Transport of cations and anions across forestomach epithelia: conclusions from in vitro studies.
Six Angus x Simmental steers, surgically equipped with abomasal and ileal cannulae, were used during three metabolism trials. Steers were blocked according to weight ad were allotted to three dietary K levels (.6, 2.4 and 4.8% dry basis) for each trial with the restriction that no animal would receive the same level during two successive trials. Each trial consisted of a 5-d transition, a 10-d preliminary period, a 7-d collection of feed, feces and urine and a 6-d sampling of feed, feces and abomasal and ileal fluid. Chromic oxide (.5%) was used as an indicator to measure flow through the digestive tract. Magnesium absorption decreased linearly (P less than .05) with increasing level of dietary K. The primary site of Mg absorption was the preintestinal region followed by a net secretion into the small intestine. Preintestinal Mg absorption was decreased 39% when 4.8% K was fed. Serum Mg was lower at high levels of dietary K. Potassium absorption increased linearly when expressed as g/d and curvilinearly when expressed as percentage of intake, with increasing levels of dietary K. When .6% K was fed, the primary site of K absorption was the small intestine, but with the high levels of K, the preintestinal region was also an important site of absorption. The effect of high K level on Mg absorption in steers was similar to that observed previously in sheep.