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

In order to determine the molecular weight cut-off (MWCO) for the atrial wall filtration into kidneys of the , we employed five magnetic resonance (MR) tracers: manganese chloride (Mn), gadolinium chloride (Gd), 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×0.1 ml) into the visceral mass, -weighted MR imaging (-MRI) and the longitudinal relaxation rates (1/=) were measured at 20°C. The MR tracers were distributed uniformly in the visceral mass within 1 h after injection. The -MRI intensity and of the kidney () were increased by Mn and MnEDTA, with urine concentrations estimated at 210 and 65 µmol l, respectively. The rest of the tracers showed only minimal or no increase. When the mussels were additionally incubated in seawater with 10 µmol l MnCl, was increased in the GdDTPA group, but not in the GdCl group. Therefore, Gd might have inhibited renal accumulation of Mn and Gd Incubation in seawater with 10 µmol l MnEDTA showed no increase in the , but additional incubation with 10 µmol l MnCl caused an increase in It is suggested that injected MnEDTA was filtrated as MnEDTA per se, and not likely separated into free Mn Thus, we concluded that the MWCO of the atrial wall of the 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.