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An energy-rich diet led to enhance ruminal Na absorption, which is associated with elevated plasma IGF-1 levels and increased number of IGF-1 receptors in rumen papillae. This study examined the in vitro effect of IGF-1 on Na transport across the ruminal epithelium of hay-fed sheep. At a concentration ranging from 20 to 100 μg/l, serosal LR3-IGF-1 rapidly (within 30 minutes) stimulated the mucosal to serosal Na flux (JmsNa) and consequently the net Na flux (JnetNa). Compared with controls, JnetNa increased by about 60% (p<0.05) through the serosal application of LR3-IGF-1 (20 μg/l). The IGF-1-induced increment of JmsNa and JnetNa was inhibited by mucosal amiloride (1 mmol/l). Neither IGF-1 nor amiloride altered Gt or Isc of the isolated ruminal epithelium. These data support the assumption that the stimulating effect of serosal-added IGF-1 on Na transport across the rumen epithelium is mediated by Na+/H+ exchange (NHE). A further study performed with cultured ruminal epithelial cells (REC) and a fluorescent probe (BCECF) to estimate the rate of pHi recovery after acid-loading. The pHi of isolated REC was 6.43 ± 0.15 after butyrate loading and recovered by 0.26 ± 0.02 pH units/15 min. Application of LR3-IGF-1 (20 μg/l) significantly increased the rate of pHi recovery to 0.33 ± 0.02 pH units/15 min. Amiloride administration reduced the recovery rate in both control and IGF-1-stimulated cells. These results show, for the first time, that an acute effect of IGF-1 on Na absorption across ruminal epithelium results from increased NHE activity. IGF-1 is thus important also for the fast functional adaptation of the ruminal Na transport via NHE.