Poly(A)+ RNA from sheep omasal epithelium induces expression of a peptide transport protein(s) in Xenopus laevis oocytes.

To verify research from this laboratory indicating that sheep omasal epithelium contains mRNA encoding for a peptide transporter(s) and to determine di- to octapeptide transport capability, we injected poly(A)+ RNA isolated from sheep omasal epithelium into Xenopus laevis oocytes. 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 hexa-, 1 hepta-, and 1 octapeptide) transport capability was measured by impaling oocytes with a microelectrode to monitor membrane potential (Vm). Oocytes were maintained in pH 5.5 buffer. Peptide transport was identified as being expressed when, in the presence of a buffered peptide substrate (1 mM), the oocyte membrane showed persistent depolarization (a more positive Vm). In the absence of peptide transport, the membrane became depolarized with the addition of buffered substrate, but it rapidly repolarized to the resting potential. Peptide transport was expressed for some di-, tri-, and tetrapeptides. Measured depolarization ranged from 9.6 mV to 42.1 mV. Larger peptides were not transported by the oocytes. When transport expression was measured with the substrates in a pH 7.5 buffer, no transport occurred, indicating that transport was dependent on a proton gradient. Thus, sheep omasal epithelium contains mRNA that codes for a protein(s) capable of proton-dependent di-, tri-, and tetrapeptide transport. Results from the present study provide further evidence that absorption of peptides from the ruminant stomach is possible.