Einfluss bioaktiver Pflanzenlipide auf Kalziumresorptionsmechanismen im Gastrointestinaltrakt von Schafen
Influence of plant bioactive lipid compounds on calcium absorption mechanisms in the gastrointestinal tract of sheep
Stimulation of calcium absorption can counteract hypocalcemia of dairy cows in early lactation. Plant bioactive lipid compounds (PBLC), most notably menthol, are known as agonists for nonselective cation channels of the transient receptor potential (TRP) family and were recently detected in the ruminal epithelium. PBLC acutely stimulated ruminal calcium absorption ex vivo and resulted in higher blood calcium concentrations in cattle in vivo. The present thesis aimed at reproducing the positive menthol-induced effects from cattle in the model animal sheep and, furthermore, at elucidating calcium absorption mechanisms in the gastrointestinal tract. For this purpose, 24 sheep were divided into a control (Kon), PBLC-L and PBLC-H group, which received 0, 80 and 160 mg/d of a blend of menthol-rich PBLC. After four weeks, blood samples were taken from all animals and specific in vivo parameters were analyzed. The ruminal and jejunal epithelium was analyzed for mRNA expression levels of TRPA1, TRPV3, TRPV5-6, and TRPM6-8, as well as claudin (Cldn)1, -2, -3, -4, -7, occludin (Occl), and zonula occludens (ZO)1 and -2. Calcium and fluorescein flux rates as well as electrophysiological properties of rumen and jejunum epithelia were measured in Ussing chambers in the presence and absence of mucosal sodium, and also in the absence of magnesium. Acute changes in calcium flux rates were measured after mucosal application of 50 μM menthol. Menthol-rich PBLC supplementation resulted in a trend for increased blood calcium concentrations. In the rumen, Cldn7 expression increased linearly with increasing PBLC dose, while ZO2 increased quadratically. In the jejunum, TRPA1, TRPV5, TRPV6, Cldn4, and ZO2 tended to decrease linearly with increasing PBLC dose. Significant calcium net absorption occurred in the rumen, whereas the jejunum showed high passive calcium permeability but no net transport. Ruminal calcium net flux rates increased quadratically with PBLC feeding and decreased under sodiumfree mucosal incubation. Ruminal fluorescein flux rates were not affected by feeding. In the jejunum, fluorescein transport decreased in the mucoserosal direction. In the rumen, shortcircuit current increased in both PBLC feeding groups. Acute menthol application stimulated mucoserosal and calcium net flux rates only in the rumen, with higher stimulation responses in both PBLC groups. It can be summarized that calcium transport in the rumen occurs as an active and transcellular process. TRPV3 is most likely significantly involved in this process, which can be stimulated by menthol. Supplementation with menthol-rich PBLC increases transcellular calcium resorption in the rumen and sensitizes it to acute stimulation by menthol. In contrast, both a functional and structural PBLC effect is not detectable in the jejunum at the doses administered. The use of a combination of long-term PBLC supplementation with acute menthol administration around the parturition period appears to be a promising alternative or complement to current hypocalcemia prevention measures, although final evaluation requires studies in cows around the parturition period.