Taurine transport by rat intestine

Rat intestinal transport of taurine was studied using jejunal brushborder membrane vesicles and everted jejunal sacs. Membrane vesicle experiments showed that the initial rate (at 10 s) of 10 microM taurine uptake was stimulated 3.5-fold by an inwardly directed Na+ gradient when compared with a K+ gradient. The Cl- salt of Na+ supported uptake to a significantly greater degree than did more (SCN-) or less (SO2-4) permeant salts. Na+-stimulated uptake at 1 min achieved a value 2.5 times greater than equilibrium ("overshoot"). When 10 s taurine uptake was determined over a range of taurine concentrations (10-1,000 microM) and plotted using the Woolf-Hofstee equation, a diffusive and carrier-mediated component was apparent. Half-maximal, carrier-mediated uptake occurred at 25 +/- 9 microM taurine (Km). An inside-negative, valinomycin-induced K+ diffusion potential stimulated 10-s taurine uptake when compared with voltage-clamped conditions (4.9 vs. 2.6 pmol/mg protein; P less than 0.05). Incubation with a structural analogue, hypotaurine, reduced 10-s taurine uptake by 89%. Uptake by everted jejunal sacs also demonstrated Na+ stimulation and inhibition by hypotaurine. These data confirm the existence of a high-affinity rat jejunal brush-border membrane-associated taurine transport mechanism that is electrogenic, stimulated by a Na+ gradient, and modified by external Cl-.

Distribution and partitioning of cholesterol and beta-sitosterol in micellar bile salt solutions

The distribution of cholesterol and beta-sitosterol in micellar bile salt solutions was studied using an ultrafiltration technique to separate micellar aggregates from monomers present in the intermicellar aqueous phase. The partitioning and the rates of influx across the brush-border membrane of these two sterols from micellar solutions were determined using polyethylene discs and everted jejunal sacs, respectively. Micellar solubilities of cholesterol and beta-sitosterol were not significantly different, whereas the monomer concentration of beta-sitosterol in the intermicellar aqueous phase was significantly lower than that of cholesterol [0.21 +/- 0.02 microM for beta-sitosterol and 25.0 +/- 2.71 (SE) microM for cholesterol, P less than 0.001]. There was no difference in the partitioning nor was there a difference in the rates of influx between cholesterol and beta-sitosterol from micellar solutions containing a similar amount of the two sterols. It is concluded that monomer concentration of beta-sitosterol in the intermicellar aqueous phase is extremely low compared with that of cholesterol, but their partitioning and rates of influx across the membrane are similar. This is primarily attributable to a much higher membrane/monomer partition coefficient of beta-sitosterol than cholesterol and to a direct interaction between micelle and membrane.

Intestinal transport of 5-methyltetrahydrofolate

The mechanism of intestinal absorption of 5-methyltetrahydrofolate (5- CH3THF ) has been the topic of some controversy. In the present study, we have used enzymatically prepared 5- CH3THF to characterize transport by rat intestinal loops in vivo and everted jejunal sacs in vitro. Transport of 5- CH3THF is saturable (Km = 5.2 microM) and highly pH dependent, with the rate of maximal transport occurring at pH 5.8. Transport is competitively inhibited by folic acid (Ki = 4.2 microM) and methotrexate (Ki = 4.65 microM). Metabolic poisons and anaerobiosis greatly reduce 5- CH3THF transport. We conclude that 5- CH3THF transport in the rat intestine occurs by the same structure-specific mechanism responsible for the transport of unreduced folic acid and other monoglutamyl folates.

The validity of assessing changes in intestinal absorption mechanisms for dietary sugars with non-metabolizable analogues (glucalogues)

1. Transfer potentials were obtained from everted jejunal sacs prepared from fed euthyroid, fasted euthyroid, fed hypothyroid and fasted hypothyroid rats by addition of serial concentrations of the dietary sugars glucose and galactose and the glucalogues 3-O-methyl glucose, α-methyl glucoside and 6-deoxy-D-glucose to the incubation fluids. The kinetic parameters of ‘apparent Michaelis constant’ (apparent Km) and maximum transfer potential difference (pdmax) obtained from the results were used to characterize the changes in the electrogenic transfer mechanisms for these substrates.2. Analysis of the significant differential changes in values for ‘apparent Km’ and pdmax for the two dietary sugars and the three glucalogues indicated heterogeneity in the mechanisms for sugar transfer across the intestine and suggested a minimum of four possible carriers.3. The validity of using glucalogues to characterize changes in the transfer mechanisms for the dietary sugars in different dietary and hormonal states was assessed. None of the kinetic parameters for electrogenic glucalogue transfer matched those for the dietary sugars in all the experimental conditions. The employment of glucalogues to assess changes in electrogenic transfer mechanisms for dietary sugars can thus lead to invalid conclusions.4. Fasting decreased the ‘apparent Km’ of the dietary sugars and the glucalogues. However, the pdmax values for glucose, galactose, and α-methyl glucoside decreased whereas those for 3-O-methyl glucose and 6-deoxy-D-glucose did not.5. Hypothyroidism showed different effects in fed and fasted intestine. Because hypothyroidism induced a reduction in food intake, separation of the direct effects of the condition on electrogenic transfer from reduced food intake effects was not possible.

The Influx of Uric Acid and other Purines into Everted Jejunal Sacs of the Rat and Hamster

The in vitro transport of [2-14C]uric acid, [8-14C]hypoxanthine, and [8-14C]xanthine, each dissolved in Krebs–Ringer bicarbonate buffer, was studied with everted jejunal sacs from rat and hamster. No evidence could be obtained for the development of a concentration gradient between the intracellular fluid and the incubation medium or between the sac contents and the incubation medium, for any of the three oxypurines. Inhibitors of active transport, such as anaerobiosis or dinitrophenol, had no significant effect on the rate of transport. A large percentage of hypoxanthine and xanthine was oxidized to uric acid in the sac-wall homogenate, sac contents, and incubation medium during the course of the incubation. This oxidation could be prevented by addition of allopurinol (3 mM) to the incubation medium, but concentration gradients were still not obtained. No active transport mechanism could be demonstrated for uric acid, hypoxanthine, or xanthine in rat or hamster jejunum.