BACKGROUND Paracellular permeability to solutes across the descending colon is much higher in cattle than sheep. This is a possible route for transmission of infective materials, such as scrapie prion.

AIMS To compare the permeabilities of labelled scrapie prion protein and other macromolecules in bovine and ovine descending colons in vitro.

RESULTS The comparative bovine to ovine permeability ratio of the probes increased with molecular weight: from 3.1 (0.13) for PEG400 to 10.67 (0.20) (p<0.001) for PEG4000; and from 1.64 (0.17) for microperoxidase to 7.03 (0.20) (p<0.001) for horseradish peroxidase (HRP). The permeability of¹²⁵I-labelled inactivated Syrian hamster scrapie prion protein (ShaPrP^sc) was 7.02 (0.33)-fold higher in bovine than ovine colon (p<0.0025). In each species, the probe permeabilities decreased according to the formula: P = P_o.exp(−K.ra). The “ideal” permeabilities, P_o are similar, however, K_(ovine) = 2.46 (0.20) cm/h/nm exceeds K_(bovine) = 0.85 (0.15) cm/h/nm (p<0.001) indicating that bovine colon has a higher proportion of wide pores than ovine. Image analysis confirmed that HRP permeated through the bovine mucosal layer via a pericryptal paracellular route much more rapidly than in sheep.

CONCLUSIONS These data may imply that scrapie prion is transmitted in vivo more easily across the low resistance bovine colonic barrier than in other species.