In vitro studies on calcium absorption from the gastrointestinal tract in small ruminants

Unidirectional flux rates of Ca2+ across gastrointestinal tissues from sheep and goats were measured in vitro by applying the Ussing-chamber technique. Except for the sheep duodenum, mucosal to serosal Ca2+ flux rates (Jms) exceeded respective flux rates in the opposite direction (Jsm) in both species and in all segments of the intestinal tract. This resulted in net Ca2+ flux rates (Jnet = Jms-Jsm) ranging between -2 and 9 nmol.cm-2.h-1 in sheep and between 10 and 15 nmol cm-2.h-1 in goats. In sheep, only Jnet in jejunum, and in goats, Jnet in duodenum and jejunum were significantly different from zero. Using sheep rumen wall epithelia, significant Jnet of Ca2+ of around 5 nmol.cm-2.h-1 could be detected. Since the experiments were carried out in the absence of an electrochemical gradient, significant net Ca2+ absorption clearly indicates the presence of active mechanisms for Ca2+ transport. Dietary Ca depletion caused increased calcitriol plasma concentrations and induced significant stimulations of net Ca2+ absorption in goat rumen. Jnet of Ca2+ across goat rumen epithelia was significantly reduced by 1 mmol.l-1 verapamil in the mucosal buffer solution. In conclusion, there is clear evidence for the rumen as a main site for active Ca2+ absorption in small ruminants. Stimulation of active Ca2+ absorption by increased plasma calcitriol levels and inhibition by mucosal verapamil suggest mechanistic and regulatory similarities to active Ca2+ transport as described for the upper small intestines of monogastric species.