Rate-limiting barriers in intestinal absorption

Factors involved in absorption of organic mercuric compounds from rat small intestine: comparative study with mercuric chloride in situ

A correlation between absorption of organic mercuric compounds and their molecular weight or lipophilicity was investigated by the perfusion of rat small intestine. The intestine was perfused for 1.0 hr at different osmolarity and different pH with buffers containing 10(-4) M organic mercuric compounds, methyl mercuric chloride (MMC), phenyl mercuric chloride (PMC), p-chloromercuribenzoic acid (PCMB) and p-chloromercuriphenylsulfonic acid (PCMBS). The order of absorption of these compounds at several osmolarities and pHs (MMC greater than PMC greater than PCMB greater than PCMBS) was inversely correlated to their molecular weight. The increase in pH decreased markedly the absorption and n-octanol/water partition coefficient of PCMB, and it decreased scarcely those of MMC, PMC and PCMBS. The order of the coefficients at each pH was as follows; PMC greater than MMC greater than PCMB greater than PCMBS, in disagreement with the order of absorption. These results suggest that the molecular weight is more important factor in the intestinal absorption of organic mercuric compounds than the lipophilicity.

Effects of stretching and stirring on water and glucose absorption by canine mucosal membrane

A 'mini' canine mucosal membrane preparation permitting simultaneous determination of water (Jv) and glucose (Jg) absorption rates, microscopic examination or micropuncture of the villi was used in this study. The small membranes were more stretched than the large ones, with more than a one-fold increase in both Jv and Jg, apparently due to a change in architectural orientation between the villi and subvillous supporting tissue so as to facilitate water transport via the lymphatic system. During stirring of the bathing solution, the villi in the small membranes were widely separated from each other with more to-and-fro swaying movements than in the large ones. Stirring was seen to cause up-and-down movements of the loosely suspended large membranes but not the small ones. In the small membranes stirring caused no change in Jv but an increase in Jg due to the increase in glucose concentration in the absorbate, while in the large membranes both Jv and Jg were greatly increased. It is thus considered that the increase in absorption in the large membranes caused by stirring is mainly due to the increased membrane movements promoting lymph flow.

Passage of molecules through the wall of the gastrointestinal tract. Increased passive permeability in rat ileum after exposure to lysolecithin

The interaction between lysolecithin and mucosal cells in the distal ileum has been studied. Using a rat experimental model, we determined the intestinal permeability to fluorescent dextran 3000 after exposure to different amounts of lysolecithin. At pH 7.3, lysolecithin, 10 mg/ml, significantly enhanced the transmural passage, and at pH 3.5, even 1 mg/ml markedly increased the permeability. However, when lysolecithin was incubated with homogenized mucosal cells, a rapid disappearance of the compound occurred; this was accompanied by formation of free fatty acids and minor formation of lecithin. It appears, therefore, that high concentrations of lysolecithin can impair the intestinal barrier function in the distal part of ileum but that the mucosal cells are well equipped with activities for the rapid removal of such high concentrations. This could be physiologically significant, since the mucosal cells could otherwise be faced with large amounts of lysolecithin that might facilitate the absorption of potentially antigenic and toxic compounds.

Permeability of everted rat small intestine to lidocaine and derivatives

The transfer rates of lidocaine and five derivatives were measured across the everted rat intestine. No obvious relationship was observed between the first-order rate constants for transfer and the lipophilicity of the compounds as measured by their apparent partition coefficients. The intestinal mucosal epithelium also did not appear to be the rate-limiting barrier for the passage of these agents across the intestinal membrane regardless of the respective clearance of the compound. It is suggested that the ionized form of these agents may be capable of crossing the intestinal barrier at substantial rates, although it is not known if a similar situation exists in vivo. The transfer rate is believed to be due to the passage of intact drug and not the metabolite.

Effect of enzyme-inducing and enzyme-inhibiting agents on drug absorption. II. Influence in proadifen on 3-O-methylglucose transport in rats

Studies utilizing the in vitro everted rat jejunum were performed to investigate the effects of proadifen pretreatment and fasting on active and passive intestinal transfer of 3-O-methylglucose. Animals were pretreated 24 hr prior to the experiments with 100 mg of proadifen/kg ip. With mucosal concentrations of 0.1 mM 3-O-methylglucose and 14C-3-O-methylglucose, transfer to the sugar-free serosal buffer after pretreatment with proadifen was 50% greater than controls after a 24-hr fast and 120% greater than controls after a 48-hr fast. Everted intestinal segments obtained from unfasted control animals showed diminished ability to transfer the sugar derivative, with transfer rates approximately 50% less than those of segments obtained from 24-hr fasted control animals. Wet and dry tissue weights for 24- and 48 hr-fasted groups increased following proadifen pretreatment. The results suggest that proadifen enhances 3-O-methylglucose transport across the rat jejunum and also increases intestinal tissue weight.

Effects of short and medium chain fatty acids on absorption of lipophilic drugs from perfused rat intestine

The absorption rates of griseofulvin and prednisolone were measured from solutions that were recirculated through a segment of in situ rat jejunum. When premicellar concentrations of butyric, octanoic, or dodecanoic acid were present in the perfusate, the griseofulvin absorption rate decreased while that of prednisolone increased. The fatty acids also increased the absorption rate of water from the perfusate, and the effect of butyric acid was attributed primarily to this increase. The absorption-altering effects of octanoic and dodecanoic acids could not be attributed solely to their effects on water absorption nor to their effects on the surface tension of the perfusate. The effects of octanoic and dodecanoic acids were explained by postulating different rate-limiting barriers to the absorption of prednisolone and griseofulvin.

Effect of enzyme-inducing and enzyme-inhibiting agents on drug absorption I: 3-O-methylglucose transport in rats

The influence of preadministration of phenobarbital, benzo[alpha]pyrene, and proadifen hydrochloride on 3-O-methylglucose transfer across the everted rat small intestine was examined. The active and passive components of the sugar transport mechanism were evaluated using phlorizin, a potent inhibitor of active transport of sugars. Pretreatment of the animals with phenobarbital did not alter 3-O-methylglucose transfer characteristics. Pretreatment with intraperitoneally administered benzo[alpha]pyrene increased mucosal to serosal transfer of the sugar low (0.1 mM) sugar concentrations. Enhancement of the active transfer of 3-O-methylglucose by pretreatment with proadifen hydrochloride was noted at low sugar concentrations. The passive transfer of the sugar was reduced after pretreatment with proadifen hydrochloride.

The transport of tetracyclines across the mouse ileum in vitro: the effect of cations and other agents

The intestinal transfer of different tetracyclines dissolved in calcium- and magnesium-free Krebs bicarbonate buffer solution, pH 7.4, was studied using the everted ileum of the mouse. The rates of transfer of chlortetracycline and demethylchlortetracycline were less than those of tetracycline and oxytetracycline, the latter compounds being transferred at the same rate. Addition of calcium and magnesium to the buffer greatly reduced the transfer of tetracycline; this inhibition could be antagonized by EDTA. The presence of iron also inhibited the transfer of tetracycline. The inhibitory effect of these ions on tetracycline transfer seemed due to chelation of the drug. Glucosamine and acetylmethionine, but not acetyl glucosamine, diminished the intestinal transfer of tetracyclines. The former two agents did not influence the uptake of tissue fluids. Tetracycline was also transfered from the serous to the mucous coat in the non-everted intestinal sac of mice. The above observations suggested that the absorption of tetracyclines was not due solely to passive diffusion.

Effect of surfactant monomers on chloramphenicol association to an albumin-lecithin complex: a model for modified drug absorption

The binding of chloramphenicol to an albumin-lecithin complex in the presence or absence of premicellar concentrations of both ionic and non-ionic surfactants has been examined. Long chain, strong ionic detergents, such as sodium dodecyl sulphate or cetyltrimethylammonium bromide, severely perturb protein structure and eventually allow full separation of the complex into lecithin and albumin-detergent complexes. The dissociation process is reversible upon the removal of the detergent by exhaustive dialysis. After the splitting of the complex, the amount of antibiotic associated with the lipid-protein mixture increases. Structural alteration of the albumin-lecithin complex and the increase in the binding of chloramphenicol have an effect on the transfer rate of this antibiotic across an artificial barrier consisting of an aqueous dispersion of the same complex, as observed in a model system. It is suggested that a reversible alteration in membrane structure, and consequently in membrane permeability, might be easily effected, at the molecular level, through a reversible dissociation of structural lipoproteins into their components, operated by premicellar concentrations of ionic surfactants. This represents a tentative picture of the possible events taking place within the membrane and modifying the absorption rate of a drug, when it is associated with surfactants in a pharmaceutical preparation.

Intestinal transport of 3H-digitoxin in vitro incompatible with simple diffusion

On everted jejunal segments of mice the transfer and tissue uptake of 3H-digitoxin, over a concentration range from 2 times 10(-10)--1 times 10(-4)M, was investigated from the mucosal ("m") to the serosal ("s") side as well as in the opposite direction. 1. The time course of the absorption of 3H-digitoxin and some other compounds investigated (glucose, urea, p-aminohippurate) gave evidence of functional integrity throughout the 75 min-periods of the experiments. 2. When 3H-digitoxin was applied to the mucosal side the permeability coefficient showed a dose-dependent increase but returned to lower values at higher concentrations. When 3H-digitoxin was administered to the serosal side the permeability coefficient showed a dose-dependent decrease at high concentrations. The ratio of both coefficients "m" leads to "s"/"s" leads to "m" increased dose-dependently from 0.4--2.6. 3. The uptake of 3H-digitoxin--applied on the serosal side--into the tissue was independent of dose. However, having administered 3H-digitoxin on the mucosal side the tissue accumulation was 2--5 fold higher and the tissue/medium (T/M) ratio increased within the concentration range from 3.0-9.0 4. Under DNP (1 mM) the asymmetry and dose dependence of the permeability and tissue uptake up 3H-digitoxin observed in controls were almost abolished. Therefore it is likely that the transfer of 3H-digitoxin in the intact intestine involves a mechanism more complex than simple diffusion. The existence of more than a two compartment system and/or the contribution of an active transport mechanism is suggested.