Animal Health: Joint Animal Health/Reproduction Symposium: Transition Cow Calcium Homeostasis-Health Effects of Hypocalcemia and Strategies for Prevention

摘要

If luminal Ca concentrations are high, Ca transport across epitheliaoccurs mainly via passive, paracellular mechanisms driven by an electrochemicalgradient or solvent drag. Restricted supply as well as anincreased demand induce an endocrine response, mainly mediated bycalcitriol, resulting in a relatively greater contribution of a more efficient,active, transcellular Ca transport. In intestinal and renal epithelia, thisis a 3-step process involving apical uptake via Ca channels (TRPV5,TRPV6), protein-bound diffusion through the cell (CaBPD28K,CaBPD9K) and active extrusion at the basolateral membrane (NCX1,PMCA1b). To further improve our strategies to prevent hypocalcemia,applied studies should be combined with fundamental research on thecomplex regulation of these transport processes, especially becauseruminants show some particularities in comparison to monogastricanimals. Although the small intestine expresses the same vitaminD-dependent Ca transport proteins (TRPV6, CaBPD9K, PMCA1b), fluxrates determined using mucosal preparations from sheep and goats weredemonstrated to be much smaller than those detected in samples frompigs or horses. Nevertheless, the expression of the above mentionedtransport proteins is regulated and altered by dietary Ca, N restrictionand lactation. The rumen has also been shown to actively absorb Ca inmany in vitro and in vivo experiments. But the absence of any relevantamounts of TRPV6 or CaBPD9K, the unresponsiveness to dietary Carestriction or calcitriol treatment and the pivotal role of luminal SCFAindicate an alternative, so far unknown mechanism. Interestingly, lactationand a diet negative in DCAD increased ruminal flux rates. RenalCa excretion is generally low in ruminants and cannot be significantlydiminished to compensate for challenges of Ca homeostasis. But whenthe animals are kept on a ration negative in DCAD renal Ca resorptionis inhibited. Preliminary experiments demonstrated that the expressionof TRPV5, CaBPD28K and NCX1 is not significantly altered underthese conditions. This might explain why renal resorption is immediatelyrestored when the ration is changed p.p. An adaptation on the functionallevel occurs faster than the stimulation via the genomic pathway.