Abstract:
Fish species show distinct differences in their muscular concentrations of imidazoles and free amino acids
(FAA). This study was conducted to investigate whether metabolic response to mildly elevated water
temperature (MEWT) relates to species-dependent muscular concentrations of imidazoles and FAA. Thirteen
carp and 17 Nile tilapia, housed one per aquarium, were randomly assigned to either acclimation (25 °C) or
MEWT (30 °C) for 14 days. Main muscular concentrations were histidine (HIS; P < 0.001) in carp versus N-αacetylhistidine (NAH; P < 0.001) and taurine (TAU; P=0.001) in tilapia. Although the sum of imidazole (HIS
+NAH) and TAU in muscle remained constant over species and temperatures (P > 0.05), (NAH+HIS)/TAU ratio
was markedly higher in carp versus tilapia, and decreased with MEWT only in carp (P < 0.05). Many of the
muscular FAA concentrations were higher in carp than in tilapia (P < 0.05). Plasma acylcarnitine profile
suggested a higher use of AA and fatty acids in carp metabolism (P < 0.05). On the contrary, the concentration of
3-hydroxyisovalerylcarnitine, a sink of leucine catabolism, (P=0.009) pointed to avoidance of leucine use in
tilapia metabolism. Despite a further increase of plasma longer-chain acylcarnitines in tilapia at MEWT
(P=0.009), their corresponding beta-oxidation products (3-hydroxy-longer-chain acylcarnitines) remained
constant. Together with higher plasma non-esterified fatty acids (NEFA) in carp (P=0.001), the latter shows
that carp, being a fatter fish, more readily mobilises fat than tilapia at MEWT, which coincides with more
intensive muscular mobilization of imidazoles. This study demonstrates that fish species differ in their
metabolic response to MEWT, which is associated with species-dependent changes in muscle imidazole to
taurine ratio.
