Size-selective filtration of the atrial wall estimated from the accumulation of tracers in the kidney of the mussel Mytilus galloprovincialis.

In order to determine the molecular weight cut-off (MWCO) for the atrial wall filtration into kidneys of the Mytilus galloprovincialis, we employed five magnetic resonance (MR) tracers: manganese chloride (Mn2+), gadolinium chloride (Gd3+), manganese-ethylenediaminetetraacetic acid (MnEDTA), gadolinium-diethylenetriamine pentaacetic acid (GdDTPA) and oligomer-based contrast agent (CH3-DTPA-Gd). After injection of the MR tracers (1 or 2 mmol l-1×0.1 ml) into the visceral mass, T 1-weighted MR imaging (T 1w-MRI) and the longitudinal relaxation rates (1/T 1=R 1) were measured at 20°C. The MR tracers were distributed uniformly in the visceral mass within 1 h after injection. The T 1w-MRI intensity and R 1 of the kidney (R 1K) were increased by Mn2+ and MnEDTA, with urine concentrations estimated at 210 and 65 µmol l-1, respectively. The rest of the tracers showed only minimal or no increase. When the mussels were additionally incubated in seawater with 10 µmol l-1 MnCl2, R 1K was increased in the GdDTPA group, but not in the GdCl3 group. Therefore, Gd3+ might have inhibited renal accumulation of Mn2+ and Gd3+ Incubation in seawater with 10 µmol l-1 MnEDTA showed no increase in the R 1K, but additional incubation with 10 µmol l-1 MnCl2 caused an increase in R 1K It is suggested that injected MnEDTA was filtrated as MnEDTA per se, and not likely separated into free Mn2+ Thus, we concluded that the MWCO of the atrial wall of the M. galloprovincialis is around 0.5 kDa, which is almost 1/100 of that for vertebrate animals, and suggests a reduction in efforts to reabsorb metabolites and osmolytes from the urine.