Transactions of magnesium (Mg) along the gastrointestinal tract and the effect of change in potassium (K) intake were recorded in two in vivo experiments in sheep and cattle. Additional information on the sensitivity to K intake was obtained by comparing Mg transport and electrochemical properties of isolated rumen epithelia of sheep and cattle in 4 additional in vitro experiments. The experiment described in Chapter 2, and performed in sheep housed indoors in metabolic crates, investigated the compensatory capacity of the intestine to respond to the reduction in Mg absorption from the stomach as a consequence of increase in K intake. The animals were equipped with a ruminal cannula and two intestinal cannulae (duodenum and ileum) and flow of digesta was measured by the addition of two indigestible markers, chromium ethylenediaminetetra-acetic acid (Cr-EDTA) and ytterbium acetate (Yb). The animals were infused in a Latin square design for periods of 10 days with a solution of K bicarbonate that provided between 15 and 47 g of K/day. The diet consisted of a 50:50 combination of concentrates plus lucerne hay that provided around 3.7 g of Mg per day and 15 g of K per day. After 5 days of infusion samples of feed, faeces, urine and plasma were collected and analysed for Mg and K content. After 6 days of infusion, samples of duodenal and ileal flow were obtained. The treatments reproduced the detrimental effect of K on Mg absorption, especially in the rumen; a rise in K intake from 15 to 23 g/day reduced total Mg absorption from the gastrointestinal tract from 1.36 to 1.23 g/day, further increase in K intake to 38 and 47 g/day reduced absorption to 1.12 and 1.05 g/day, an overall reduction of around 50% in Mg apparent availability. Magnesium was mainly absorbed in the stomachs and large intestine with the small intestine a site of net secretion. Most of the reduction in Mg absorption with increase in K intake occurred in the stomachs, reducing from 1.86 to 1.11 g/day. A compensatory reduction in the net secretion of Mg from the intestines (small and large) was observed. This compensation was largely due to reduction in net secretion from the small intestine, from 0.85 to 0.22 g/day, rather than an increase in net absorption from the large intestine, although both segments acted synergistically. Results also suggested significant individual variation in plasma Mg concentration, urinary Mg excretion and in the flow and absorption of Mg along the gastrointestinal tract. It was suggested that most of that variability was due to genetic factors. Differences between species (cattle and sheep) were pursued during the course of the experiment described in Chapter three. Four triple cannulated rams and 3 triple cannulated dry cows were placed in metabolic crates, fed daily fresh-cut pasture and infused, in a total randomised design that provided the equivalent of an intake of 30,40 and 50 g of K per kg dry matter intake (DMI) per day. Solutions of K (as K bicarbonate) and markers (CrEDT A and Yb acetate) were infused continuously for a period of 10 days; after 5 days of infusion samples of pasture, faeces, urine and plasma were collected and analysed for Mg and K content. After 6 days of infusion, samples of duodenal and ileal flow were obtained. Total feed offered, refusals and water consumption were recorded daily. Results showed a greater sensitivity of cattle to the increase in K supply. A rise in K supply from 30 to 40 g per kg DMI/day reduced Mg absorption by almost 50% from 0.32 to 0.16 g per kg DMI/day, whereas only the highest treatment dose (50 g of K per kg DMI/day) produced the same deleterious effect in sheep. The absorption of Mg occurred mainly in the stomachs and large intestine; in contrast the small intestine was a site of net secretion in both species. The addition of K slightly reduced the rate of Mg absorption from the rumen, especially in cattle. Similarly, net Mg secretion within the intestines increased with increasing K intake in both species, only to be counterbalanced by a greater Mg absorption from the large intestine. The large intestine in both species (sheep and cattle) reduced faecal losses of Mg but was unable to fully compensate for the reduction in Mg absorption from the stomach or the greater net Mg secretion observed at the small intestine. Differences between species in water content of the faeces were observed to be mainly related to the moisture content of the digesta that reached the ileum rather than a result of differences in absorption in the large intestine. More evidence of species differences in Mg transport and of sensitivity to K intake were obtained by using isolated rumen epithelia and the Ussing chamber technique. Transport and electrophysiological properties of the tissues were observed in standard conditions and by adding different K concentrations to the mucosal side. Under standard conditions and open-circuit voltage, sheep isolated rumen epithelia had greater transmural potential difference (PDt), and lower conductance (Gt) but similar short-circuit current (Isc) than those from cattle. These results suggested that the rumen epithelium of cattle is leakier than that of sheep. Measurement of the transport of Mg showed that isolated rumen epithelia of cattle transported more Mg and was saturated at higher Mg concentrations (12 vs 4 mM) than sheep epithelia. These differences in Mg influx (transport from mucosa to serosa) were also observed in studies of Mg transport using stable isotopes. Magnesium influx (transport from mucosa to serosa) from the isolated rumen of cattle was greater than in sheep (57.5 ± 12.72 vs. 17.3 ± 12.72 nmol.cm⁻².h⁻¹); however this was counterbalanced by a greater Mg efflux (transport from serosa to mucosa) of 48.1 ± 12.72 vs. 9.9 ± 12.72 nmol.cm⁻².h⁻¹, for cattle and sheep respectively, when mucosal K concentrations were around 25 mM. A increase in K concentration on the mucosal side enhanced transmural potential difference (PDt) and short-circuit current (Isc) to a greater extent in sheep than in cattle, suggesting a greater effect of K on sheep than on cattle epithelia. On the other hand, the transport of Mg measured by stable isotopes suggested that net absorption of Mg (7.4 ± 12.72 vs. 11.1 ± 12.72 nmol.cm⁻².h⁻¹) in sheep epithelia was similar at 25 and 50 mM of K on the mucosal side, whereas net Mg influx in cattle was largely depressed as a consequence of a reduction in Mg influx (mucosa to serosa) from 57.7 ± 12.72 to 2.9 ± 12.72 nmol.cm⁻² h⁻¹ together with a constant Mg efflux (serosa to mucosa) 48.1 ± 12.72 and 41.2 ± 12.72 nmol.cm⁻².h⁻¹, presumably leaving through a paracellular shunt. However, this finding was based on date from a small size and caution should be applied to this conclusion. In conclusion, data collected from several comparative studies suggest differences in Mg apparent availability between sheep and cattle and also a greater sensitivity of cattle to an increase in K intake. This high sensitivity to K represents a great risk of hypomagnesaemia in dairy cattle in New Zealand where high K concentration is endemic in pastures. Most importantly, these results suggest that models for Mg metabolism in cattle should be based on measurements from cattle nutritional and physiological studies rather than on extrapolation from sheep studies.
| Identifer | oai:union.ndltd.org:ADTP/282575 |
| Date | January 2005 |
| Creators | Laporte Uribe, José Alberto |
| Publisher | Lincoln University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://purl.org/net/lulib/thesisrights |
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