The aim of the present study was to determine the thermodynamic properties of the Na⁺-K⁺-ATPase along the intestine of gilthead seabream (Sparus aurata) and to analyse the possible relationship between the fatty acid composition of the lipid microenvironment and the regional differences observed for the Na⁺-K⁺-ATPase activities. With this objective, the temperature dependence for the Na⁺-K⁺-ATPase has been examined in the proximal-distal axis of the intestine of gilthead seabream, i.e. pyloric caeca (PC), anterior intestine (AI) and posterior intestine (PI). The experiments were performed in accordance with the requirements of the European Convention for the Care and Use of Laboratory Animals.

Data derived from the Arrhenius plots showed differences in terms of temperature discontinuity points (T_d) (13.29, 16.39 and 17.48 °C for PC, AI and PI, respectively) and activation energy ratios (E_a2/E_a1) obtained at both sides of T_d (2.38, 1.98 and 1.78 for PC, AI and PI, respectively). The analyses of polar lipids showed differences in the levels of certain fatty acids among intestinal regions. The content of each fatty acid and different fatty acid ratios were correlated with the corresponding T_d and E_a2/E_a1 values. Regression analyses revealed the existence of strong negative correlations between docosahexaenoic acid (22:6n-3, DHA) or the DHA/monoenes ratio and T_d. No obvious relationships were observed for other polyunsaturated fatty acids (PUFA) nor saturated fatty acids.

The results obtained in the present study indicate that the heterogeneous values of T_d displayed by the Na⁺-K⁺-ATPase along the intestinal tract could be related to a modulatory role of certain fatty acid within the lipid microenvironment of the enzyme.