Positional dependence of enterocyte membrane potential in hamster and rabbit enterocytes

Meta-analysis of the turnover of intestinal epithelia in preclinical animal species and humans

Due to the rapid turnover of the small intestinal epithelia, the rate at which enterocyte renewal occurs plays an important role in determining the level of drug-metabolizing enzymes in the gut wall. Current physiologically based pharmacokinetic (PBPK) models consider enzyme and enterocyte recovery as a lumped first-order rate. An assessment of enterocyte turnover would enable enzyme and enterocyte renewal to be modeled more mechanistically. A literature review together with statistical analysis was employed to establish enterocyte turnover in human and preclinical species. A total of 85 studies was identified reporting enterocyte turnover in 1602 subjects in six species. In mice, the geometric weighted combined mean (WX) enterocyte turnover was 2.81 ± 1.14 days (n = 169). In rats, the weighted arithmetic mean enterocyte turnover was determined to be 2.37 days (n = 501). Humans exhibited a geometric WX enterocyte turnover of 3.48 ± 1.55 days for the gastrointestinal epithelia (n = 265), displaying comparable turnover to that of cytochrome P450 enzymes in vitro (0.96-4.33 days). Statistical analysis indicated humans to display longer enterocyte turnover as compared with preclinical species. Extracted data were too sparse to support regional differences in small intestinal enterocyte turnover in humans despite being indicated in mice. The utilization of enterocyte turnover data, together with in vitro enzyme turnover in PBPK modeling, may improve the predictions of metabolic drug-drug interactions dependent on enzyme turnover (e.g., mechanism-based inhibition and enzyme induction) as well as absorption of nanoparticle delivery systems and intestinal metabolism in special populations exhibiting altered enterocyte turnover.

Intestinal transport in megacolonic mice. Alterations in sugar absorption

This study examined jejunal sugar absorption in piebald mice with congenital megacolon and in normal littermates. Transmural potential difference, short-circuit current, and tissue conductance of flat sheets of jejunum set up in flux chambers were significantly greater in the diseased mice compared to normal siblings. In piebald mice, net absorption of 3-o-methylglucose was enhanced due to a significant increase in mucosal-to-serosal flux compared with normal littermates. Stimulation of electrogenic sodium absorption by alanine (10 mM) increased basal short-circuit currents more in piebald tissues than in tissues from normal mice, whereas stimulation of ion transport by carbachol (10 microM) evoked an increase in short-circuit current that was similar in the two groups. Alterations in intestinal mass, morphology, or Na+, K+-ATPase activity could not account for the increase in absorptive function characteristic of piebald mice.

Glucose depolarizes villous but not crypt cell apical membrane potential difference: a micropuncture study of crypt-villus heterogeneity in the rat

Brush-border membrane potentials and fractional resistances have been recorded from enterocytes at different points along the crypt-villus axis of rat ileum in vitro. Microelectrode impalements were obtained under visual control and brush-border membrane potentials were higher in crypt than in villous cells (-57 +/- 1.6 against -50 +/- 1.6 mV referred to the mucosal side). Replacing mannitol with D-glucose in the mucosal perfusate resulted in a rise in transmural potential difference (0.5 +/- 0.17 to 1.0 +/- 0.21 mV (n = 37)) and apical membrane potential was depolarized. This occurred consistently only in the upper two-thirds of the villus (-54 +/- 1.7 to -47 +/- 2.3 mV (n = 17)) and not in crypt cells (-56 +/- 2.6 to -57 +/- 2.4 mV (n = 10) or at the crypt-villus junction. The glucose-induced apical membrane depolarization in villous enterocytes was blocked by phlorizin, a competitive inhibitor of sodium-dependent glucose uptake (-50 +/- 2.1 to -53 +/- 2.8 mV (n = 9) in the presence of phlorizin and glucose). Transmural resistance, Rt, and fractional resistance, FR, were unaltered by glucose (61 +/- 3.4 to 61 +/- 3.5 omega X cm2 (n = 50] and (0.60 +/- 0.06 to 0.57 +/- 0.06 (n = 17]. This micro-puncture technique provides direct evidence for functional differentiation along the crypt-villus axis and indicates that active electrogenic accumulation of glucose is confined to villous epithelium.

Starvation-induced changes in the autoradiographic localisation of valine uptake by rat small intestine

We report here the effects of a 72-h fast on the localisation of Na-dependent [3H]-valine uptake by rat small intestine. Starvation results in the earlier appearance of valine transport during cell migration and an enhanced accumulation of the amino acid at the villus tip.

Intracellular potassium as a possible inducer of amino acid transport across hamster jejunal enterocytes

Brush border membrane potentials (Vm), intracellular K+ activity (aiK) and alanine uptake were measured in different parts of villi in mid-jejunal tissue taken from hamsters fed different amounts of food at high and low environmental temperatures. Basal villus enterocytes (Y cells) were found to have lower values for Vm and aiK than upper villus enterocytes (O cells). Alanine uptake was confined to O cells. The position on the villus where values for Vm and aiK changed, and where alanine uptake could first be seen to take place, depended on the energy intake and environmental temperature at which hamsters were kept. Na+-dependent alanine uptake and Vm were both higher in O cells of hamsters fed 10 kcal day-1 at 30 degrees C (10 k/30 degrees C) compared with animals fed 30 kcal day-1 at an environmental temperature of 12 degrees C (30 k/12 degrees C hamsters). The rates at which enterocytes migrated along the crypt-villus axis, measured separately in thymidine-labelling experiments, were 6.9 and 16.1 micron h-1 for 10 k/30 degrees C and 30 k/12 degrees C hamsters respectively. Both Vm and aiK were estimated, from these measurements, to have increased significantly by the time enterocytes became 30 h old. Alanine uptake began 15 h later. Neither of these parameters were influenced by previous adaptation conditions. The close temporal and variable positional relationship found between changes in aiK and onset of transport suggests that early changes in electrolyte composition might initiate a second phase of development enabling the enterocyte to absorb nutrients. The possibility that other ions besides K+ might also be involved in this aspect of regulation is also discussed.