Tilapias (Oreochromis mossambicus), acclimated to 25 °C and water with a Ca2+ content of 0.68 mmoll−1, were subjected to gradual water acidification (from pH 7.6 to 4.0 in 4 h), followed by 10 h of exposure to low pH (pH 4.0), gradual environmental alkalization (from pH 4.0 to 7.6 in 2 h) and 6 h of recovery at normal pH (pH 7.6). Intermediates of energy metabolism were measured in perchloric acid extracts of gill, muscle and blood. In a separate series of experiments, the intracellular pH (pHi) and the levels of high-energy phosphate compounds were continuously monitored by in vivo31P-NMR spectroscopy. We used a 10 mm surface coil, which was positioned above the gill arches or the epaxial white muscle. With the coil above the gill, splitting of the inorganic phosphate peak indicated that the signal was picked up from three different compartments. These were tentatively identified as plasma, muscle and gill epithelium. Water acidification induced a transient pH drop of the plasma (0.24 units) and the gill (0.19 units) but the pH of both compartments slowly recovered during the 10-h exposure to acid water. In contrast, the pHi of muscle tissue was only slightly affected. Alkalization of the environment caused a surprising transient decline of the plasma pH, which was not due to lactic acidosis, but may be related to the precipitation of a buffer compound like CaCO3 in scales and bone. The high-energy phosphate stores in the tissues were unchanged during the whole experimental protocol.

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