Magnesium (Mg2+) is an essential mineral without known specific regulatory mechanisms. In ruminants, plasma Mg2+ concentration depends primarily on the balance between Mg2+ absorption and Mg2+ excretion. The primary site of Mg2+ absorption is the rumen, where Mg2+ is apically absorbed by both potential-dependent and potential-independent uptake mechanisms, reflecting involvement of ion channels and electroneutral transporters, respectively. Transport is energised in a secondary active manner by a basolateral Na+/Mg2+ exchanger. Ruminal transport of Mg2+ is significantly influenced by a variety of factors such as high K+ concentration, sudden increases of ammonia, pH, and the concentration of SCFA. Impaired Mg2+ absorption in the rumen is not compensated for by increased transport in the small or large intestine. While renal excretion can be adjusted to compensate precisely for any surplus in Mg2+ uptake, a shortage in dietary Mg2+ cannot be compensated for either via skeletal mobilisation of Mg2+ or via up-regulation of ruminal absorption. In such situations, hypomagnesaemia will lead to decrease of a Mg2+ in the cerebrospinal fluid and clinical manifestations of tetany. Improved knowledge concerning the factors governing Mg2+ homeostasis will allow reliable recommendations for an adequate Mg2+ intake and for the avoidance of possible disturbances. Future research should clarify the molecular identity of the suggested Mg2+ transport proteins and the regulatory mechanisms controlling renal Mg excretion as parameters influencing Mg2+ homeostasis.

To analyze the elemental composition and topology of the extracellular compartments of the compound eye, the eyes of blowflies Calliphora vicina were rapidly frozen and ultrathin cryosections were freeze dried. Three zones of an ommatidium, peripheral cytosol of visual cells, rhabdomeres, and ommatidial cavities were analyzed by X-ray microprobe analysis. The ommatidial cavity was found to contain sodium and potassium in proportion similar to that in the blowfly hemolymph. Potassium-to-sodium ratio in a cytosol was typical for a cytosol. The rhabdomeres displayed an electrolyte content intermediate between the above compartments. Three topologically connected extracellular compartments were characterized by the experiments with tracers, monastral blue and lanthanum: (1) common intercellular space of ommatidia including peripheral clefts between the visual cells, both tracers entered this compartment; (2) the ommatidial cavity, which is not accessible for monastral blue, however, as revealed by our X-ray microanalysis, it was reachable for lanthanum; (3) rhabdomeric loops, which were accessible for lanthanum entering either via the cavity or from the common intercellular clefts. The above characteristics of the ionic content and topology of ommatidial compartments might suggest higher sodium and lower potassium content in the microvilli as compared with the cytosol. The rhabdomeric and “cavital” plasma membranes are assumed to be permeable for these ions so that a voltage of only 25–30 mV, negative inside, is probably formed across them, much lower than the known resting potential −60 mV across the peripheral plasma membrane of a visual cell.