CHARACTERIZATION OF THE BIVALVE ULTRAFILTRATION SYSTEM IN MYTILUS-EDULIS, CHLAMYS HASTATA, AND MERCENARIA-MERCENARIA

The ultrafiltration barrier of Mytilus edulis, Chlamys hastata, and Me rcenaria mercenaria was studied (1) to resolve whether the basal lamin a or the substructure of the ultrafiltration slits (slit diaphragm) is the principal filter and (2) to address the question of permselective behavior of the ultrafiltration barrier. The ultrafiltration slits in M. edulis are 15-22 nm wide. In grazing sections of the filtration ba rrier, projections from adjoining pedicels protrude into the ultrafilt ration slits. These projections have a periodicity of about 20 nm and occur in both staggered and opposing patterns. However, considerable v ariability characterizes the organization of the slit diaphragm. Trace r experiments (mostly in Chlamys hastata and Mercenaria mercenaria) re vealed that horseradish peroxidase (HRP, 40 kDa) is passed through the ultrafiltration barrier; colloidal gold (16 nm) and native ferritin ( 400 kDa), on the other hand, are retained at the level of the basal la mina. In addition, the distribution of HRP indicates that podocyte and pedicel surfaces are negatively charged. Highly anionic sites in the basal lamina were detected with ruthenium red. We conclude that (1) th e basal lamina is the principal filter, and (2) the cutoff range for u ltrafiltration is larger than 40 but smaller than 400 kDa. We hypothes ize that the negative charges are important in filtration barrier func tion and maintenance. Finally, comparing bivalve ultrafiltration and m ammalian glomerular filtration, we conclude that they share important characteristics: the basal lamina/basement membrane is the principal u ltrafiltration barrier and the basal lamina/basement membrane and podo cyte/pedicel surfaces are negatively charged.