Calcium transfer across the outer mantle epithelium in the Pacific oyster, Crassostrea gigas

Calcium transport is essential for bivalves to be able to build and maintain their shells. Ionized calcium (Ca²⁺) is taken up from the environment and eventually transported through the outer mantle epithelium (OME) to the shell growth area. However, the mechanisms behind this process are poorly understood. The objective of the present study was to characterize the Ca²⁺ transfer performed by the OME of the Pacific oyster, Crassostrea gigas, as well as to develop an Ussing chamber technique for the functional assessment of transport activities in epithelia of marine bivalves. Kinetic studies revealed that the Ca²⁺ transfer across the OME consists of one saturable and one linear component, of which the saturable component fits best to Michaelis-Menten kinetics and is characterized by a K_m of 6.2 mM and a V_max of 3.3 nM min^-1 The transcellular transfer of Ca²⁺ accounts for approximately 60% of the total Ca²⁺ transfer across the OME of C. gigas at environmental Ca²⁺ concentrations. The use of the pharmacological inhibitors: verapamil, ouabain and caloxin 1a1 revealed that voltage-gated Ca²⁺-channels, plasma-membrane Ca²⁺-ATPase and Na⁺/Ca²⁺-exchanger all participate in the transcellular Ca²⁺ transfer across the OME and a model for this Ca²⁺ transfer is presented and discussed.