CHARACTERIZATION OF UREA TRANSPORT MECHANISM ACROSS THE OMASUM EPITHELIUM IN SHEEP

Abstract

The transport of urea has been studied in a variety of epithelial tissues
including the rumen, where carrier-mediated transport mechanisms have
been reported. No such information concerning the transport of urea in the
omasum is known. We have therefore carried out this study to
characterize the transport mechanism of urea across the isolated omasum
epithelium in sheep. This study was conducted under short circuit
condition using conventional Ussing-chamber technique on 42 pieces of
omasal
epithelial tissues isolated from 5 sheep. Tissues bathing buffer solutions
contained a mixture of antibiotics and the urease inhibitor
phenylphosphorodiamidate (PPD). Our
results show significant correlation (r2
= 0.85) between the transepithelial
conductance
(
Gt
) and the serosal to mucosal urea flux rates (
sm J
).
Moreover, mucosal application of the actin cytoskeleton inhibitor
cytochalasin D (10µM)
significantly (p<0.05) increased both the tissue conductance (
Gt
) and the
serosal to mucosal (
sm J
) flux rates of urea. However, reducing the
mucosal pH from 7.4 to 6.4 did not induce a significant effect on the
bidirectional urea flux rates across the omasum epithelium. The results
indicate that transport of urea across the omasum epithelium involves a
passive paracellular pathway mechanism.
Key words: Sheep; Omasum; Urea; Cytochalasin D