Technical note: a new facility for continuous respiration measurements in lactating cows.

An open-circuit indirect calorimetry system consisting of 4 climate-controlled respiration chambers for cattle has been constructed and validated. The system allows for the continuous monitoring of O(2), CO(2), and CH(4) concentrations in chamber air, and the simultaneous determination of feed and water intake, overall physical activity, position changes, standing and lying times, and animal behavior. For complete balance trials, feces, urine, and milk can be collected quantitatively. Most importantly, lactating cows can be milked in the chamber, and blood samples can be drawn from permanent catheters without disruption of the measurements. The investigator, on entering the chamber, wears a facemask connected to the ambient air during the whole milking process. Data are routed to a data acquisition system with appropriate data evaluation software developed in our research unit. Thus, dynamic changes of the above-named parameters during the course of the day or of longer time periods can be monitored. Such data are critical for understanding the complex regulation and interplay of feed intake, energy metabolism, climatic conditions, and milk production.

Indication of intracellular magnesium deficiency in lactating dairy cows revealed by magnesium loading and renal fractional excretion.

Nine non-pregnant, lactating dairy cows were used to study plasma and urinary magnesium concentrations ([Mg]pl; [Mg]u), and the urinary fractional excretion of magnesium (FE(Mg)) before, during and after an 120 min intravenous magnesium (Mg) administration (2.5 mg/kg body weight). Animals received a total mixed ration, and Mg content of the diet was within recommended range. Basal mean [Mg]pl, [Mg]u and FE(Mg) were 0.89 +/- 0.09 mm, 5.92 +/- 2.99 mm and 8.3 +/- 9.7% respectively. For all parameters, a substantial inter-individual variation was observed. Three cows showed suboptimal [Mg]pl and/or [Mg]u as well as low FE(Mg) values of approximately 2% indicating an insufficient Mg supply to these animals (depressed feed intake, reduced absorption of Mg). The applied Mg challenge induced no significant change of mean [Mg]pl in the cows because part of the excess Mg was excreted in the urine. But in five out of nine cows, a decrease of the FE(Mg), during and after an intravenous Mg load was observed showing that part of the infused Mg is used to replenish intracellular Mg pools. Thus, the existence of an intracellular Mg deficiency in these cows was unmasked by performing the Mg loading test only. Because a reduced free intracellular [Mg] impairs cell and tissue functions, the results highlight the importance of an accurate definition of the intracellular Mg status. The Mg loading test is a suitable procedure, however, for practical purposes less expensive and time consuming methods must be developed.

Measurements of heat production for estimation of maintenance energy requirements of Hereford steers.

This study aimed to test the hypothesis that maintenance energy requirement (MEm) can be estimated from continuous heat production measurements during a change from a near maintenance feeding level to far below maintenance for two consecutive days. The MEm of eight Hereford steers weighing 286 +/- 5 kg (mean +/- SE) was determined in a balance trial. In addition, during the 10-d collection period, the animals were kept in open-circuit respiration chambers to measure 24-h gas exchange continuously at 10-min intervals. During the balance trial, the animals were fed dried chopped grass twice daily at an estimated level of 1.2 x MEm. After termination of the collection period on the 11th d of the balance trial, the steers were offered 2 kg/d of wheat straw while only gas exchange was measured. Estimates of MEm were derived from heat production (HP) data. The analyses included values of 24-h HP, HP of the nocturnal period (0000 to 0630), HP of the nocturnal period (excluding HP caused by standing) during the grass-feeding period and 24-h HP, nocturnal HP, and nocturnal HP (excluding HP caused by standing) during the straw feeding period. The MEm predicted from estimates of HP measurements were 536 +/- 9, 470 +/- 8, 441 +/- 8, 435 +/- 8, 393 +/- 9, and 373 +/- 9 kJ.kg of BW(-0.75).d(-1), respectively, whereas MEm calculated from data of the balance trial were 416 +/- 9 kJ.kg of BW(-0.75).d(-1). Values predicted for nocturnal HP (excluding HP caused by standing) of grass fed animals, 24-h HP, and nocturnal HP during straw feeding did not differ significantly from MEm. The differences in MEm among animals were reflected by all estimates of HP, whereas the correlation with the 24-h HP during straw feeding reached 0.9 (P = 0.002). We conclude that the method described is adequate to determine MEm with a sufficient degree of accuracy.

[Influence of potassium on Mg- and Ca-metabolism in cows: effects and side effects of scientific research]. / Effekte des Kaliums auf den Mg- und Ca-Stoffwechsel der Kuh: Wirkungen und Nebenwirkungen wissenschaftlicher Tätigkeit.

Results of scientific studies are obtained by analysing of the present knowledge of a current problem and a corresponding new experimental set-up. Under ideal conditions the data of the new study agree with the deduced working hypothesis. This general consideration is true for the well established correlation between K content and growth rates of plants. At low K concentrations (up to 3% of dry matter) K causes a linear increase of growth and finally a saturation. This positive effect of K on growth rates of plants is accompanied by some side effects. There is no doubt that a high intake of K is involved in the pathogenesis of grass tetany and of milk fever. The present publication gives some information about this correlation and discusses the discrepancy between the intention of a scientific study and possible "side effects", which cannot be predicted in many cases.

No evidence for active peptide transport in forestomach epithelia of sheep.

The transport of peptides was studied with isolated preparations of rumen and omasum tissue of sheep by using the conventional Ussing-chamber method and isolated ruminal cells (REC). Mucosal addition of glycyl-L-glutamine, captopril (angiotensin converting enzyme inhibitor) or cefadroxil (beta-lactam antibiotic) did not change the short-circuit current (I(sc)), or tissue conductance (G(t)). The intracellular pH, pH(i), in isolated REC was not influenced by the addition of peptides to the buffer solution. These findings do not support the assumption of proton-coupled or electrogenic peptide transport. The determination of unidirectional flux rates of the peptide D-phenylalanyl-L-alanine (2,3-(3)H) showed that the flux rate in the serosal-mucosal direction, J(sm), was greater than J(ms), leading to a small net secretion of peptide. Transport was not significantly inhibited by the serosal addition of ouabain. Enhancing the paracelluIar permeability by an increase of osmotic pressure in the mucosal solution (FREYER and MARTENS, Proc. Soc. Nutr. Physiol. 8, 80, 1999) caused an increase of G(t) and significantly higher transport rates of peptide. The flux rates of peptides (in the nanomolar range) may therefore represent passive and possibly paracellular diffusion and are not of nutritional importance.

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.

Magnesium transport in the gastrointestinal tract.

Magnesium is an essential (macro) mineral in vertebrates with many biochemical and physiological functions including activation of enzymes, involvement into metabolic pathways, regulation of membrane channels and muscle contraction. Despite these important functions, Mg++ homeostasis is not regulated by hormones, but depends on absorption from the gastrointestinal tract, requirement of the body, and excretion via the kidneys. The present review summarizes data on epithelial Mg++ transport in the gut via paracellular and cellular pathways. Paracellular movement of Mg++ is only important in leaky epithelia as in the small intestine. The transcellular transport of Mg++, luminal uptake and basolateral extrusion, require membrane proteins which increase the low permeability of the membranes and facilitate the movement of Mg++ through these lipid bilayers. Proposals have been made how these proteins could mediate Mg++ transport. There is now a growing body of evidence for a PD-dependent luminal Mg++ uptake via a carrier or channel. Furthermore, PD-independent uptake mechanisms have been demonstrated which may be represented by Mg++/2cation+ exchange or co-transport of Mg++ with anions. The mechanism of a basolateral extrusion is not clear. A Na+/Mg++ exchange, well characterized in non-polar cells, has been suggested which leads to the proposal that there is a secondary active transport system for Mg++. It can readily be learned from this fragmentary knowledge of transepithelial Mg++ transport that future research must be directed to a study of the relevant membrane proteins (carriers or channel for Mg++) in order to close the gap between the incompletely described epithelial Mg++ transport mechanisms and the well established transport systems, e.g. , sodium or glucose.

[Electrophysiologic changes in rumen epithelium in their effect on magnesium transport--a review]. / Elektrophysiologische Veränderungen des Pansenepithels und deren Auswirkungen auf den Magnesiumtransport--eine Ubersicht.

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.

Mechanisms of Mg(2+) transport in cultured ruminal epithelial cells.

Net Mg(2+) absorption from the rumen is mainly mediated by a transcellular pathway, with the greater part (62%) being electrically silent. To investigate this component of Mg(2+) transport, experiments were performed with isolated ruminal epithelial cells (REC). Using the fluorescent indicators mag-fura 2, sodium-binding benzofuran isophthalate, and 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, we measured the intracellular free Mg(2+) concentration ([Mg(2+)](i)), the intracellular Na(+) concentration ([Na(+)](i)), and the intracellular pH (pH(i)) of REC under basal conditions, after stimulation with butyrate and HCO(-)(3), and after changing the transmembrane chemical gradients for Mg(2+), H(+), and Na(+). REC had a mean resting pH(i) of 6.83 +/- 0.1, [Mg(2+)](i) was 0.56 +/- 0. 14 mM, and [Na(+)](i) was 18.95 +/- 3.9 mM. Exposure to both HCO(-)(3) and HCO(-)(3)/butyrate led to a stimulation of Mg(2+) influx that amounted to 27.7 +/- 5 and 29 +/- 10.6 microM/min, respectively, compared with 15 +/- 1 microM/min in control solution. The increase of [Mg(2+)](i) was dependent on extracellular Mg(2+) concentration ([Mg(2+)](e)). Regulation of pH(i) has been demonstrated to be Na(+) dependent and is performed, for the most part, by a Na(+)/H(+) exchanger. The recovery of pH(i) was fully blocked in nominally Na(+)-free media, even if [Mg(2+)](e) was stepwise increased from 0 to 7.5 mM. However, an increase of [Mg(2+)](i) was observed after reversing the transmembrane Na(+) gradient. This rise in [Mg(2+)](i) was pH independent, K(+) insensitive, dependent on [Mg(2+)](e), imipramine and quinidine sensitive, and accompanied by a decrease of [Na(+)](i). The results are consistent with the existence of a Na(+)/Mg(2+) exchanger in the cell membrane of REC. The coupling between butyrate, CO(2)/HCO(-)(3), and Mg(2+) transport may be mediated by another mechanism, perhaps by cotransport of Mg(2+) and HCO(-)(3).

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.

[Neural regulation of food absorption--review]. / Nervale Regulation der Nahrungsaufnahme--Ubersichtsreferat.

The periodical food intake (discrete meals) demands a control system, which includes signals for hunger and satiety. Satiety and hunger change with the absorptive and postabsorptive state of the delivery of nutrients to the organism. The brain areas involved in the regulation of food intake receive informations from three sources: periphery, environment and memory. Hypothalamic structures and pathways of neurotransmitters are considered especially. Beside these, the limbic structures are mainly responsible for the development of motivated feeding behaviour. Disturbances in the regulation of feeding behaviour are prone to cause obesity and anorexia nervosa.

[The effect of short-term fasting on sleep in pigs]. / Vliv krátkodobého hladovení na spánek prasat.

Trials were conducted with twelve piglets of the German Large White breed, intended for fattening. Their age was 56 +/- 2 days and their average weight was 21 +/- 3.7 kg. The piglets were studied for the effect of short fasting on the time and nature of sleep. After 18 hours of fasting (skipping the morning feeding), the piglets were fixed to obtain a combined record, including the electroencephalogram (EOG). This record was used for analyzing the time of waking and sleep, both the NREM stage and REM episodes. The same observations were performed in the piglets after feeding. The differences in the obtained data between the groups of animals were subjected to statistical evaluation. The fasting animals slept for about 15% of all the time under study and their sleep included short NREM periods interrupted by waking. No REM sleep was observed in the fasting piglets. After feeding the piglets slept significantly longer (p less than 0.01), more than half the period of study. Episodes of REM sleep occurred repeatedly in four animals. It is indicated by the results that fasting reduces the length of sleep and the nature of sleep.