Biology Reference
In-Depth Information
smoltifi cation (Richman III et al., 1987). Chloride cells from the opercular
membrane (an epithelium lining the branchial side of the operculum)
increased nearly two-fold in late May, concurrent with the second increase
in gill Na+/K+-ATPase activity and SW tolerance. The gill fi lament surface,
which is rough initially, became smoother during smoltifi cation and rough
again toward the end of smoltifi cation (Richman III et al., 1987). Two
mitochondrion-rich (chloride) cell types were present in the gill epithelium
during smoltifi cation: the electron-lucent type I cell contained large, circular
mitochondria, while the electron-dense type II cell contained thin, elongate
mitochondria (Richman III et al., 1987). Chloride cells are the sites of active
electrolyte transport (Payan et al., 1984). Chloride cell totally changes at the
end of smolting with the development of a specifi c associated accessory
cell (Pisam et al., 1988).
An increase in gill Na+/K+-ATPase activity during smoltifi cation was
fi rst demonstrated in coho salmon by Zaugg and McLain (1970) and later
reported in other salmonids (Zaugg and McLain, 1972; Zaugg and Wagner,
1973; McCartney, 1976; Lasserre et al., 1978; Boeuf and Harache, 1982).
McCormick et al. (1989b) found that gill Na+/K+-ATPase activity increased
along with the mitochondrial enzymes (citrate synthase and cytochrome
C oxidase) between March and May during smoltifi cation. The Na+/K+-
ATPase activity remained high until August, while mitochondrial activity
had declined. A further study reported a fi vefold increase in gill Na+/
K+-ATPase activity following transfer of Atlantic salmon to seawater, but a
decrease in citrate synthase and cytochrome C oxidase activities in the gill
(McCormick et al., 1989a). The enzyme Na+/K+-ATPase being composed
of α and β subunits, an increase in the expression of α subunit mRNA in
gill tissue was shown during smoltifi cation in Atlantic salmon (D'Cotta et
al., 1996, 2000; Seear et al., 2010).
Recently, five isoforms of claudins, tight junction proteins, were
identifi ed in gill libraries of Atlantic salmon and only the expression of
branchial isoform 10e varied during smoltifi cation, peaking in May at
optimal seawater tolerance (Tipsmark et al., 2008).
6.2.4.2 Intestine
In salmonids, the intestine is the most important part of the digestive tract
for osmoregulation, notably for water absorption. Indeed, whereas FW
fi sh do not drink water and their gut is virtually watertight, in SW fi sh
drink the ambient water continually and salt water is absorbed across the
gut to reach the blood. During smoltifi cation, the intestinal function must
change from its FW role of preventing water infl ow, to that of actively
absorbing ions and water. Using an
in vitro
sac preparation, measurements
of intestinal net fl uid absorption (Jv) showed that FW coho salmon parr
