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

摘要

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) x0.1 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 degrees 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 mu mol l(-1), respectively. The rest of the tracers showed only minimal or no increase. When the mussels were additionally incubated in seawater with 10 mu mol 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 mu mol l(-1) MnEDTA showed no increase in the R-1K, but additional incubation with 10 mu mol 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.