|dc.description.abstract||Aspects of magnesium (Mg) metabolism were investigated in mature female sheep housed indoors in metabolism crates. Three in vivo experiments and one in vitro experiment were conducted. Experiment 1 involved studies on the interactions between dietary Mg and potassium (K) intakes and their effects on net Mg absorption. In Experiment 2 the sites of Mg absorption within the large intestine were investigated. Prior to Experiment 4 an in vitro study was undertaken to determine the response of Mg solubility to pH manipulation in rumen and caecal digesta. Finally in Experiment 4 the effect of pH change on Mg solubility and net Mg absorption from the large intestine was studied in vivo.
In Experiment 1 intraruminal infusions, 12 d in duration, were used to supply 16 sheep with 4 K (16-46 g/kg DM/d) and 4 Mg (1.3-3.1 g/d) levels within the range likely to be experienced in New Zealand pastures. Net absorption of Mg was estimated from changes in plasma Mg concentration, urinary and faecal Mg excretion and duodenal Mg flow.
Duodenal Mg flow was used to estimate Mg absorption from the rumen. At low Mg (1.3g/d) and high K (46 g/kg DM) intakes only 0.08 ± 0.120 g Mg/d was absorbed from the rumen compared with 0.46 ± 0.191 g/d at high Mg (3.1 g/d) and low K (16 g/kg DM/d) intakes. The decline in Mg absorption from the rumen across the range of K treatments was associated with a 0.3 unit increase in rumen digesta pH and a decline in Mg, solubility from 0.38 to 0.30.
Urinary Mg excretion was used to estimate net absorption of Mg from the whole digestive tract. Increasing K intake from 16 to 46 g K/kg DM decreased urinary Mg excretion by between 0.14 and 0.30 g/d for all Mg treatments. The decline in urinary Mg excretion was associated with a decline in the coefficient of absorption, viz. the proportion of total daily Mg intake excreted in urine and entering the extracellular fluid Mg pool, from 0.30 allow K intake to between 0.13 and 0.22 for Mg intakes of 1.3 to 3.1 g/d, at high K intakes.
Increasing K intake resulted in a decline in net absorption of Mg from the whole digestive tract, supporting the findings in the literature.
The results of Experiment 1 suggest that high K intakes impair Mg absorption from the rumen and that some site distal to the rumen is involved in compensatory absorption when Mg absorption from the rumen is low. It was speculated that the large intestine is the distal site with possible reasons being discussed. Magnesium absorption at high K intakes was increased by increasing Mg intake, which is an important practical finding.
Secondly, having observed that Mg absorption from sites distal to the rumen appear to be more important as net absorption of Mg from the rumen declined, Experiment 2 was designed to investigate the potential sites of Mg absorption within the large intestine. Four grams of Mg were infused into either the terminal ileum, proximal colon or rectum during 10 h. Apparent absorption of Mg was estimated from changes in plasma Mg concentration and urinary Mg excretion.
Mean increases in urinary Mg excretion of 53.9, 59.1 and 81.2 mg/h for ileal, colonic and rectal infusions, respectively, were observed in samples collected between 6 and 12 h from the start of the treatment period. Plasma Mg concentrations were elevated by (10 to 14 mg/l) after 4, 5 and 6 h of rectal, colonic and ileal infusions, respectively. Between 0.15 and 0.18 of infused Mg was excreted in urine from all 3 infusion treatments while the increase in the extracellular fluid Mg pool equated to 10 % of urinary Mg excretion. Overall apparent absorption of Mg infused into the large intestine was 0.17-0.20.
Net Mg absorption occurred from all regions of the large intestine at a constant and relatively low rate of uptake.
Thirdly, Experiment 3 involved manipulation of rumen and caecal digesta pH, in vitro, in order to provide data to generate response curves of Mg solubility versus pH. Rumen and caecal digesta were collected, post euthanasia, from animals offered either hay or concentrate diets, or grazing pasture. Subsamples of digesta were added to a series of plastic containers and mixed with sulphuric acid or sodium hydroxide solutions of various concentrations, incubated at 39 ºC for 1 h and the pH determined. The Mg concentration in the 30 000 g supernatant fraction was determined and Mg solubility, over a range of pH values, calculated.
Magnesium solubility in the 30 000 g supernatant fraction of both rumen and caecal digesta was a function of pH. With rumen digesta Mg solubility declined from 0.86 at pH 5 to 0.30 at pH 7 and differences in response between diets were small. Conversely, in caecal digesta differences were observed among diets in both the maximum solubility observed and the rate of decline in solubility with increasing pH. At pH 5, 0.90 of Mg from hay concentrated was soluable compared with only 0,80 for herbage. By pH7 Mg solubility in digesta from hay and concentrate fed animals was almost double (0.64 vs 0.36) that in digesta from grazing animals.
Finally, to help understand post ruminal absorption mechanisms Experiment 4 was undertaken to determine the nature of the response of Mg solubility to pH change, in vivo. Five volatile fatty acid (VFA) treatments, 0-880mmol/d, were infused into the terminal ileum for 24 h. Blood and urine samples were used to estimate changes in net absorption of Mg and colonic digesta samples collected to monitor digesta pH and Mg solubility changes.
Digesta pH decreased by 2.1 ± 0.48 pH units and Mg solubility increased from 0.21 ± 0.03 to 0.52± 0.01 with the 880 mmol VFA infusion. Urinary Mg excretion increased during the first 4 h of infusion on all treatments but this increase was not sustained so that by 24 h urinary Mg excretion was at or below levels prior to infusion. Increasing VFA infusion tended to decrease plasma Mg concentration.
Results indicated that, in vivo, proximal colonic digesta is responsive to pH. The lack of response of Mg absorption to increased Mg availability is difficult to explain but possible reasons are discussed.
In summary, although high K intakes can impair Mg absorption from the rumen Mg absorption from the entire digestive tract can be increased by increasing Mg supply. The importance of post ruminal absorption of Mg appears to increase as rumen absorption of Mg decreases. Magnesium can be absorbed from throughout the large intestine. The lack of information on the physiological changes occurring the large intestine during Experiments 2 and 4 make it difficult to draw firm conclusions on the transport mechanism involved in Mg uptake from the large intestine. The limited evidence available suggests that an energy dependent carrier system may be involved. Within the large intestine Mg availability is responsive to pH and it is possible that significant differences occur between diets which are not directly attributable to pH.||en