The absorption of calcium ions from the ovine reticulo-rumen

Abstract

Net Ca2+ and Mg2+ absorption rates were measured in vivo from buffer solutions placed in the washed reticulo-rumen, isolated in situ in 30 conscious, trained sheep. An increase in concentration of short chain fatty acids (SCFA) in the buffer, over the range 0-50 mM, was shown to stimulate the net rates of absorption of Ca2+ and Mg2+ ions from the rumen. Similarly, the results of in vitro experiments, carried out with ovine rumen epithelium mounted in short-circuited Ussing chambers, showed that the absence of SCFA from the chamber fluid resulted in a reduction in Jnet Ca2+ caused by reduced flux of Ca2+ ions in the mucosal to serosal direction (Jms Ca2+). The addition of 1 mM acetazolamide, an inhibitor of carbonic anhydrase, to the ruminal buffer used in the in vivo experiments led to significant reductions in the net absorption rates of Ca2+ and Mg2+ ions in the presence of SCFA (50 mmol x l(-1)) but not in the absence of SCFA. However, in the in vitro experiments, the addition of 60 microM ethoxyzolamide had no significant effect on Jnet Ca2+. A reduction in pH of the intraruminal buffer in vivo from 6.8 to 5.4 led to significant increases in the net absorption rates of Ca2+ and Mg2+ ions, an effect which was duplicated for Ca2+ in preliminary in vitro experiments in which the pH of the mucosal buffer was reduced from 7.4 to 5.4. This stimulatory effect was confined to Jms Ca2+ and Jnet Ca2+. Ussing chambers were also used to demonstrate that Jnet Ca2+ was reduced by a high transmural potential difference (PD), caused by voltage clamping, independently of the mucosal K+ concentration. Both unidirectional Ca2+ fluxes consisted of a PD-dependent and a K+-insensitive PD-independent component. The latter may be represented by a Ca2+/ 2H+ antiporter. It is postulated that SCFA, and to a lesser extent H2CO3, can stimulate Jms Ca2+ by activation of an apical Ca2+/2H+ antiporter through the provision of protons within the ruminal epithelial cell. A mild reduction in ruminal pH may also lead to a similar stimulation of this putative electroneutral exchange.

Transport of cations and anions across forestomach epithelia: conclusions from in vitro studies

“…When Ca 21 intake rises to 8 g/day in sheep or to 120 g/day in cattle, absorption of Ca 21 is shifted to preintestinal regions (Grace et al, 1974;Khorasani et al, 1997;Schrö der and Breves, 2006 (Hö ller et al, 1988a and1988b;Schrö der et al, 1997;Wadhwa and Care, 2000). Active ruminal Ca 21 absorption can be increased by concentrate feeding (Uppal et al, 2003), in the presence of SCFAs (Schrö der et al, 1997 andWadhwa and Care, 2000;Uppal et al, 2003, LeonhardMarek et al, 2007a or by increasing the amounts of luminal chloride (Leonhard-Marek et al, 2007a), whereas sulfate ions seem to have stimulatory and depressing effects with no net change of Ca flux rates in vitro (Leonhard-Marek et al, 2007b). An increased absorption of undissociated SCFA will provide protons to the cell interior, while the ruminal uptake of SCFA 2 or Cl 2 via anion exchange mechansims is accompanied by a secretion of bicarbonate (Gä bel et al, 1991a) and an increase in surface pH (Leonhard-Marek et al, 2006).…”

Mechanisms and regulation of calcium absorption from the gastrointestinal tract in pigs and ruminants: comparative aspects with special emphasis on hypocalcemia in dairy cows

“…The potential role of the reticulorumen has also been examined by applying the washed rumen technique and a positive correlation between ruminal Ca concentration and Ca net absorption could be demonstrated (Höller et al, 1988). Active mechanisms have been shown to make significant contributions to Ca transport in caprine as well as ovine rumen, by measuring Ca flux rates in Ussing chambers (Schröder et al, 1997(Schröder et al, , 1999Wadhwa and Care, 2000;Uppal et al, 2003). These mechanisms could only be detected when short chain fatty acids (SCFAs) were present in the luminal buffer solutions.…”

Dietary magnesium increases calcium absorption of ovine small intestine in vivo and in vitro

“…Furthermore, the significant net flux rates of Ca were recently reported in the caprine duodenum and jejunum and in the ovine jejunum using the Ussing chamber technique [33]. In addition, it has been recently reported that there existed significant net Ca absorption in the rumen, and this is increased by 1,25 (OH) 2 D 3 and short-chain fatty acids (SCFAs) [5,38]. On the contrary, it has been believed that the rumen is the major site of Mg absorption in the ruminant.…”

“…Martens et al reported that the mechanism of net Mg absorption in the rumen is dependent on the transepithelial potential difference by the difference of K concentration between mucosal and serosal [25,26]. Also, it was recently reported the net Mg absorption was influenced by CO 2 , SCFAs and phosphate in the rumen [12,38].…”