Influence of blood flow on intestinal absorption of drugs and nutrients

Chlordecone: development of a physiologically based pharmacokinetic tool to support human health risks assessments

Chlordecone (CD; Kepone™) is a carcinogenic organochlorine insecticide with neurological, reproductive, and developmental toxicity that was widely used in the French West Indies (FWI) from 1973 to 1993 to fight banana weevils. Although CD has not been used there for more than 25 years, it still persists in the environment and has polluted the waterways and soil of current and older banana fields. Today, human exposure to CD in the FWI mainly arises from consuming contaminated foodstuffs. The aims of this study were to develop a physiologically based pharmacokinetic (PBPK) model in the rat and extrapolate it to humans based on available pharmacokinetic data in the literature. A comparison of simulations using the rat model with published experimental datasets showed reasonable predictability for single and repetitive doses, and, thus, it was extrapolated to humans. The human PBPK model, which has seven compartments, is able to simulate the blood concentrations of CD in human populations and estimate the corresponding external dose using the reverse dosimetry approach. The human PBPK model will make it possible to improve quantitative health risk assessments for CD contamination and reassess the current chronic toxicological reference values to protect the FWI population.

A PBPK model describing the pharmacokinetics of γ-HBCD exposure in mice

The brominated flame retardant, hexabromocyclododecane (HBCD), is added-but not bound-to consumer products and is eventually found in the environment and human tissues. Commercial-grade HBCD mixtures contain three major stereoisomers, alpha (α), beta (β), and gamma (γ), that are typically at a ratio of 12%:6%:82%, respectively. Although HBCD is widely used, the toxicological effects from its exposure in humans are not clearly understood. Using a physiologically based pharmacokinetic (PBPK) model could help improve our understanding of the toxicity of HBCD. The aim of this work was to develop a PBPK model, consisting of five permeability limited compartments (i.e., brain, liver, adipose tissue, blood, and rest of the body), to evaluate the pharmacokinetics of γ-HBCD in C57BL/6 mice. Physiological parameters related to body size, organ weights, and blood flow were taken from the literature. All partition coefficients were calculated based on the log Kow. The elimination in urine and feces was optimized to reflect the percent dose eliminated, as published in the literature. Compared with data from the literature for brain, liver, blood, and adipose tissue, the model simulations accurately described the mouse data set within 1.5-fold of the data points. Also, two examples showing the utility of the PBPK model supplement the information regarding the internal dose that caused the health effects observed during these studies. Although this version of the PBPK model expressly describes γ-HBCD, more efforts are needed to clarify and improve the model to discriminate between the α, β, and γ stereoisomers.

Physiologic responses to sensory stimulation by food: nutritional implications

The importance of sensory factors (both the sensory properties of foods and sensory abilities of consumers) in food selection is widely accepted. Less recognized and understood are the effects of sensory stimulation on physiologic processes. Mere exposure to the sight, smell, taste, and textural attributes of foods elicits myriad digestive, endocrinologic, thermogenic. cardiovascular, and renal responses. The responses are rapid (generally occurring within minutes of sensory stimulation), small (relative to the magnitude achieved when food is actually being metabolized), and transient (returning to near-baseline levels within minutes). Nevertheless, they have been hypothesized to prime the body to better absorb and use ingested nutrients. They are termed preabsorptive or cephalic phase responses. This review summarizes current knowledge of stimuli that trigger cephalic phase responses and selected response features (eg, magnitude and type of enzymes or hormones released). Mechanisms by which cephalic phase responses may influence nutrient absorption and use are proposed. Current knowledge of the behavioral and physiologic importance of cephalic phase responses is limited, thereby hampering development of dietary recommendations for preventive or therapeutic interventions. Further study of several identified promising areas of application and other easily proposed situations may provide a basis for expanded medical nutrition therapy.

Pharmacokinetic modelling of the effect of activated charcoal on the intestinal secretion of theophylline, using the isolated vascularly perfused rat small intestine

The effect of activated charcoal administration on the secretion of theophylline from the blood into the intestinal lumen has been examined by use of the rat isolated vascularly perfused small intestine. A closed two compartment model was used to analyse the vascular and luminal concentration-time curves obtained. An equation was derived to calculate the time-dependent intestinal clearance. From control experiments it was concluded that theophylline is secreted by a diffusional transport system through the intestinal wall. The intestinal clearance declined rapidly with time as a result of the concomitant increase in luminal theophylline concentration. After 120 min a steady state between the vascular and luminal perfusate was established. Administration of activated charcoal in the lumen had a profound effect on the kinetics of the drug. The vascular steady state concentration was depressed dramatically. The theophylline clearance remained nearly constant with time, because the blood to lumen concentration gradient was maximized. The maximal value for the intestinal theophylline clearance was estimated to be 0.88 mL min-1 and it equalled the value for the intestinal blood flow at the absorptive site. By use of the concept of absorptive site blood flow, the maximal effect of charcoal on systemic theophylline clearance could be adequately predicted for rats, dogs and man. Activated charcoal administration is only useful to enhance the systemic clearance of drugs or toxicants if that clearance is of the same order of magnitude as the absorptive site blood flow or lower.

Characterization of mitomycin C-induced gastrointestinal damage. I. In situ recirculation experiment

The effects of mitomycin C (MMC), an anti-tumor agent, on the intestinal absorption of various drugs in rats were investigated. Based on microscopic observations, preadministration of a single intravenous dose of MMC (3 mg/kg) caused serious degeneration of epithelial cells, villous atrophy, and mitotic arrest in crypts at 48 hr after pretreatment. At this time point, absorption of sulfanilamide, salicylic acid, cephalexin, and L-tryptophan was shown to be significantly decreased by means of an in situ recirculation technique. The histological changes and the decrease in absorption of sulfanilamide, a model for passively absorbed drugs, were shown to depend on the timing of MMC pretreatment. Maximal effects were observed 48 hr after dosing. The MMC-induced reduction in the absorption of drugs was not a result of differences between treated and control animals with respect to pH of the drug solution, binding of drugs with intraluminal macromolecules, or intestinal mucosal blood flow. The absorption of sulfanilamide from the small intestine in the in situ system correlated well with the wet weight of the small intestine regardless of pretreatment dose or route. This suggests that the change in absorptive surface area of the intestinal mucosa may play a major role in the MMC-induced decrease in absorption capacity of the intestine.

Differential regional changes of prostacyclin and thromboxane A2 synthesis in the intestinal tract of the fasted and semistarved rat

The synthesis of prostacyclin (PGI2) and thromboxane A2 (TXA2) by the mucosal and muscular portions of the duodenum, jejunum, ileum and ascending colon, as well as that by mesenteric vessels, was investigated in starved and semistarved rats. The jejunal mucosa and muscularis showed a marked increase in PGI2 synthesis after fasting for 48 h and 72 h or semistarvation for 9 days when compared with controls. Jejunal TXA2 synthesis did not alter. In contrast, PGI2 and TXA2 synthesis in ileal mucosa and muscularis was significantly reduced after fasting for 48 h, 72 h and semistarvation for 9 days. PGI2 and TXA2 synthesis by duodenal and colonic muscularis was unaffected by fasting or semistarvation. PGI2 synthesis in mesenteric vessels was significantly increased by fasting and semistarvation. No changes in PGI2 or TXA2 were detected at 24 h in fasted rats in any of the tissues studied when compared with controls. These selective changes in PGI2/TXA2 secretion may be important mediators of adaptive changes in the small intestine in response to starvation.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Relationship between antipyrine absorption and blood flow rate in rat jejunum, ileum, and colon

The appearance rate of antipyrine in intestinal venous blood was measured in anesthetized rats during perfusion (0.2 ml/min) of a buffered solution with 1 mmol/l labeled antipyrine through a jejunal, ileal, or colonic segment (length: 2-5 cm). When the blood flow rate was increased from 0.9-1.2 to 1.6-2.0 ml min-1 g-1 by raising the systemic blood pressure from 80 to 130 mm Hg, the absorption of antipyrine increased only in the colon. Stepwise reduction of the blood flow rate from 1.4-1.7 to 0.2-0.3 or stepwise raise from 0.2-0.3 to 1.4 ml min-1 g-1 by constriction or release of the mesenteric artery decreased or increased the absorption rate of antipyrine. The relation between absorption and flow rate can be described by curves which ascend at low and level off into a horizontal section at high flow rates. At the same blood flow rate the regional absorption rate decreased in the order jejunum, ileum, and colon with the largest step between ileum and colon. Model analysis yielded the following results for jejunum, ileum, and colon, respectively: permeability-surface area product 0.083, 0.074, and 0.037 ml min-1 g-1; fraction of absorptive site blood flow rate 0.24, 0.19, 0.08. The differences can be attributed mainly to the change of the surface area from jejunum to ileum and colon.

Vasopressin effects on the small intestine: a possible factor in paralytic ileus?

Experiments were performed on luminally perfused, isolated, canine, jejunal segments in vivo to determine the effects of vasopressin on intestinal motility, myoelectrical activity and absorption. Intravenous vasopressin abolished spike activity and sometimes disrupted slow wave activity. The jejunum became atonic with intraluminal pooling of perfusate. Transit time was prolonged and intestinal absorption of water was decreased. Radionuclide imaging of the intestinal contents showed a doubling of intraluminal volume with vasopressin and confirmed the loss of intestinal motility. It is suggested that the high plasma levels of vasopressin which are known to follow laparotomy may be a factor in the development of postoperative ileus.

In vivo studies of mucosal-serosal transfer in rat jejunum

In anesthetized rats, the appearance rates of a series of labeled substances in jejunal venous blood (phi B) and serosal bath (phi S) were measured in vivo (intestinal blood flow rate 1.5 ml min-1 g-1) after intraluminal administration of 0.5 ml buffer solution (initial concentration 1 mmol/1 or 1 GBq/1 tritiated water) into a closed jejunal segment (length 4-5 cm). Between 32% (erythritol) and 93% (salicylic acid) of the administered activity (unchanged substance and possible metabolites) appeared in the intestinal venous blood within 60 min. The fraction recovered from the serosal bath after 15 (60) min was 11 (6)% for tritiated water, 7 (4)% for aniline, 3 (7)% for aminopyrine, 5 (4)% for butanol, 3 (3)% for benzyl alcohol, 2 (4)% for benzylamine, 1-2% for benzoic acid, theophylline, methyl-alpha-D -glucopyranoside, L-lysine, antipyrine, and urea, and less than 1% for L-phenylalanine, D-galactose, erythritol, and salicylic acid. During single pass perfusion of a jejunal segment (length 3-4 cm) the fraction of serosal transfer phi S/(phi B + phi S) was 19% for tritiated water, 4.9% for antipyrine, 0.5% for benzoic acid, and 0.08% for salicylic acid. Distension of the intestinal wall by administration of 1 ml buffer solution instead of 0.5 ml increased the appearance rate of benzoic acid and antipyrine in intestinal venous blood by a factor of 2 and serosal transfer by a factor of approximately 3.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of citalopram in test animals; drug exposure in safety studies

1. Plasma levels of citalopram and its metabolites were assayed after single oral and intravenous doses to baboons (4 mg/kg), dogs (1, 4, 5, and 10 mg/kg), rats (8 mg/kg), and mice (24 mg/kg). Kinetic parameters were estimated. 2. Half-lives were short (estimate for baboon 3, dog 3 1/2-8, rat 3, and mouse 1 1/2 hours) and systemic plasma clearance high (baboon 39, dog 37-14, male rat 82, female rat 103, male mouse 87, and female mouse 116 ml/min/kg body weight). Considerable first-pass metabolism was demonstrated. 3. Drug level data were obtained in long-term safety studies in dogs (1, 3, and 8 mg/kg), rats (32 and 320 mg/kg), and mice (640 mg/kg). The high-dose citalopram level in dogs and rats exceeded that of patients by a factor of 10; the factor for mice was 40.

Enteric transport of chlordecone (Kepone) in the rat

Disposition of chlordecone (Kepone) in the rat is quantitated. Particular attention is devoted to the role of the intestinal tract in excretion, as well as absorption, of the parent form of the halogenated pesticide. A detailed physiological pharmacokinetic model for the GI tract is presented in which the organs are segmented into a series of well-mixed compartments representing stomach, small intestine, cecum, and large intestine. The model is applied to the early time behavior of data from the following two types of studies in the rat: (1) the movement of a nonabsorbable tracer along the GI tract, and (2) the enteric transport of parent chlordecone. Model parameter values for the gut wall permeability-area products for parent chlordecone determined for the rat are used to estimate the corresponding values for man based on scale-up considerations. The enhancement of excretion rates through use of orally administered adsorbents is discussed.