Molecular charge effects on the protein permeability of the guinea-pig placenta

The effect of molecular charge on protein permeability of the dually perfused guinea-pig placenta has been investigated by measurement of the permeability surface area products (PS) and observation of the ultrastructural localization of cationic horseradish peroxidase (cHRP) and anionic horseradish peroxidase (aHRP) molecules. Steady-state PS calculated from the experimental data was 0.032 +/- 0.0045 and 0.0045 +/- 0.0008 ml min-1 (mean +/- s.e.m.) for cHRP and aHRP respectively (P less than 0.01). The PS for a diffusional marker, 51Cr-ethylenediaminetetraacetic acid, showed no significant difference between the two groups. Ultrastructurally, placentae perfused with cHRP showed fewer microvilli and a dilated interstitial space compared with placentae perfused with aHRP; large vacuoles were also found in the syncytiotrophoblast in the former but not in the latter tissue. Reaction product in placentae perfused with cHRP was localized to the maternal-facing plasma membrane of the syncytiotrophoblast, in vacuoles and vesicles of the syncytiotrophoblast, and also in the basement membrane of the interstitial space, whereas placentae perfused with aHRP only had reaction product in vesicles in the syncytiotrophoblast. These results suggest that anionic sites in the guinea-pig placenta affect its permeability to charged proteins, cationic molecules inducing structural changes associated with increased permeability.