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Apical uptake of acetate into ruminal epithelia of sheep comprises at least three components differing in their HCO3-dependence and NO3-sensitivity. The present study aimed to elucidate whether butyrate has related transport modes.
Materials and Methods:
The transepithelial flux rates of [3H]-acetate and [14C]-butyrate (10 mM each) were determined in parallel in isolated ruminal epithelia of 17 sheep using Ussing chambers. In separate epithelia from the same sheep, apical uptakes of [3H]-acetate and [14C]-butyrate were determined in parallel. The chloridefree incubation buffer had a mucosal and serosal pH of 6.1 and 7.4, respectively. Bicarbonate-dependent (BD) flux or uptake was calculated as the difference between measurements in the presence (24 mM) and absence of HCO3-Bicarbonate-independent (BI) flux or uptake was further divided into NO3-sensitive NS) and NO3-insensitive (NI) components based on flux or uptake inhibition by 40 mM NO3-.
The total flux rate was higher for butyrate compared to acetate (2.70 ± 0.15 vs. 1.17 ± 0.10 μmol· cm-2 · h-1; P < 0.05). This was linked to higher BD (1.14 ± 0.14 vs. 0.68 ± 0.10 μmol · cm-2 · h-1; P < 0.05) and BI-NI components of flux rate (1.84 ± 0.15 vs. 0.40 ± 0.02 μmol · cm-2 · h-1; P < 0.05). By contrast, the BI-NS component of flux rate was smaller and negative for butyrate compared to acetate (-0.43 ± 0.19 vs. 0.084 ± 0.18 μmol · cm-2 · h-1; P < 0.05). Total flux rates of butyrate and acetate (r = 0.91), as well as their BD (r = 0.69) and BI-NI components (r = 0.61) were positively correlated (P < 0.01), while the BI-NS components were negatively correlated (r = -0.53; P < 0.05). Similar results were obtained for the uptakes of butyrate and acetate, except for an absent negative correlation between the BI-NS components (r = 0.34).
The results point to a common bicarbonate-dependent transport mechanism for butyrate and
acetate. The BI-NS component could represent a nitrate-sensitive protein that is capable of acetate/butyrate exchange, at least, under steady state conditions and in the absence of chloride and bicarbonate.