[Electrophysiologic changes in rumen epithelium in their effect on magnesium transport--a review]

The forestomach is the main site of Mg2+ absorption in the gastrointestinal tract of ruminants and maintains Mg2+ homeostasis. It has long been known that an increase in K+ intake and, consequently, in ruminal K+ concentration ([K+]) decreases the apparent digestibility of Mg2+, which increases the risk of hypomagnesemia and tetany. The present review summarizes new findings on the mechanisms of Mg2+ absorption across the rumen epithelium. It has been shown that transcellular and active Mg2+ transport is the predominant pathway for Mg2+ transport from lumen to blood. It is well established that the apical uptake of Mg2+ is mediated by a PD-independent of K(+)-insensitive and by a parallel working PD-dependent, K+ sensitive mechanisms. The predominant driving force for the electro-diffusive Mg2+ uptake is PDa, the potential difference across the apical membrane of the rumen epithelium, that amounts to -50 mV under physiological conditions, permitting an effective Mg2+ absorption even at very low luminal Mg2+ concentrations. The antagonism between K+ and Mg2+ absorption can be explained by K+ dependent electrophysiological changes of the rumen epithelium. An elevation of the ruminal [K+] has two different effects that are responsible for the observed reduction of net Mg2+ absorption; (1) It depolarizes PDa and thereby reduces the driving force for the electro-diffusive Mg2+ uptake into the ruminal epithelial cells, hence decreases the cytosolic [Mg2+] and the transcellular component of Mg2+ absorption; (2) It increases the transepithelial potential difference (PDt; blood-side positive) and, hence causes a small, passive backflow of Mg2+ via the paracellular route from the blood side into the lumen. The second, PD-independent uptake mechanism is primarily working at high ruminal [Mg2+]. Therefore the negative effect of K+ can be compensated by this K+ insensitive Mg2+ absorption, if high [Mg2+] are present in the ruminal fluid.