POLY(A)(-TRANSPORT PROTEIN(S) IN XENOPUS-LAEVIS OOCYTES() RNA FROM SHEEP OMASAL EPITHELIUM INDUCES EXPRESSION OF A PEPTIDE)

To verify research from this laboratory indicating that sheep omasal e pithelium contains mRNA encoding for a peptide transporter(s) and to d etermine di- to octapeptide transport capability, we injected poly(A)( +) RNA isolated from sheep omasal epithelium into Xenopus laevis oocyt es. Poly(A)(+) RNA was functionally expressed in Xenopus oocytes 4 to 7 d after injection. Peptide (5 di-, 10 tri-, 6 tetra-, 2 penta-, 1 he xa-, 1 hepta-, and 1 octapeptide) transport capability was measured by impaling oocytes with a microelectrode to monitor membrane potential (V-m). Oocytes were maintained in pH 5.5 buffer. Peptide transport was identified as being expressed when, in the presence of a buffered pep tide substrate (1 mM), the oocyte membrane showed persistent depolariz ation (a more positive V-m). In the absence of peptide transport, the membrane became depolarized with the addition of buffered substrate, b ut it rapidly repolarized to the resting potential. Peptide transport was expressed for some di-, tri-, and tetrapeptides. Measured depolari zation ranged from 9.6 mV to 42.1 mV. Larger peptides were not transpo rted by the oocytes. When transport expression was measured with the s ubstrates in a pH 7.5 buffer, no transport occurred, indicating that t ransport was dependent on a proton gradient. Thus, sheep omasal epithe lium contains mRNA that codes for a protein(s) capable of proton-depen dent di-, tri-, and tetrapeptide transport. Results from the present s tudy provide further evidence that absorption of peptides from the rum inant stomach is possible.