AVT is involved in the regulation of ion transport in the intestine of the sea bream (Sparus aurata)

• A partial sequence of AVT V1a2-type receptor was cloned in the sea bream.
• The AVT V1a2-type receptor is distributed in the whole gastrointestinal tract.
• The AVT V1a2-type receptor expression is sensitive to salinity.
• AVT controls ion transport in the intestine as demonstrated in Ussing chambers.
• AVT action is mediated by a bumetanide-sensitive mechanism, likely NKCC2.

The intestine of marine fish plays a crucial role in ion homeostasis by selective processing of ingested fluid. Although arginine vasotocin (AVT) is suggested to play a role in ion regulation in fish, its action in the intestine has not been demonstrated. Thus, the present study investigated in vitro the putative role of AVT in intestinal ion transport in the sea bream (Sparus aurata). A cDNA encoding part of an AVT receptor was isolated and phylogenetic analysis revealed it clustered with the V1a2-type receptor clade. V1a2 transcripts were expressed throughout the gastrointestinal tract, from esophagus to rectum, and were most abundant in the rectum regardless of long-term exposure to external salinities of 12, 35 or 55 p.p.t. Basolateral addition of AVT (10−6 M) to the anterior intestine and rectum of sea bream adapted to 12, 35 or 55 p.p.t. mounted in Ussing chambers produced rapid salinity and region dependent responses in short circuit current (Isc), always in the absorptive direction. In addition, AVT stimulation of absorptive Isc conformed to a dose–response curve, with significant effects achieved at 10−8 M, which corresponds to physiological values of plasma AVT for this species. The effect of AVT on intestinal Isc was insensitive to the CFTR selective inhibitor NPPB (200 μM) applied apically, but was completely abolished in the presence of apical bumetanide (200 μM). We propose a role for AVT in the regulation of ion absorption in the intestine of the sea bream mediated by an absorptive bumetanide-sensitive mechanism, likely NKCC2.