Abstract
From various in vivo and in vitro studies it has been shown that the rumen represents a significant site of Ca2+ absorption in sheep and goats. It was the aim of the present study to further characterize the underlying mechanisms. Unidirectional flux rates of Ca2+ across rumen wall epithelia of sheep were measured in vitro by applying the Ussing-chamber technique in the absence of electrochemical gradients. Under these conditions, significant Ca2+ net flux rates (Jnet) clearly indicate the presence of active mechanisms for Ca2+ transport. Short chain fatty acids (SCFAs) caused highest stimulation of Ca2+ Jnet (6.3 +/- 1.9 nmol.cm-2.h-1) when used as a mixture of acetate, proprionate and butyrate in physiological proportions (36, 15, 9 mmol.l-1, respectively). The effect of 30 mmol.l-1 butyrate (3.2 +/- 0.6 nmol.cm-2.h-1) was higher than respective amounts of propionate and acetate (0.6 +/- 0.8 nmol.cm-2.h-1 and 0.9 +/- 0.8 nmol.cm-2.h-1, respectively). Eliminating SCFAs resulted in Ca2+ Jnet of 0.4 +/- 1.1 nmol.cm-2.h-1. Addition of Ca channel blocker verapamil (mucosal 1 mmol.l-1) had no significant effect on SCFA-stimulated Jnet of Ca2+, whereas application of Na+/H- inhibitor amiloride (mucosal 1 mmol.l-1) further enhanced the Ca2+ Jnet by > 65%. The Ca(2+)-pump inhibitor vanadate had no significant effect on Jnet of Ca2+. Dietary Ca depletion enhanced calcitriol plasma concentrations but had no effect on active Ca2+ absorption across the rumen wall of sheep. In addition, no effect on active Ca2+ absorption could be observed during early lactation. In conclusion, there is clear evidence for the rumen as a main site for active Ca2+ absorption in sheep. Our results suggest the presence of a Ca2+/H+ exchange mechanism in the apical membrane of rumen epithelial cells which depends on SCFA absorption and which does not seem to be under the control of calcitriol. Basolateral Ca2+ extrusion occurs independently from Ca(2+)-pump activity and may be accomplished via Na+/Ca2+ exchange.
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