The rete mirabile of the eel: A useful model for the study of transcapillary passage of MR contrast agents

Our purpose was to study the capillary leakage of MR contrast media using a pure capillary model. the rete mirabile of the eel. The rete is a counterc urrent-exchange organ composed of an arterial and a venous capillary system that can be catheterized and perfused. Substances are introduced at the ar terial input by a constant Infusion, and their steady-state concentrations are measured at the arterial and venous outputs, The capillary leakage of f our MR contrast agents-Gd-DOTA (MW = 561 D), carboxymethyldextran-Gd-DTPA ( MW = 38,900 D), albumin-Gd-DTPA (MW = 92,000 D), AMI-227 (400,000 D < MW < 900,000 D)-was characterized by reference to radioactive tracers ((HHO)-H-3 , Na-22, C-14-sucrose, I-125-albumin) by two parameters. These parameters w ere the concentration ratio of the venous output over the arterial input [C -VOUT(%)] and the permeability coefficient (P), The transcapillary pathway mechanisms for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA were studie d by electron microscopy, P values for Gd-DOTA (9.4 +/- 3.6 10(-7) cm/s) an d albumin-Gd-DTPA (11.8 +/- 5.5 10(-7) cm/s) were close to P values for C-1 4-sucrose, while P values for carboxymethyldextran-Gd-DTPA (6.4 +/- 4.9 10( -7) cm/sec) were similar to P values for 125I-albumin. The lowest permeabil ity was observed with AMI-227 (2.7 +/- 2 10(-7) cm/sec), Vesicular transpor t was demonstrated for carboxymethyldextran-Gd-DTPA and albumin-Gd-DTPA. Th e transcapillary passage of several MR contrast agents can be characterized with the rete mirabile model. Molecular weight is the major factor influen cing transport, J. Magn. Reson. Imaging 1999;9:353-361, (C) 1999 Wiley-Liss , Inc.