Calcium transport by the normal rat colon in vivo and in vitro

We studied calcium transport in vivo and in vitro in large bowel of normally fed young male rats. Solutions containing zero, 1.7, 3.4 and 5.0 mM calcium and isotopic 45Ca as tracer were perfused in vivo through the lumen of cecum and descending colon. In vitro flux studies were also carried out in cecum with 2.0 and 0.2 mM calcium solutions utilizing the technique of Ussing and Zerahn whereby both chemical and electrical gradients across planar sheets of cecal tissues were nulled. The combined findings demonstrated that (i) cecum was more permeable to calcium than descending colon indicating a proximal to distal absorption gradient in large bowel; (ii) the transport of calcium in both segments increased with increasing luminal calcium concentration, and (iii) calcium transport in both cecum and descending colon was passive only. Thus the large bowel appears to play only a minor role in calcium homeostasis in the normally fed rat.