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+ flu x rates (J(ms)) exceeded respective flux rates in the opposite directi on (J(sm)) in both species and in all segments of the intestinal tract . This resulted in net Ca2+ flux rates (J(net) = J(ms) - J(sm)) rangin g 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 J(net) in jejunum, and in goats, J(net) in duodenum and jejunum were significantly different fr om zero. Using sheep rumen wall epithelia, significant J(net) of Ca2of around 5 nmol . cm(-2). h(-1) could be detected. Since the experime nts were carried out in the absence of an electrochemical gradient, si gnificant 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. J(net) of Ca2+ across goat rumen epithelia was significantly reduced by 1 mmol . l(-1) verapamil in th e 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 re gulatory similarities to active Ca2+ transport as described for the up per small intestines of monogastric species.