The regulation of the intracellular 6 (pHi) in the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. during an acidic and alkaline load was studied by 31P-NMR spectroscopy. Under aerobic conditions, a total intracellular buffer capacity of 26.5±0.7mmoll−1 pH unit−1 (N=3) was estimated. In the absence of serotonin, active acid extrusion was by a Na+-independent, primary active transport mechanism coupled to anion transport. Base equivalents appear to be extruded by a Cl/HCO3 exchanger. Serotonin (10−5 moll−1) induced an increase in pHi through additional activation of a Na+/H+ exchanger.

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