Effect of dehydration and acidemia on the potassium content of muscle tissue and erythrocytes in calves with neonatal diarrhea.

Disturbances of extracellular potassium (K) homeostasis in calves with severe neonatal diarrhea have been studied extensively. Although total body depletion of this predominantly intracellular electrolyte is generally thought to occur in diarrheic calves, the mechanisms through which K depletion occurs are poorly understood. The aim of this study was to investigate how intracellular K homeostasis is affected by dehydration and acidemia, the 2 most important metabolic disturbances in calves with naturally occurring diarrhea. Twenty-seven calves with naturally occurring neonatal diarrhea, pronounced dehydration, and acidemia, and 2 groups of 10 healthy control calves were included in this study. Blood samples and muscle biopsies were obtained immediately before initiation of treatment (T0) and after complete rehydration and correction of acidemia (T1) from diarrheic calves. Blood samples were used to perform blood gas, blood biochemical, and hematological analyses and to determine K content in erythrocytes. Muscle biopsies were used to determine muscle tissue K content and tissue dry matter. Controls were used to determine values for erythrocyte and muscle tissue K content in healthy neonatal calves for comparison with diarrheic calves. As defined by the inclusion criteria, diarrheic calves showed pronounced acidemia and dehydration at T0. Mean muscle tissue K content and tissue dry matter remained unchanged between sampling times and did not differ from values measured in healthy control calves. Erythrocyte K content increased from 73.63 ± 13.73 to 77.64 ± 15.97 mmol/L (±standard deviation) but was associated with a concomitant decline in erythrocyte volume. Values measured at both sampling times in diarrheic calves did not differ from erythrocyte K measured in healthy control calves. The plasma K concentration (median [interquartile range]) decreased from 5.44 [4.76-6.17] to 4.16 [3.99-4.31] mmol/L between T0 and T1. Although changes in plasma [K] were associated with the degree of dehydration, neither dehydration nor acidemia was associated with changes of K content in muscle tissue or erythrocytes. In conclusion, severe dehydration and acidemia in diarrheic calves were not associated with notable changes in K content of muscle tissue or erythrocytes. These results do not support the concept of pronounced K depletion occurring in calves with neonatal diarrhea. Erythrocytes are a poor surrogate tissue in which to measure changes of intracellular K content in diarrheic calves because of concomitant changes in erythrocyte volume that complicate the interpretation of results.

CD117- and vimentin-positive telocytes in the bovine teat sphincter.

Mastitis is a common economically relevant problem in dairy farming. As the major entry for pathogens is the papillary duct, one of the first defence mechanisms is the teat sphincter. This sphincter shows a rhythmic contractility of yet unknown origin. Searching for possible modulatory pacemaker cells, teat sphincters of eight cows were stained immunohistochemically with antibodies against CD117 and vimentin and evaluated microscopically for the presence of telocytes. CD117- and vimentin-positive telocytes with telopodes were found in close contact with smooth muscle cells. Our findings present a first evidence of telocytes in the teat of bovines.

A dynamic model to simulate potassium balance in dairy cows.

High-performing dairy cows require a particular composition of nutritional ingredients, adapted to their individual requirements and depending on their production status. The optimal dimensioning of minerals in the diet, one being potassium, is indispensable for the prevention of imbalances. Potassium balance in cows is the result of potassium intake, distribution in the organism, and excretion, and it is closely related to glucose and electrolyte metabolism. In this paper, we present a dynamical model for potassium balance in lactating and nonlactating dairy cows based on ordinary differential equations. Parameter values were obtained from clinical trial data and from the literature. To verify the consistency of the model, we present simulation outcomes for 3 different scenarios: potassium balance in (1) nonlactating cows with varying feed intake, (2) nonlactating cows with varying potassium fraction in the diet, and (3) lactating cows with varying milk production levels. The results give insights into the short- and long-term potassium metabolism, providing an important step toward the understanding of the potassium network, the design of prophylactic feed additives, and possible treatment strategies.

Effects of ß-hydroxybutyrate and different calcium and potassium concentrations on the membrane potential and motility of abomasal smooth muscle cells in cattle.

The left displacement of the abomasum (LDA) is a common disease in periparturient dairy cows. Plasma ß-hydroxybutyrate (BHBA) levels above the reference range are regarded as risk factors for the occurrence of LDA. Additionally, hypokalemia and hypocalcemia have been observed in LDA cows. The aim of the present study was to characterize the membrane potential and the slow waves in abomasal smooth muscle cells by a microelectrode technique and to determine possible effects of BHBA and of various calcium and potassium concentrations on the membrane potential. Subsequently, the results obtained by the microelectrode technique were combined with in vitro motility experiments of abomasal smooth muscles. Strips of the abomasal circular smooth muscles were prepared and incubated in different buffer solutions. For the microelectrode technique, healthy bulls and cows that underwent surgery for LDA were sampled. These measurements showed a frequency of the slow waves between 3.5 and 10.9 per minute (for amplitudes ≥ 3mV) and between 0.6 and 4.5 per minute (for amplitudes ≥ 5mV). The frequency of contractions (1.8 to 3.1 per minute) were in the same order as the frequency of the slow waves with amplitudes ≥ 5 mV. Blocking potassium conductance with barium chloride induced a depolarization of the basal membrane potential (from -43±2.9 to -37±4.1mV; mean ± standard error of the mean) without affecting the frequency or the height of the slow waves. The reduction in the potassium concentration from 5.4 to 2 mmol/L resulted in a nominal decrease in the activity of contractions (from 22.2 to 18.6 mN/min). The subsequent addition of 1 mmol of KCl/L induced a nominal increase in contraction activity (from 18.6 to 25.7 mN/min). An effect of BHBA (5 mmol/L) could not be demonstrated, neither on the electric nor on the motility parameters. A simulated hypocalcemia (1.2 mmol/L total, 0.9 mmol/L ionized Ca) did not change slow waves and motility. In conclusion, changes in membrane potential ≥ 5 mV correlated with contractions of abomasal muscles. Hypokalemic conditions may reduce abomasal contraction activity via an effect on the membrane potential. An exclusive increase in BHBA or a slight hypocalcemia (0.9 mmol/L ionized Ca) had no effects on the motility of healthy abomasal smooth muscle cells. A prolonged or severe exposition of muscles to a combination of low Ca and high BHBA concentrations might nevertheless be able to affect abomasal motility.

Potassium and insulin affect the contractility of abomasal smooth muscle.

Abomasal displacement is a frequent and important disease of high yielding dairy cows. Although several factors are related to its occurrence, the pathogenesis of the condition is still inadequately understood, particularly in regard to K(+) and insulin homeostasis. For this reason the aim was to investigate the effects of K(+) and insulin concentrations on in vitro motility of abomasal smooth muscle. The second aim was to determine whether the in vivo change in K(+) and insulin levels might be sufficient to induce reduced abomasal motility. Muscle strips were isolated from the abomasum of slaughtered cows and incubated in buffer solution under isometric conditions. Results show that a decrease in extracellular K(+) (between 5 and 1 mmol/L) or an increase in extracellular insulin concentrations (to 21 mU/L or higher) were able to affect the contraction activity of abomasal muscles. Contraction activity given as median (25th, 75th percentiles) changed from 28.1 mN/min (2.5, 49.9) at 5 mmol/L of K(+) to 9.4 mN/min (0.6, 35.7) at 1 mmol/L of K(+), and from 34.5 mN/min (10.8, 112.4) at 0 mU/L of insulin to 12.0 mN/min (7.6, 49.8) at 120 mU/L of insulin. Because the effect of insulin could be abolished by barium, glybenclamide, or ouabain, the underlying mechanisms of the insulin action could be an increased K(+) conductance or an increased Na/K-ATPase activity or both. Low K(+) or high insulin concentrations both reduced the activity of the circular muscle of the abomasal corpus (i.e., of the part that is responsible for the propulsion of abomasal chymus) and might play an important role in the pathogenesis of abomasal displacement.

Transport of cations and anions across forestomach epithelia: conclusions from in vitro studies.

Secretion of saliva as well as absorptive and secretory processes across forestomach epithelia ensures an optimal environment for microbial digestion in the forestomachs. Daily salivary secretion of sodium (Na+) exceeds the amount found in plasma by a factor of 2 to 3, while the secretion of bicarbonate (HCO3-) is 6 to 8 times higher than the amount of HCO3- in the total extracellular space. This implies a need for efficient absorptive mechanisms across forestomach epithelia to allow for an early recycling. While Na+ is absorbed from all forestomachs via Na+/H+ exchange and a non-selective cation channel that shows increased conductance at low concentrations of Mg2+, Ca2+ or H+ in the luminal microclima and at low intracellular Mg2+, HCO3- is secreted by the rumen for the buffering of ingesta but absorbed by the omasum to prevent liberation of CO2 in the abomasum. Fermentation provides short chain fatty acids and ammonia (NH3) that have to be absorbed both to meet nutrient requirements and maintain ruminal homeostasis of pH and osmolarity. The rumen is an important location for the absorption of essential minerals such as Mg2+ from the diet. Other ions can be absorbed, if delivered in sufficient amounts (Ca2+, Pi, K+, Cl- and NH4+). Although the presence of transport mechanisms for these electrolytes has been described earlier, our knowledge about their nature, regulation and crosstalk has increased greatly in the last years. New transport pathways have recently been added to our picture of epithelial transport across rumen and omasum, including an apical non-selective cation conductance, a basolateral anion conductance, an apical H+-ATPase, differently expressed anion exchangers and monocarboxylate transporters.

Integrity of gastric mucosa in reared piglets--effects of physical form of diets (meal/pellets), pre-processing grinding (coarse/fine) and addition of lignocellulose (0/2.5%).

The aim of the present study was to investigate the potential effects of different particle fractions in non-pelleted (meal) and pelleted diets on the development of pre-ulcerative gastric alterations. Furthermore, the effect of increased crude fibre supply (lignocellulose) on the integrity of gastric mucosa were investigated. For that purpose, 49 piglets were divided into eight feeding groups and fed pelleted diets differing in grinding intensity (very coarse/coarse/fine/very fine) and addition of lignocellulose (0/2.5%) for 6 weeks. A coarsely ground meal was used as control diet. Mucosal integrity of the pars non-glandularis was characterised by macroscopical and histological score and basal epithelial conductance. Feed structure was assessed by sieve analysis (wet/dry). The use of coarsely ground meal (25% >2 mm, 29% <0.4 mm) had almost no negative effects on the gastric wall: three of seven pigs had slight histological and none had macroscopical lesions. Irrespective of the original grinding intensity before pelleting, offering pelleted diets led to mucosal changes similar in severity (one out of seven pigs fed coarsely ground and pelleted diets had no macroscopical alterations, whereas all pigs fed finely ground and pelleted diets showed altered tissues). Increasing the proportion of coarse particles in the pellet (from 25 to 29% >2 mm) did not show any ulceroprotective effect. An increase of crude fibre content (42-54 g/kg dm) by adding lignocellulose did not result in a decreased ulcerogenity. Unpelleted diets are recommended as more favourable for alleviating the problem of gastric ulcers in pigs as the pelleting process is equal to a secondary grinding process. According to our results, an upper level of fine particles seems to be reasonable (a minimum level of coarse particles is not ulceroprotective). In this study, an amount of 30% <0.4 mm resulted in higher risks for ulcerations.

Characterisation of electrogenic nutrient absorption in the Cftr TgH(neoim)Hgu mouse model.

Most cystic fibrosis (CF) patients show an exocrine pancreatic insufficiency that results in lower enzyme and bicarbonate secretion. To test whether an altered function of nutrient transporters might additionally attribute to the lower bodyweight of CF patients we investigated electrogenic absorption of alanine, glycyl-glutamine, glucose and the effect of pH on nutrient absorption by Ussing chambers in a CF mouse model carrying the Cftr TgH(neoim)Hgu mutation. The electrogenic transport of all three nutrients was similar between the D2.129P2(CF/3)-Cftr TgH(neoim)Hgu congenic strain and DBA/2J mice as well as between the B6.129P2(CF/3)-Cftr TgH(neoim)Hgu congenic strain and C57BL/6J mice. This indicates that the Cftr TgH(neoim)Hgu mutation does not affect the electrogenic absorption of alanine, glycyl-glutamine and glucose. In contrast, electrogenic nutrient absorption was reduced in the CF/1-Cftr TgH(neoim)Hgu and CF/3-Cftr TgH(neoim)Hgu inbred strains compared to the HsdOla:MF1, D2.129P2(CF/3)-Cftr TgH(neoim)Hgu and B6.129P2(CF/3)-Cftr TgH(neoim)Hgu strains, whereas no difference was found among the wild-type strains. This indicates that not the Cftr TgH(neoim)Hgu mutation but differences in the genetic background of the CF/1-Cftr TgH(neoim)Hgu and CF/3-Cftr TgH(neoim)Hgu strains compared to HsdOla:MF1, D2.129P2(CF/3)-Cftr TgH(neoim)Hgu and B6.129P2(CF/3)-Cftr TgH(neoim)Hgu strains are associated with the differences in electrogenic nutrient absorption. The electrogenic absorption of alanine, glycyl-glutamine and glucose was not influenced by an acidic pH (5.4) compared to absorption at pH 7.4.

Chloride, gluconate, sulfate, and short-chain fatty acids affect calcium flux rates across the sheep forestomach epithelium.

In ruminants, more than 50% of overall gastrointestinal Ca absorption can occur preintestinally, and the anions of orally applied Ca salts are thought to play an important role in stimulating ruminal Ca absorption. This assumption is based mainly on ion-exchange studies that have used gluconate as the control anion, which may bind Ca2+ ions and interfere with treatment effects. In the present study, we investigated the distinct effects of different anions on Ca absorption across the sheep rumen and on the concentration of free Ca2+ ions ([Ca2+]ion). We showed that gluconate, sulfate, and short-chain fatty acids (SCFA) remarkably reduced [Ca2+]ion in buffer solutions. Nevertheless, increasing the Cl or SCFA concentration by 60 mM stimulated net ruminal Ca absorption 5- to 7-fold, but these effects could be antagonized by gluconate. Therefore, ion-exchange experiments must be (re)evaluated very carefully, because changes in [Ca2+]ion in the presence of gluconate, sulfate, or SCFA not only might entail an underestimation of Ca flux rates, but also might have effects on other cellular pathways that are Ca2+ dependent. Concerning the optimal Ca supply for dairy cows, the present study suggests that CaCl2 formulations and Ca salts of the SCFA stimulate Ca absorption across the rumen wall and are beneficial in preventing or correcting a Ca deficiency.

Characterisation of chloride currents across the proximal colon in CftrTgH(neoim)1Hgu congenic mice.

It was the aim of the present study to investigate chloride secretion across the proximal colon of Cftr (TgH(neoim)1Hgu) congenic mice. Stripped epithelia were incubated in Ussing chambers and the electrophysiological data were compared between cystic fibrosis (CF) animals and wild type (WT) animals. In comparison with the control animals, all Cftr (TgH(neoim)1Hgu) congenic mice had a distinctly reduced basal chloride secretion and a reduced chloride secretion after stimulation with carbachol and forskolin. When comparing chloride secretion across the proximal colon between WT animals, all mice showed a comparable pattern of response to carbachol and forskolin but quantitative differences, BALB/c exhibiting the highest and HsdOla:MF1 exhibiting the lowest increase in Cl current. Likewise, all CF animals showed the same reaction pattern to carbachol and forskolin, but there was no distinct difference that lasted for the whole measurement. To investigate interferences between Ca- and cyclic adenosine monophosphate-activated pathways of Cl secretion in CF animals, we studied epithelia from CF/3CF/1F1 animals with a mixed background. In these animals, the levels of the carbachol or forskolin-induced chloride currents did not depend on the prestimulation with the respective other secretagogue. 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, which blocks calcium-activated chloride channels, reduced the current response to carbachol by about 23%. This result, obtained in BALB/c-Cftr (TgH(neoim)1Hgu) mice, indicates that alternative chloride channels might be present in the proximal colon of these mice. In contrast, there was no evidence for alternative chloride conductances in BALB/c WT animals, but we cannot exclude that in WT mice a higher chloride secretion via Cftr-channels may have masked an alternative chloride secretion.

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep.

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.

Divalent cations reduce the electrogenic transport of monovalent cations across rumen epithelium.

The rumen epithelium of sheep and goats showed an increase in short circuit current ( Isc) and transepithelial conductance (gt) upon mucosal removal of divalent cations. A divalent-sensitive Isc and gt were present in Na+, K+ or Rb+ buffer, but nearly abolished in mucosal NMDG+ (N-methyl-D-glucamine) buffer. High K buffer, addition of BaCl2 or of ouabain on the serosal side also reduced or abolished the divalent-sensitive Isc. Mucosal Ca2+ was more potent in blocking Isc, but had the same potency as Mg2+ in blocking gt. A prolonged mucosal deprivation of Mg2+ ions increased gt, potential difference and basal as well as the Ca2+-sensitive Isc. Mucosal addition of Mg2+ had a smaller effect on gt after serosal preincubation with Ba. The data suggest that rumen epithelial cells exhibit an apical non-selective cation conductance, which permits the passage of monovalents in the mucosal absence of divalents. The development of a divalent-sensitive Isc in Na buffer requires Na+/K+ pumps and K+ recycling through Ba2+-sensitive K+ conductances on the basolateral side. This Isc is blocked by extracellular Ca2+ and both extracellular and intracellular Mg2+ ions. A prolonged deprivation of mucosal Mg2+ alone seems to affect intracellular Mg2+ in this Mg2+-absorbing tissue.

Colonic fermentation as affected by antibiotics and acidic pH: Application of an in vitro model.

Antimicrobial substances such as vancomycin or metronidazole suppress normal gut flora, thereby preventing physiological fermentation of colonic substrates that may promote mucosal inflammation. This study was designed to establish an in vitro model of microbial metabolism in the colon under control and disturbed conditions (acidic pH) to investigate specific effects of vancomycin and metronidazole on the production of short chain fatty acids (SCFA), which play a pivotal role in maintaining homeostasis in the colon. The experiments were carried out with the colon simulation technique (Cositec) representing an in vitro model for the semi-continuous incubation of defined colon contents. Inocula and fermentable substrates were sampled from cecal contents of fistulated pigs. Disturbed microbial metabolism was generated by reduction of pH in the fermentation vessels from 6.7 to 5.8 and 5.1. In general, application of either vancomycin or metronidazole resulted in a significant decrease of SCFA production rates indicating substantial disturbance of the homeostasis of microbial metabolism. With low doses of vancomycin acetate and butyrate production rates were reduced and with high doses of the antibiotic propionate production was inhibited to a greater extent. Treatment with metronidazole inhibited butyrate production almost completely. Similarly, low pH caused a reduction in total SCFA production, which was mainly due to respective decrease of acetate synthesis. Metronidazole effects were not consistently changed at low pH. The Cositec system provides an excellent facility to test the effects of different antibiotics under defined conditions. In this study, both vancomycin and metronidazole affected microbial metabolism to a considerable extent. Both substances may thus be responsible for disturbances of colon function in vivo.

Do forestomach epithelia exhibit a Mg2+/2H(+)-exchanger?

In ruminants the forestomachs (reticulum, rumen and omasum) represent the main site of Mg absorption. Readily fermentable carbohydrates enhance Mg availability for ruminants in vivo. The beneficial effect of carbohydrate addition on Mg absorption occurs preintestinally and is associated with changes in the rumen fluid (short chain fatty acids, SCFA, luminal pH, carbon dioxide, NH3, lactate, osmolarity). SCFA and HCO3-/CO2 increase Mg efflux from the isolated reticulorumen in vivo and stimulate 28Mg flux from mucosal to serosal across rumen epithelium in vitro. The stimulatory effect of SCFA on Mg absorption differs between acids, being a function of SCFA absorption and possibly SCFA metabolism. The stimulatory effect of HCO3-/CO2 is mediated by carbonic anhydrase activity. Experimental data suggest that the availability of protons inside the cell might be involved in the stimulation of Mg absorption. The contribution of Mg/H exchange to Mg2+ uptake across the apical membrane of rumen epithelium is discussed in comparison with other possible ways of stimulation (Mg/Na exchange, Mg/anion cotransport, electrogenic and metabolic effects of SCFA). It remains to be shown that Mg2+ uptake into rumen epithelium is directly linked to an efflux of protons. However, an apical Mg/H exchange (in contrast to other suggested Mg transporters) would explain a variety of in vivo and in vitro observations on ruminal Mg absorption.

Effects of short chain fatty acids and carbon dioxide on magnesium transport across sheep rumen epithelium.

Short chain fatty acids (SCFAs) and CO2 have been shown to stimulate net Mg2+ efflux from the isolated reticulorumen in vivo. To investigate the underlying mechanisms of Mg2+ transport we performed Ussing chamber and microelectrode experiments and measured 28Mg2+ fluxes across sheep rumen epithelium in vitro. In the presence of SCFAs mucosal-to-serosal Mg2+ flux (Jm-sMg) amounted to 82.3 +/- 7.8 nmol cm-2 h-1 and serosal-to-mucosal Mg2+ flux (Js-mMg) to 3.2 +/- 0.7 nmol cm-2 h-1. Replacing SCFAs with gluconate caused a 50% reduction of Jm-sMg, whereas Js-mMg was not affected. Among the SCFAs, n-butyrate was more effective in stimulating Jm-sMg than acetate, propionate or iso-butyrate. Eliminating HCO3(-)-CO2 from SCFA-containing solutions did not affect Mg2+ fluxes, whereas the same replacement in SCFA-free solutions led to a further reduction in Jm-sMg. Jm-sMg decreased after the addition of ethoxyzolamide to SCFA-free, bicarbonate buffered solutions. Decreasing mucosal pH from 6.4 to 5.4 increased Jm-sMg in SCFA-free, bicarbonate buffered solutions. SCFAs had no effect on the apical membrane potential of rumen epithelial cells. The experiments show that both SCFAs and CO2 stimulate Mg2+ transport through an increase in Jm-sMg, most probably via stimulation of a Mg(2+)-2H+ exchange mechanism. SCFAs may have additional metabolic effects on Mg2+ transport.

Electro- and transportphysiological changes in pig proximal colon during parasitic infection.

Oesophagostomum dentatum, one of the most common nematodes in pigs, causes the formation of subepithelial granuloma in the large intestine. To investigate possible changes in epithelial function or response during the infection we incubated epithelia of pig proximal colon in Ussing chambers at different days post infectionem (p.i.). Transepithelial conductance, gt, and the Cl flux from serosal to mucosal, JsmCl, were increased on day 2 p.i., when the nematodes penetrate the epithelium of the large intestine, and declined toward control levels thereafter. Histamine, PGE2 and carbachol caused transient increases in short circuit current, Isc, and conductance that could partly be attributed to a higher JsmCl. The Isc responses were highest on the days of nematode penetration in or out of the epithelium (days 2 and 14 p.i.) and did decline on day 7 p.i. during the histotropic development of the parasite. This reduced epithelial reaction on day 7 p.i. might be an adaptation to secretory stimuli released from the inflammatory cells in the intestinal wall or might reflect modulation by the parasite and could be responsible for the absence of marked clinical signs during the infection.

Effects of potassium on magnesium transport across rumen epithelium.

Increasing ruminal K concentration impairs active Mg transport across the forestomach epithelium of ruminants. We used Ussing chamber and microelectrode techniques to test the hypothesis that high mucosal K decreases the driving force for Mg2+ uptake by depolarizing the apical membrane. Serosal Ba enhanced net 42K secretion. Under open-circuit conditions, rumen epithelial cells showed an apical membrane potential (Va) of -67.3 +/- 1.5 mV. An increase in mucosal K concentration depolarized Va and decreased mucosal-to-serosal 28 Mg flux (Jm-->sMg). An increase in the transepithelial potential difference (Vt) depolarized Va, greatly decreased Jm-->sMg, and slightly increased Mg serosal-to-mucosal flux (Js-->mMg). Jm-->sMg exhibited an electrogenic and an electroneutral component. Mucosal verapamil depolarized Va and reduced Jm-->sMg, without affecting Js-->mMg. The study shows that rumen epithelial cells exhibit apical and basolateral K conductances and that transcellular Mg absorption has a distinct electrogenic component. This supports the assumption that a change in Va represents the link between increased ruminal K concentration and decreased Mg absorption. The results are discussed with regard to the development of hypomagnesemia in ruminants.