Importance of molecular charge for the passage of endogenous macromolecules across continuous capillary walls,studied by serum clearance of Lactate Dehydrogenase (LDH) isoenzymes

Electrostatic capillary barrier characteristics was studied in the isolated maximally vasodilated rat hindquarter by use of a modified “tissue uptake” technique (Rippe et al. 1979). The hindquarters were artificially perfused with oxygenated horse serum at isogravimetry. As tracers two isoenzymes of lactate dehydrogenase (LDH) were used, having indentical size (41 Å, M_w? 140000) but with differing molecular charge and labelled with two separable isotopes. LDH-H₄ (¹²⁵I) is negatively charged and LDH-M₄ (¹³¹I) slightly positive, at physiological pH. The negatively charged protein LDH-H₄ was more retarded in its transcapillary passage than LDH-M₄. Net clearance of H₄ was 0.0242±0.0045 ml/min × 100 g and that of M₄ was 0.0748±0.0092 ml/min × 100 g (n= 11, p<0.001). This difference is suggested to be due to an interaction of the polyanionic tracer with a barrier of negative molecular charge, most effective at the small pore equivalent. Clearance data for H₄ and for albumin (Rippe et al. 1979) are compatible with an equivalent large pore radius of 520 Å. Neither vesicular transport (Palade 1953) nor the impact of fibre pore matrix (Michel 1980) is considered to be involved in the transcapillary passage of proteins. Negatively charged proteins probably pass through the large pore equivalent exclusively, while neutral macromolecules also utilize part of the small pore equivalent, for their transcapillary passage.