Rumen epithelial Na(+)/H(+) exchanger (NHE) catalyzes the exchange of
extracellular Na(+) for intracellular H(+). Thus, it is of importance in the
maintenance of Na and pH homeostasis of rumen epithelial cells. We have tested
the hypothesis that an increase in energy and protein intake induces
alterations of NHE isoform 1, 2, and 3 (NHE1, NHE1, and NHE3, respectively)
mRNA abundance in the rumen epithelium of goats. Goats (n = 26) were randomly
allocated to 2 experiments (n = 16 in Exp. 1, and n = 10 in Exp. 2) and fed
either peanut straw ad libitum [PNS, n = 8 in Exp. 1, and n = 5 in Exp. 2; 600
kJ of ME/(kg(0.75)·d)] or PNS + concentrate [CF, n = 8 in Exp. 1, and n = 5 in
Exp. 2; 1,000 kJ of ME/(kg(0.75)·d)] for 42 d. Concentrate (400 g/d) was given
daily (0800 to 1700 h) in 4 equal portions at 3-h intervals. In Exp. 1, the
goats were euthanized 2 h after the last portion of concentrate was fed, and
in Exp. 2, the goats were euthanized after a fasting period of 16 h. In Exp.
1, goats in the CF treatment exhibited a greater ruminal short-chain fatty
acid (SCFA) concentration (140.6 ± 1.30 mM) compared with those in the PNS
treatment (114.3 ± 3.11 mM; P < 0.001), and pH decreased from 6.9 ± 0.09 to
5.9 ± 0.04 (P < 0.001). Correspondingly, the mRNA expression of NHE1 and NHE3
in the rumen epithelium was greater by 20% (P = 0.041) and 25% (P = 0.043) for
goats in the CF treatment than for those in the PNS treatment. However, in
Exp. 2, 16 h of fasting abolished differences in ruminal SCFA concentration,
pH, and NHE mRNA expression between goats in the CF and PNS treatments. In
both Exp. 1 and 2, a positive correlation was observed between ruminal SCFA
concentration and expression of mRNA in NHE1 and NHE3, whereas expression was
negatively correlated with ruminal pH. In in vitro studies with isolated rumen
epithelial cells from goats fed dried grass, exposure to pH of 6.8 or to 20 mM
SCFA increased (P < 0.01) NHE1 and NHE3 mRNA expression, as compared with
exposure to pH of 7.4 or the absence of SCFA. A combination of reduced pH
(6.8) and SCFA (20 mM) further enhanced (P < 0.05) NHE1 and NHE3 mRNA
expression, indicating synergism between an increased concentration of SCFA
and low pH (P < 0.05). Messenger RNA expression of NHE2 did not vary in vitro
with pH (6.8) or SCFA (20 mM) or in vivo in Exp. 1 and 2. Thus, diet-dependent
rumen epithelial NHE1 and NHE3 expression is probably related to ruminal SCFA
concentration and pH, but that is not the case with NHE2