The absorption of magnesium from the forestomachs, stomach and small intestine of sheep

Metabolic Diseases in Beef Cattle

Beef cattle are less prone to metabolic diseases as compared with dairy cattle; however, there are disease entities of concern in feedlot and cow-calf beef cattle operations. In one study, a prevalence of 2% was found for ruminant acidosis in a feedlot; however, there is little prevalence information published with regard to metabolic diseases in beef cattle.1 Metabolic diseases covered in this article are hypomagnesemia, ruminal acidosis, and all of the common sequelae, polioencephalomalacia, manganese deficiency, and protein-energy malnutrition (PEM).

Pathophysiology of grass tetany and other hypomagnesemias. Implications for clinical management

Magnesium is an essential mineral with many physiologic and biochemical functions. Surprisingly, Mg homeostasis is not regulated by a hormonal feedback system, but simply depends on inflow (absorption) from the gastrointestinal tract and outflow (endogenous secretion, requirement for milk production, uptake by tissues). Any surplus (inflow greater than outflow) is excreted via urine. Conversely, if the outflow (mainly milk secretion and endogenous loss) exceeds inflow, hypomagnesemia occurs because of the lack of hormonal mechanisms of homeostasis. The major reason for insufficient inflow is a reduced absorption of Mg from the forestomachs. Recent studies from our laboratory and data from the literature permit the proposal of a putative transport model for the secondary active transport of Mg across the rumen epithelium. This model includes two uptake mechanisms across the luminal membrane (PD-dependent and PD-independent) and basolateral extrusion via a Na/Mg exchange. The well-known negative interaction between ruminal K concentration and Mg absorption can be explained on the basis of this model: an increase of ruminal K depolarizes the potential difference of the luminal membrane, PDa, and as the driving force for PD-dependent (or K-sensitive) Mg uptake. Because Na deficiency causes an increase of K concentration in saliva and ruminal fluid, Na deficiency should be considered a potentially important risk factor. The data obtained from in vitro and in vivo studies on the association of Mg transport, changes of ruminal K concentration, and PDa are extensive and confirm the model, if the ruminal Mg concentrations are below 2 to 3 mM. It is further proposed by the model that the PD-independent Mg uptake mechanism is primarily working at high ruminal Mg concentration (above 2 mM). Mg absorption becomes more and more independent of ruminal K with increasing Mg concentration, which can be considered as an explanation for the well-known prophylaxis of hypomagnesemia by increasing oral Mg intake. Fermentation products, NH4+ and SCFA, influence Mg absorption. The possible meaning regarding the pathogenesis of hypomagnesemia is not quite clear. A sudden increase of ruminal NH4+ should be avoided, because high NH4+ concentrations transiently reduce Mg absorption. The most prominent signs of hypomagnesemia are excitations and muscle cramps, which are closely correlated with the Mg concentration in the CSF. It is suggested that the clinical signs are caused by spontaneous activation of neurons in the CNS at low Mg concentrations, which leads to tetany. Prophylactic measures are discussed in context with the known effects on ruminal Mg absorption.

Impaired absorption of magnesium in the aetiology of grass tetany

Magnesium is absorbed mainly from the reticulo-rumen and there are a number of factors reducing its absorption. The chief of these is the increased potential difference across the rumen epithelium caused by increased intraruminal potassium concentration. A significant amount of magnesium leaves the extracellular fluid each day as saliva. As only a portion of it is reabsorbed the rest is lost through the endogenous faecal excretion of magnesium. Thus, during impaired magnesium absorption, saliva could play an important role in the aetiology of hypomagnesaemia especially during dietary sodium depletion and the resultant increase in the potassium content of the saliva.