Hidefumi Wakashin, Eriko Seo, and Yoshiteru Seo

In order to determine the molecular weight cut-off (MWCO) for the atrial wall filtration into kidneys of the Mytilus galloprovincialis, we employed 5 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 mM x 0.1 mL) into the visceral mass, T1-weighted MR imaging (T1w-MRI) and the longitudinal relaxation rates (1/T1=R1) were measured at 20°C. The MR tracers were distributed uniformly in the visceral mass within 1 h after injection. The T1w-MRI intensity and R1 of the kidney (R1K) were increased by Mn2+ and MnEDTA, with urine concentrations estimated at 210 and 65 µM, respectively. The rest of tracers showed only minimal or no increase. When the mussels were additionally incubated in seawater with 10 µM MnCl2, R1K 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 µM MnEDTA showed no increase in the R1K, but additional incubation with 10 µM MnCl2 caused an increase in R1K. 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 Mytilus 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.

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