Thiocyanate transport across fish intestine (Pleuronectes platessa)

Summary When bathed on both sides with identical chloride-containing salines thein vitro preparation of the plaice intestine maintains a negative (serosa to mucosa) short-circuit current of 107±11 A/cm², a transepithelial potential difference of 5.5±0.6 mV (serosa negative), and a mean mucosal membrane potential of –45.4±0.6 mV. Under these conditions the intracellular chloride activity is 32mm.If chloride in the bathing media is partially, or completely substituted by thiocyanate the measured electrical parameters do not change but transepithelial flux determinations show a reduction in chloride fluxes and the presence of a significant thiocyanate flux. The addition of piretanide (10^–4 m) reduced the short-circuit current and the mucosa-to-serosa fluxes of chloride and thiocyanate; this inhibition is similar to the effect of piretanide on chloride transport in this tissue.The results indicate that thiocyanate is transported in this tissue via the piretanide-sensitive chloride pathway and are compared with the effects of thiocyanate on other tissues reported in the literature.     

Taurine transport across the small intestine of adult and suckling rats

1. Taurine accumulation in intestinal cells of adult and suckling rats reached steady-state after 60 min with an In/Out ratio of 1.46 and 4.66 in the adult and suckling rats respectively. 2. The accumulative capacity of the intestinal strips isolated from suckling rats is almost four times higher than that of adult rats. 3. The steady-state uptake of taurine by the adult and suckling rats intestinal cells is saturable, sodium-dependent and inhibited by ouabain. 4. The calculated Vmax of the mediated component of the steady-state uptake in the suckling rats is three times greater than that of the adult rats, and the affinity is seven fold greater in the suckling as compared to the adult. 5. Taurine influx across the mucosal membrane in the suckling rat is significantly greater than that of the control adult.     

Effect of enzymatic digestion on phenylalanine transport across the small intestine

Phenylalanine accumulation in mucosal strips isolated from rat small intestine was significantly inhibited (P less than 0.01) after preincubation with trypsin, chymotrypsin, phospholipase D and neuraminidase. Unidirectional phenylalanine influx across the small intestine was significantly reduced (P less than 0.01) when the mucosal strips were preincubated with the above mentioned enzymes. Intestinal cell water and volume were not significantly changed (P greater than 0.6) when the intestinal tissues were preincubated with these enzymes.     

Enzymatic inhibition of lysine transport across the small intestine in vivo

1. Trypsin, at different concentrations, significantly inhibited lysine absorption (P less than 0.05) in a dose-dependent pattern. 2. Maximum inhibition equivalent to 35% below control value was reached with 10 micrograms/ml (100 BAEE units) trypsin with a non-reversible inhibitory effect. 3. Chymotrypsin at 10 micrograms/ml produced a significant decrease (P less than 0.05) of lysine absorption although it did not exceed 5%. Perfusion of both enzymes did not show an additive inhibitory effect. 4. Lysine absorption showed a 39% decrease with 10 micrograms/ml trypsin and 1 X 10(-4) M ouabain, whereas ouabain alone produced 34% inhibition. 5. Lysine absorption showed a 71% decrease with 10 micrograms/ml trypsin in a sodium-free medium, and 70% inhibition with Na-free medium alone. 6. The inhibition of lysine absorption after trypsin treatment could be due to inhibition of the active component of lysine transport.     

Direct measurement of ferrous ion transport across membranes using a sensitive fluorometric assay

The fluorophore, Phen Green SK (PGSK), was assessed for its suitability to be used in an assay for ferrous ion transport into membrane vesicles. The long wavelengths of excitation and emission (506 and 520 nm, respectively) enable PGSK fluorescence to be detected in membranes, such as the chloroplast inner envelope, that contain high levels of carotenoids which absorb light at lower wavelengths. At low concentrations of Fe2+, less than 3 microM, the interaction between PGSK and Fe2+ appears to result in both static and dynamic quenching of the PGSK fluorescence. The characteristics of this quenching were used to develop a calibration curve to determine the concentration of free Fe2+ at these low concentrations. Pronounced quenching of PGSK fluorescence entrapped within chloroplast inner envelope membrane vesicles was observed when Fe2+ was added. The extent of quenching of PGSK fluorescence trapped inside asolectin vesicles on Fe2+ addition was much less. The kinetics of the quenching of PGSK fluorescence by Fe2+ in vesicles was quite different from that for PGSK and Fe2+ in solution. Using the calibration curve developed for interaction of PGSK and low Fe2+ concentrations the initial rates of iron transport could be determined for the chloroplast inner envelope membranes.     

Effect of somatostatin on D-galactose transport across the small intestine of rats

Somatostatin was found to diminish control and theophylline-treated tissue sugar accumulation as well as control and also to diminish theophylline mucosal to serosal D-galactose fluxes. When Na+ was removed from the bath solution, sugar transport was unaltered by the presence of somatostatin. The same results were obtained with phlorizin in the medium. These results seem to suggest that the action of somatostatin is restricted to the Na+-dependent sugar carrier located on the brush border of the intestinal epithelium.     

Imino acid transport across the brush-border membrane of the guinea-pig small intestine

The transport of imino and non-alpha-amino acids across the brush-border membrane of the guinea-pig small intestine has been examined. It was found that the guinea pig is without a transport system for non-alpha-amino acids. The transport of imino acids was characterized using methylaminoisobutyrate (MeAIB) as a substrate. This choice was validated by lack of kinetic evidence that more than one transport system was involved in the transport of MeAIB, by the identical values of the estimates of the passive permeability of MeAIB, the magnitude of its proline-resistant transport, and the permeability of mannitol. The transport system for MeAIB is moderately stereospecific. It does not accept cationic amino acids. It accepts alpha-amino-monocarboxylic acids but N-methylation increases the affinities of these amino acids by an order of magnitude. The length of the side-chain of the aliphatic imino acids seems of little importance for the affinity for the transport system, but the data on inhibition of the transport of MeAIB by proline and piperidine-2-carboxylic acid indicate that it is sharply increased by ring formation.     

Inhibition of sugar active transport across rat intestine in vivo by cadmium

Galactose absorption by rat jejunum perfused in vivo is inhibited by 0.5 mM Cd2+. This effect is explained by impairment of the phlorizin-sensitive sugar transport system, as Cd does not modify the absorption of L-sorbose or that of galactose in the presence of 0.5 mM phlorizin. Cd inhibition is observed as early as in the 1st minute, does not increase by previous exposure of the mucosa to the metal and does not decrease after washing with saline solution, but it is entirely reversed by EDTA or dithioerythritol. Results agree with a Cd2+ binding to HS- groups of membrane proteins at the brush border, appertaining or functionally related to the phlorizin-sensitive and Na+ dependent transport system for sugars.     

Cytoskeletal control of alanine transport across the rat and turtle small intestine

Procaine inhibited significantly (P less than 0.01) alanine accumulation in the rat intestinal strips in a concentration-dependent pattern, whereas it showed no effect on alanine uptake by the turtle intestinal cells. Colchicine and Vinca alkaloids at 5 X 10(-4) and 1.5 X 10(-6) M respectively caused a significant inhibition (P less than 0.01) of intracellular alanine concentration in the rat with no effect noticed in the turtle. Unidirectional influx of alanine across the brush border membrane of the rat was significantly (P less than 0.01) reduced in the presence of procaine, colchicine and vincristine in the preincubation medium. The same drugs did not show any effect on alanine influx into the turtle small intestine. Electron microscopy showed major structural alterations in the cytoskeletal organization of the turtle intestine in response to procaine, colchicine or vincristine treatment. It is proposed that microtubular system may participate in the overall transport mechanism of alanine across the small intestine.     

Regulation of ion and water transport across the eel intestine: effects of acetylcholine and serotonin

Summary Both acetylcholine (ACh) and serotonin (5-HT) lowered the serosa-negative transepithelial potential difference (PD) and the short-circuit current (I_sc), accompanied by a decrease in NaCl and water absorption across the eel intestine. These inhibitory effects of ACh and 5-HT were blocked by atropine, a muscarinic receptor antagonist, and ICS-205930, a 5-HT₃ receptor antagonist, respectively. Even after blocking the ACh receptor with atropine, 5-HT inhibited the PD and I_sc, and ACh lowered them after blocking the 5-HT receptor with ICS-205930, indicating that ACh and 5-HT act independently. Similar inhibition in the PD and the I_sc was observed after electrical field stimulation (EFS) which is expected to release endogenous regulators. These effects of EFS were reduced by 70% after simultaneous addition of atropine and ICS-205930. Since atropine and ICS-205930 block ACh and 5-HT receptors, respectively, these results suggest that endogenous ACh and 5-HT are released by EFS.Abbreviations ACh acetylcholine - EFS electrical field stimulation - 5-HT serotonin - I _sc short-circuit current - PD transepithelial potential difference - R _t tissue resistance - TTX tetrodotoxin     

pH-sensitive transport of Fe2+ across purified brush-border membrane from mouse intestine

Recent studies of Fe2+ uptake by mouse proximal intestine brush-border membrane vesicles revealed low-affinity, NaCl-sensitive and high-affinity, NaCl-insensitive, components of uptake (Simpson, R.J. and Peters, T.J. (1985) Biochim. Biophys. Acta 814, 381-388). In this study, the former component is demonstrated to show a strong pH dependence with an optimum of pH 6.8-6.9. Studies at pH 6.5, where the low affinity component is inhibited by more than 25-fold compared with pH 7.2, suggest that the pH-sensitive component represents transport across the brush-border membrane followed by intravesicular binding. Cholate extracts of brush-border membrane vesicles contain pH- and NaCl-sensitive Fe2+ binding moieties which may be involved in the transfer of Fe2+ across the intestinal brush-border membrane and subsequent binding inside the vesicles. Fe2+ uptake by brush-border membrane vesicles from the duodenum of hypoxic mice is higher than uptake by vesicles from control-fed animals, suggesting the existence of a regulable brush-border membrane Fe2+ carrier.