Magnesium Absorption From the Digestive Tract of Sheep

Abstract

SUMMARYFactors affecting absorption of Mg from the ovine rumen have been studied using either a pouch constructed from part of the dorsal rumen or by an isolated washed rumen technique in vivo. Net absorption of Mg against the prevailing electrochemical gradient was observed. An increase in the K/Na ratio within the rumen led to an increase in the potential difference across the rumen wall, blood positive, and to a decrease in the net efflux of Mg from the rumen. This decrease was due to an increase in Mg influx into the rumen. The addition of ammonium chloride (30 mmol/l) to the rumen contents also led to a reduction in net Mg absorption but to no significant change in potential difference. The effects of high K/Na ratio and high ammonium ion concentration within the rumen were additive in causing decreases in net effluxes of both Mg and Na. An inverse relationship was demonstrated between the Ca concentration in the rumen and the net absorption rate of Mg. It was concluded that the efflux of Mg across the rumen wall depends at least in part on a functional system for Na transport.

Hypomagnesemia Tetany in Cattle

“…Many studies results established that the absorption of Mg from the temporarily isolated rumen was severely reduced with increasing NH4 + concentrations from 0-40 mmol.l -1 [36]. This decrease agrees with the observation of Care et al [32] in studies with a rumen pouch in sheep. Mg absorption was decreased by ammonia in acute experiments, and this effect was additive to the known depression of K. But it is inconsistent with the findings of [51] who had not noticed any effect for high protein intake or high ruminal NH4 + concentration on apparent Mg digestibility or urinary Mg excretion in conventional balance studies.…”

“…The (PD)-dependent magnesium absorption is therefore also called potassiumsensitive. The extent of the negative effect of potassium on magnesium absorption depends on the ruminal concentration of magnesium and it has been shown that supplemental magnesium can counteract the inhibitory effect of high levels of dietary potassium [32,33]. The effect of K appears to be dose dependent [34].…”

“…In addition, the absorbed Na cannot be stored and is rapidly excreted via urine [42]. Care et al [32] showed that in sheep the K:Na ratio affects the absorption of Mg from a solution placed in a rumen pouch. A high K:Na ratio significantly depresses the rate of absorption which is related to the PD across the rumen wall (blood positive) compared with a solution with a low K:Na ratio.…”

Quantitative Prediction of Magnesium Absorption in Dairy Cows

“…It is well known that both dietary Mg and K are important determinants of Mg absorption in ruminants (Brown et al, 1978;Martens, 1979;Care et al, 1984;Martens et al, 1987;Leonhard et al, 1989;LeonhardMarek and Martens, 1996). However, Kemp (1960) suggested that dietary CP might interfere with Mg absorption because signs of tetany were observed when cows were fed herbage heavily fertilized with N. However, under controlled feeding conditions, dietary CP did not affect Mg absorption (Grace and MacRae, 1972;Moore et al, 1972;Fontenot et al, 1973;Gä bel and Martens, 1986).…”

Magnesium transport in the gastrointestinal tract

“…Under these conditions osmotic water and solvent drag of Mg ++ through the paracellular pathway probably reverses, leading to Mg ++ losses and clinically observed hypomagnesemia (24). (17,65) and have shown that the reduced net Mg ++ transport at high ruminal K + concentrations is closely correlated with electrophysiological changes within the rumen epithelium. a) There is a positive correlation between the (log) K + concentration of the ruminal fluid and the transepithelial potential difference (PD t ) (23, 45, 92; see Table 3 Table 2).…”