Effects of seasonal changes in food quality and food intake on the transport of sodium and butyrate across ruminal epithelium of reindeer.

Transport of 22Na and 14C-butyrate across the ruminal epithelium of captive reindeer fed a concentrate diet in summer (n=5) and in winter (n=5) and from free-ranging reindeer taken from summer (n=3) and winter pasture (n=5) was measured in vitro in Ussing chambers. Significant amounts of both Na+ and butyrate were transported across the isolated epithelium without any external driving force. The ruminal transport of Na+ and butyrate were interacting, as evidenced by both the observed amiloride-induced reduction of net butyrate-transport and by the positive correlation between net transport of butyrate and Na+. Amiloride also reduced the net transport of Na+ without significantly affecting the short-circuit current, indicating the presence of an apical Na+/H+ exchanger in the ruminal epithelium of reindeer. The captive reindeer increased the dry matter intake of a constant quality concentrate from winter to summer, but this neither affected their ruminal transport capacity nor their ruminal surface enlargement factor (SEF). Free-ranging reindeer increased their ruminal transport capacity for Na+ and butyrate from summer to winter but simultaneously reduced their ruminal SEF. The present data indicate that this food-induced increase in transport capacity was attributed to changes in the nutrient composition of the diet.

Ruminal transport and metabolism of short-chain fatty acids (SCFA) in vitro: effect of SCFA chain length and pH.

The unidirectional transport and metabolism of 14C-labeled acetate, propionate and butyrate across the isolated bovine rumen epithelium was measured in vitro by the Ussing chamber technique. There was a significant, but relatively small, net secretion of acetate and propionate, and a large and significant net absorption of butyrate. The results demonstrate that the mucosal-serosal (MS) pathway for short-chain fatty acids (SCFA) is different from the serosal-mucosal (SM) pathway, and that butyrate is treated differently from acetate and propionate by the epithelium. The results support that the main route for epithelial SCFA transport is transcellular. The correlation between SCFA lipophility and the flux rate was positive but weak at both pH 7.3 and 6.0. Decreasing pH increased all SCFA fluxes significantly, but not proportionally to the increase of protonized SCFA in the bathing solution. There was a significant and apparently non-competitive interaction between the transport of acetate, propionate and butyrate. It seems that mediated transport mechanisms must be involved in epithelial SCFA transport in the bovine rumen, but the data do not exclude that passive diffusion could account for a significant part of the flux. The metabolism of SCFA in the Ussing chamber system was considerable, and there was a clear preference for excretion of CO2 from this metabolism to the mucosal side, while side preference for non-CO2 metabolite excretion was not studied. Of the propionate and butyrate transported in the MS direction, 78 and 95% was metabolised, while only 37 and 38% was metabolised in the SM direction (acetate metabolism could not be measured). There was, however, no simple relation between the degree of metabolism and the transport rate or the transport asymmetry of the SCFA.

Transport of butyrate across the isolated bovine rumen epithelium--interaction with sodium, chloride and bicarbonate.

The Ussing chamber technique was used for studying unidirectional fluxes of 14C-butyrate across the bovine rumen epithelium in vitro. Significant amounts of butyrate were absorbed across the bovine rumen epithelium in vitro, without any external driving force. The paracellular pathway was quantitatively insignificant. The transcellular pathway was predominately voltage-insensitive. The serosal to mucosal (SM) pathway was regulated by mass action, whereas the mucosal to serosal (MS) pathway further includes a saturable process, which accounted for 30 to 55% of the MS flux. The studied transport process for 14C-butyrate across the epithelium could include metabolic processes and transport of 14C-labelled butyrate metabolites. The transport of butyrate interacted with Na+, Cl- and HCO3-, and there was a linear relationship between butyrate and sodium net transport. Lowering the sodium concentration from 140 to 10 mmol l-1 decreased the butyrate MS flux significantly. Amiloride (1 mmol l-1) did, however, not reduce the butyrate flux significantly. Chloride concentration in itself did not seem to influence the transport of butyrate, but chloride-free conditions tended to increase the MS and SM flux of butyrate by a DIDS-sensitive pathway. DIDS (bilateral 0.5 mmol l-1) did further decrease the butyrate SM flux significantly at all chloride concentrations. Removing bicarbonate from the experimental solutions decreased the MS and increased the SM flux of butyrate significantly, and abolished net butyrate flux. There were no significant effects of the carbonic anhydrase inhibitor Acetazolamide (bilateral 1.0 mmol l-1). The results can be explained by a model where butyrate and butyrate metabolites are transported both by passive diffusion and by an electroneutral anion-exchange with bicarbonate. The model couples sodium and butyrate via CO2 from metabolism of butyrate, and intracellular pH.

Birth and prematurity influence intestinal function in the newborn pig.

Developmental changes in intestinal function occur in the perinatal period of many species. We investigated the hypothesis that gestational age at delivery and the mode of delivery influence intestinal function. Newborn pigs (106-108 or 113-115 days gestation, term = 115 +/- 2 days) were either delivered by caesarean section or born vaginally following induction of parturition with a prostaglandin F2 alpha analogue. The pigs were killed at birth and used for measurements of intestinal ion transport in vitro (using Ussing chambers) or killed at 2 days of age, after being fed porcine colostrum to follow the absorption of intact proteins into plasma. The results indicate that premature birth is associated with increased paracellular permeability to ions. The uptake and net absorption of chloride were higher in the term, vaginally-delivered pigs than in the remaining pigs. Among the newborn pigs, the preterm caesarean-delivered pigs exhibited the lowest chloride secretion in response to the secretagogue, theophylline. The latter pigs also absorbed the lowest amounts of immunoglobulin G and albumin from colostrum. In conclusion, gestational age at delivery and the mode of delivery have significant effects on intestinal transport of ions and intact proteins. However, the observed variation in the magnitude and the direction of responses indicate that (a) prematurity and birth influence the transport of ions and intact proteins through independent regulatory mechanisms and (b) the absorption pathways for ions and intact proteins in the neonatal pig intestine are not closely associated.

Feed-induced changes in transport across the rumen epithelium.

The effect of feeding strategy on rumen epithelial growth and transport capacity were studied in sixteen dairy cows. There was a significant effect of feeding strategy on transport of butyrate, sodium and chloride ions, which could not be explained by changes in epithelial surface area, structure or resistance.

Absorption and fate of L- and D-lactic acid in ruminants.

Lactic acid is produced in significant amounts in the rumen on rations rich in easily digestible carbohydrates such as starch and sugars. If high amounts of concentrates, containing starch and sugars, are fed to high lactating dairy cows, lactic acid concentrations in rumen fluid up to 80 mmol/l can be found. If such high concentrations of lactic acid are consistent during longer periods of time, rumen acidosis may arise, causing disfunction and necrosis of rumen epithelium.

Lactate transport across the bovine rumen epithelium in vitro.

The in vitro transport of L-lactate across rumen epithelium was found to be a passive, mass-action regulated diffusion, and no saturable transport mechanisms were involved at concentrations from 2-20 mmol/L. During this increase in L-lactate concentrations the epithelial resistance and potential difference increased significantly, indicating an epithelial change towards a more tight epithelium. The net sodium transport was significantly reduced by 45% at 20 mmol/L lactate as compared to lactate free conditions.

Transport of sodium across the isolated bovine rumen epithelium: interaction with short-chain fatty acids, chloride and bicarbonate.

Unidirectional transport rates of sodium (22Na+) and chloride (36Cl-) across bovine rumen epithelium were measured in vitro by the Ussing chamber technique. The active and short-chain fatty acid (SCFA)-stimulated sodium transport was shown to fit Michaelis-Menten kinetics, and was rate limited mainly by one transport system, characterized by a Km of 43 mmol l-1 Na+ and a Jmax (maximal transport rate) of 6.2 mumol cm-2 h-1 Na+. It was confirmed that the basolateral Na+,K(+)-ATPase was essential for active sodium transport, and that an apical amiloride-sensitive sodium transport system (Na(+)-H+ exchange) was involved in a minimum of 60-70% of the active sodium transport in the presence of SCFAs (butyrate). The main part of both the mucosal-serosal (MS) and serosal-mucosal (SM) sodium flux was sensitive to an applied electrical potential difference (PD). It is noteworthy that an applied PD, equal to the in vivo PD (+30 mV, lumen as reference), abolished net transport of sodium. The stimulating effect of a mixture of acetate, propionate and butyrate on active sodium transport was confirmed, and it was further shown that the stimulating effect of each of the three SCFAs was nearly equal. Analogues of naturally occurring SCFAs (isobutyrate and 2-ethyl-butyrate) did not stimulate active sodium transport, but inhibited the stimulating effect of butyrate. The stimulating effect of butyrate was clearly concentration dependent and showed a maximum at approximately 20 mmol l-1 butyrate. Above this limit active sodium transport was decreased with increasing butyrate concentration. This suggests that there was a limit to the amount of butyrate that could be handled by the epithelium. The active sodium transport was clearly correlated with the chloride concentration, and was significantly reduced, but not abolished, by replacement of chloride with gluconate. Active transport of chloride was stimulated by butyrate and reduced by the Na(+)-H+ exchange inhibitor amiloride (3 mmol l-1). There was no effect of the Cl(-)-HCO3- exchange inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid; 0.5 mmol l-1) on sodium transport. HCO3- (13 mmol l-1) and CO2 (5%) themselves had only a small and non-significant stimulating effect on sodium fluxes, however, in the presence, but not the absence of HCO3- and CO2 in the experimental solutions acetazolamide (1 mmol l-1) significantly reduced active sodium transport. It is concluded that SCFAs could stimulate the active sodium and chloride transport as a result of their metabolism. The CO2 produced could stimulate apical Na(+)-H+ and Cl(-)-HCO3- exchangers running in parallel via increased H+ and HCO3- gradients.

Sodium and chloride transport across the rumen epithelium of cattle in vitro: effect of short-chain fatty acids and amiloride.

Isolated mucosal sheets of cattle rumen were studied in vitro in Ussing chambers using a computer-controlled voltage clamp. Unidirectional fluxes of Na+ and Cl- were measured in standard (short-chain fatty acid (SCFA)-free) or SCFA-containing Ringer solution under zero voltage clamp conditions. Net NaCl absorption was observed under zero voltage clamp conditions, was stimulated by SCFAs and was not reduced by 1 mmol l-1 amiloride, although short-circuit current was significantly reduced by this concentration of the blocker. A constant coupling ratio of Na+ and Cl- absorption was observed.

Intestinal transport of sodium, glucose and immunoglobulin in neonatal pigs. Effect of glucocorticoids.

The effect of cortisol on plasma immunoglobulin G (IgG) levels and on jejunal transport of sodium and glucose has been investigated in neonatal pigs (0-7 days of age). Five pigs were delivered vaginally (VD) at term (114-115 days gestation) and thirty-five by caesarean section (CS) 2-4 days before term (to circumvent a neonatal cortisol surge). Postnatally, twenty-seven CS pigs were injected with adrenocorticotrophic hormone (ACTH; n = 13), metyrapone (an inhibitor of adrenal 11 beta-hydroxylase activity, n = 7) or physiological saline (n = 7). Pigs were killed at birth, at 3 days, or at 6-7 days of age. Plasma concentrations of cortisol and IgG were determined and unidirectional fluxes of sodium and glucose measured across the proximal jejunum in vitro. Newborn CS pigs had significantly lower plasma cortisol but higher net fluxes of sodium and glucose than newborn VD pigs. At 3-7 days, metyrapone-treated pigs had significantly lower concentrations of cortisol and IgG in plasma and lower net fluxes of sodium and glucose than ACTH-treated pigs. Differences in net fluxes between treatment groups resulted from changes in both electrogenic and non-electrogenic sodium transport. Furthermore, sodium-glucose co-transport accounted for a major proportion of net sodium transport. High unidirectional fluxes in newborn CS pigs may be explained by the passage of large amounts of sodium through tight junctions and paracellular pathways in the prepartum period. The results indicate that cortisol stimulates net transport of sodium, glucose and immunoglobulins in the postnatal period of caesarean-delivered pigs. However, cortisol is unlikely to play a major regulatory role in the postnatal maturation of intestinal transport in normally delivered pigs.