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Submitted on March 14, 2005
Accepted on August 17, 2005
Department of Zoology, University of Hong Kong, Pokfulam Road, Hong Kong
* To whom correspondence should be addressed. E-mail: olwong{at}hkucc.hku.hk.
Pituitary adenylate cyclase-activating polypeptide (PACAP), a member of the glucagon/ secretin peptide family, has been recently proposed to be the ancestral growth hormone (GH)-releasing factor. Using grass carp as a model for bony fish, we examined the mechanisms for PACAP regulation of GH synthesis and secretion at the pituitary level. Nerve fibers with PACAP immunoreactivity were identified in the grass carp pituitary overlapping with the distribution of somatotrophs. At the somatotroph level, PACAP was shown to induce cAMP synthesis and Ca2+ entry through voltage-sensitive Ca2+ channels (VSCC). In carp pituitary cells, PACAP but not VIP increased GH release, GH content, total GH production, and steady-state GH mRNA levels. PACAP also enhanced GH mRNA stability, GH promoter activity, and nuclear expression of GH primary transcripts. Increasing cAMP levels, induction of Ca2+ entry, and activation of VSCC were all effective in elevating GH secretion and GH mRNA levels. PACAP-induced GH secretion and GH mRNA expression, however, were abolished by inhibiting adenylate cyclase (AC) and PKA, removing [Ca2+]e or VSCC blockade, or inactivating calmodulin (CaM)-dependent protein kinase II (CaM kinase II). Similar sensitivity to VSCC and CaM kinase II blockade was also observed by activating cAMP production as a trigger for GH release and GH gene expression. These results suggest that PACAP stimulate GH synthesis and secretion in grass carp pituitary cells through PAC1 receptors. These stimulatory actions probably are mediated by the AC/cAMP/PKA pathway coupled to Ca2+ entry via VSCC and subsequent activation of CaM/CaM kinase II cascades.
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