CALCIUM SPECIATION AND EXCHANGE BETWEEN BLOOD AND EXTRAPALLIAL FLUID OF THE QUAHOG MERCENARIA-MERCENARIA (L.)

Calcium and small organic molecules (e.g., tyrosine, MW 181 Da) introd uced into the extrapallial fluid (EPF) of the quahog Mercenaria mercen aria exhibit rapid fluxes across the outer mantle epithelium and are d istributed throughout the circulatory system within 3 h. Larger molecu les (e.g., bovine serum albumin, MW 66,000 Da) are less readily exchan ged between EPF and blood. The protein compositions of blood plasma an d EPF are different, with at least seven protein bands expressed more prominently in the EPF. Equilibrium dialysis experiments reveal that C a2+ constitutes only 2% of the total Ca in plasma; most of the Ca (85% ) is bound to macromolecules, and the remaining 13% is present as dial yzable low molecular weight moieties. This distribution cannot be expl ained by speciation of inorganic Ca alone, since the MINTEQA2 equilibr ium speciation model predicts that 79%-86% of the Ca should be present as Ca2+, with the remainder as CaSO4 (20% - 13%). However, inclusion of a weakly Ca-binding organic molecule (log(10) K-a approximate to 2 M-1) into MINTEQA2 could fully reconcile modeling with experimental me asurements. Results suggest that calcium transport in blood plasma and EPF is mediated by a suite of proteins and small organic ligands with a low affinity for Ca.