Demonstration of peptide and free amino acid absorption by sheep forestomach epithelium using parabiotic chambers and identification of H⁺/peptide and free amino acid transport proteins in sheep omasal epithelium and b^o,+ amino acid transport proteins in pig jejunal epithelium by expression of mRNA in Xenopus laevis oocytes

The absorption of methionine and methionylglycine (Met-Gly) across sheep (average BW = 38 kg) ruminal and omasal epithelia was studied using parabiotic chambers. Ruminal tissue demonstrated a greater ability to accumulate both substrates. Omasal tissue demonstrated a greater ability to translocate methionine and Met-Gly and a greater total absorption of both. Intact Met-Gly was transferred across both tissues. More was hydrolyzed by omasal epithelia. Within tissues, the total absorption of substrates did not differ. Evidence for carrier-mediated absorption was not observed. The ability to express exogenous mRNA in defolliculated Xenopus laevis oocytes was developed using sucrose gradient size-fractionated poly(A)⁺ RNA (RNA) isolated from the jejunal epithelial tissue of pig (average BW = 33.8 kg). Compared to water-injected oocytes, RNA injected oocytes displayed greater rates of Nat-independent lysine and leucine absorption. RNA-induced uptake of lysine (Kt = 52 uM) and leucine (Kt = 97 uM) was inhibited by 5 mM leucine and lysine, respectively, by .2 mM cysteine, but not by 5 mM glutamate. RNA-induced lysine and leucine absorption also was inhibited when oocytes were injected with RNA plus DNA oligomers that were complementary to the cloned human kidney b⁰‘⁺ transporter. Oocytes were injected with RNA isolated from omasal epithelial tissue of sheep (average BW = 67.5 kg) to identify potential peptide and amino acid transport proteins. Injection of specific RNA fractions induced greater rates of glycylsarcosine (Gly-Sar) uptake, as compared to water injection of oocytes. Media pH of less than 6.5 was required for induced Gly-Sar uptake. Induced Gly-Sar uptake required a pH of less than 6.5, was saturable (Kt = .40 mM), and was inhibited by 5 mM carnosine, Met-Gly, glycylleucine, but not by glycine. The RNA-induced Gly-Sar absorption was completely inhibited when oocytes were co-injected with RNA and DNA oligomers that were complementary to the cloned rabbit H⁺/peptide cotransporter. When oocytes were assayed for their ability to absorb lysine, RNA-induced lysine absorption was determined to be Na⁺-independent and to display b⁰‘⁺-like transport activity. Collectively, these results indicate that sheep omasal epithelia possess the potential to absorb free and peptide-bound amino acids by non-mediated processes and possess mRNA that encode for Ht-dependent dipeptide and b⁰‘⁺ transport protein activity. mRNA that encodes for b⁰‘⁺-like transport activity was identified in the jejunal epithelium of growing pigs.