responses elicited by maximal effective concentrations of two endogenous gonadotropin releasing hormones (GnRHs), a subthreshold concentration of mPACAP potentiated GnRH and PKC activator stimulation of GTH-II secretion. Similarly, submaximal concentrations of forskolin potentiated the GTH-II response to the PKC activator, tetradecanoyl phorbol acetate. These data suggest that PACAP and its cAMP-dependent signalling mechanisms provide an alternate stimulatory input to goldfish gonadotropes and may influence the effectiveness of the major neuroendocrine control exerted by the PKC-dependent GnRH signalling pathway.
Chang
J P
JP
Department of Biological Sciences, CW405 Biological Sciences Building, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada. john.chang@ualberta.ca
Wirachowsky
N R
NR
Kwong
P
P
Johnson
J D
JD
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006054:Goldfish / Q000378:metabolism*; D006065:Gonadotropins, Pituitary / Q000378:metabolism*; D008297:Male; D009479:Neuropeptides / Q000494:pharmacology*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D010902:Pituitary Gland / Q000187:drug effects* / Q000378:metabolism*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000502:physiology; D015398:Signal Transduction
D000067956:Adenylyl Cyclase Inhibitors; D002121:Calcium Channel Blockers; D004791:Enzyme Inhibitors; D006065:Gonadotropins, Pituitary; D009479:Neuropeptides; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D005576:Colforsin; D015124:8-Bromo Cyclic Adenosine Monophosphate; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C