The relationships between potassium intakes, transmural potential difference of the rumen epithelium and magnesium absorption in wethers

In vitro studies with isolated sheep rumen epithelium have shown that an increase in the lumen K concentration induces an increase in the transmural potential difference across the rumen epithelium (serosal side: positive), which is associated with a decrease in Mg transport. However, at lumen K concentrations > 80 mmol/l, Mg transport across the epithelium became independent of the lumen K concentration. The present study was carried out to determine whether this observation also occurs in vivo. Four ruminally fistulated wethers were fed four rations supplemented with KHCO₃ (15.7, 37.6, 59.4 or 77.4 g K/kg DM) in a 4 × 4 Latin square design. Increased K intakes significantly increased the rumen K concentration. For all data combined, Mg absorption expressed as % intake was negatively correlated with the rumen K concentration. However, apparent Mg absorption either expressed in absolute terms (g/d) or as % intake was not significantly affected when the dietary K concentration was increased from 59.4 to 77.4 g/kg DM. Rumen K concentration was inversely correlated with the transmural potential difference (blood side: positive) (Pearson's r - 0.709; R_adj² 0.468, P=0.002, n 16). It is concluded that in wethers apparent Mg absorption becomes independent of the dietary K concentration when the K concentration is > 60 g/kg DM or equivalent to a postprandial rumen K concentration of about 125 mmol/l.