Native corn starch versus either cellulose or glucose in the diet and the effects on apparent magnesium absorption in goats

Magnesium homeostasis in cattle: absorption and excretion

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.

Effects of two different feeding strategies during dry-off on certain health aspects of dairy cows

With increasing milk production and short calving intervals, high daily milk yields at dry-off are rather common, making the dry-off procedure difficult and increasing the risk for health problems during the dry-off period. The objective of the following study was to compare the effects of 2 dry-off protocols, using different nutrient supplies, on health, as measured by clinical findings, intramammary infections, milk somatic cell count, and plasma concentrations of cortisol, serum amyloid A, haptoglobin, and Mg, in dairy cows. Twenty-one primi- and multiparous dairy cows were randomly assigned to 2 different feeding treatments. One group was fed ad libitum straw (straw), whereas the other group was fed 4 kg of DM silage daily and ad libitum straw (silage) during dry-off (i.e., for 5 d). All cows were milked in the morning of d 3 and 5 during this period. At the start of dry-off (d 0), the average daily milk yield was 17.1 +/- 0.8 kg. The plasma cortisol concentration increased during dry-off only in cows fed straw. There was no significant effect of treatment on plasma serum amyloid A, but the concentration increased during dry-off in both groups. The plasma Mg concentration decreased during dry-off, and the values tended to be lower in the straw group. The milk somatic cell count increased in both groups during dry-off but did not differ between groups. In both groups the heart rate decreased at dry-off, but the decrease was more pronounced in the straw group. Overall, this study (together with a previous report) shows that the common dry-off procedure of feeding straw only may give rise to metabolic disturbances. However, this might be avoided without any apparent negative effects on udder health if a limited amount of silage is added during dry-off.

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.

Time courses of plasma magnesium concentrations and urinary magnesium excretion in cows subjected to acute changes in potassium intake

Hypomagnesaemic tetany in cows develops occasionally after an acute increase in K intake such as can occur when cows are transferred to spring grass. There is evidence that under these conditions plasma Mg concentrations are only transiently decreased. In this study the questions addressed were whether the plasma Mg concentration, indeed adapts to a high K intake as only dietary variable, and whether urinary Mg excretion is associated with this adaptation. Dry cows were fed rations containing either 26 or 50 g K/kg dm, the extra K being in the form of KHCO3. When the cows were acutely transferred from the low to the high K ration, plasma Mg concentrations fell slightly, but significantly from 0.86 to 0.76 mmol/l within five days, but rose again to 0.80 mmol/l after another 23 days, this rise being also statistically significant. None of the animals developed tetany. The decrease in plasma Mg concentration in individual animals after five days on the high-K ration ranged from 6 to 21%. The time course of urinary Mg excretion resembled that of plasma Mg concentration; minimum Mg excretion was seen after four to six days on the high-K ration with a subsequent increase thereafter. To explain the transient lowering of plasma Mg concentration, it is suggested that the K-induced decrease in Mg status caused a delayed increase in the carrier-mediated component of Mg absorption, which in turn caused an increase in urinary Mg excretion. When the cows were acutely switched from the high to the low K ration, plasma Mg concentration and urinary Mg excretion rose, but no transient changes were seen.

Dietary potassium bicarbonate and potassium citrate have a greater inhibitory effect than does potassium chloride on magnesium absorption in wethers

We addressed the question whether the type of anion in potassium salts affects magnesium absorption and the transmural potential difference by using wethers (n = 8) fed a control diet and diets supplemented with equimolar amounts of KHCO(3), KCl or K-citrate according to a Latin-square design. The control diet contained 10.9 g K/kg dry matter and the high K diets contained 41.3 g K/kg dry matter. Compared with the control diet, KHCO(3) and K-citrate significantly reduced apparent Mg absorption by 9.5 and 6.5%, respectively. Supplemental KCl tended to reduce (P = 0.070) group mean magnesium absorption by 5.5%. Consumption of supplemental KHCO(3) and K-citrate produced a significant increase in the transmural potential difference (serosal side = positive) by 17.1 and 20.7 mV, respectively, whereas the addition of KCl to the diet did not. The individual values for the four diets tended to show a negative correlation (r = -0.336, n = 32, P = 0.060) between the transmural potential difference and apparent magnesium absorption. We conclude that different potassium salts have different effects on magnesium absorption in ruminants as caused by different effects on the transmural potential difference.

Mg(2+) transport in sheep rumen epithelium: evidence for an electrodiffusive uptake mechanism

The potential difference (PD)-dependent component of transcellular Mg(2+) uptake in sheep rumen epithelium was studied. Unidirectional (28)Mg(2+) fluxes were measured at various transepithelial PD values, and the unidirectional mucosal-to-serosal (28)Mg(2+) flux (J(Mg)(ms)) was correlated with the PD across the apical membrane (PD(a)) determined by mucosal impalement with microelectrodes. PD(a) was found to be -54 +/- 5 mV, and J(Mg)(ms) was 65.9 +/- 13.8 nmol. cm(-2). h(-1) under short-circuit conditions. Hyperpolarization of the ruminal epithelium (blood-side positive) depolarized PD(a) and, most noticeably, decreased J(Mg)(ms). Further experiments were performed with cultured ruminal epithelial cells (REC). With the aid of the fluorescence probe mag-fura 2, we measured the intracellular free Mg(2+) concentration ([Mg(2+)](i)) of isolated REC under basal conditions at various extracellular Mg(2+) concentrations ([Mg(2+)](e)) and after alterations of the transmembrane voltage. Basal [Mg(2+)](i) was 0.54 +/- 0.08 mM. REC suspended in media with [Mg(2+)](e) between 0.5 and 7.5 mM showed an increase in [Mg(2+)](i) that was dependent on [Mg(2+)](e) and that exhibited a saturable component (Michaelis-Menten constant = 1.2 mM; maximum [Mg(2+)](i) = 1.26 mM). Membrane depolarization with high extracellular K(+) (40, 80, or 140 mM K(+)) and the K(+) channel blocker quinidine (50 and 100 microM ) resulted in a decrease in [Mg(2+)](i). On the other hand, hyperpolarization created by K(+) diffusion (intracellular K(+) concentration > extracellular K(+) concentration) in the presence of valinomycin induced a 15% increase in [Mg(2+)](i). None of the manipulations had any effect on intracellular Ca(2+) concentration and intracellular pH. The results support the assumption that the membrane potential acts as a principal driving force for Mg(2+) entry in REC and suggest that the pathway for this electrodiffusive Mg(2+) uptake across the luminal membrane is a channel or a carrier.

Effects of intrinsic potassium in artificially dried grass and supplemental potassium bicarbonate on apparent magnesium absorption in dry cows

Literature data indicate that the form of K in the ration can affect its inhibitory influence on Mg absorption in ruminants. We tested whether identical amounts of K either intrinsically present in artificially dried grass or present in added KHCO3 have different effects on Mg absorption in dry cows. In a 3 x 3 Latin square design, six cows were fed rations consisting of low-K grass and concentrate with or without KHCO3 or a ration consisting of high-K grass with concentrate without added KHCO3. Each ration was given for a period of 4 wk. The ration low in intrinsic K contained 26 g of K/kg of dry matter, the ration low in intrinsic K plus KHCO3 contained 43 g of K/kg of dry matter, and the ration high in intrinsic K also contained 43 g of K/kg of dry matter. The three rations were balanced for crude protein, crude fat, crude fiber, Mg (2.2 g/kg of dry matter), Ca, P, and Na. Apparent Mg absorption was 10.8 +/- 1.54% of intake (mean +/- SE, n = 6) when the cows were fed the low-K ration, but dropped to 1.9 +/- 3.4 and 2.1 +/- 1.9% of intake, respectively, when the rations high in KHCO3 and high in intrinsic K were fed. The two high-K rations induced similar increases in ruminal K concentrations both before and after feed consumption. The feeding of KHCO3 did not influence ruminal pH. The intake of extra K may raise ruminal K concentrations, which increases the transmural potential difference so that Mg transport across the rumen epithelium becomes depressed. Thus, intrinsic and added K had identical effects on ruminal K concentrations and on Mg absorption. Feeding trials with ruminants in which K intakes are manipulated with the use of KHCO3 may reflect those cases when concentrations of K intrinsically present in feedstuffs may vary.