Effect of different beta-adrenergic agonists on the intestinal absorption of galactose and phenylalanine

Preclinical models for colonic absorption, application to controlled release formulation development

Oral controlled release (CR) formulations have many benefits and have become a valuable resource for the local and systemic administration of drugs. The most important characteristic of these pharmaceutical products is that drug absorption occurs mainly in the colon. Therefore, this review analyses the physiological and physicochemical features that may affect an orally administered CR product, as well as the different strategies to develop a CR dosage form and the methods used to evaluate the formulation efficacy. The models available to study the intestinal permeability and their applicability to colonic permeability determinations are also discussed.

Neural regulation of intestinal nutrient absorption

The nervous system and the gastrointestinal (GI) tract share several common features including reciprocal interconnections and several neurotransmitters and peptides known as gut peptides, neuropeptides or hormones. The processes of digestion, secretion of digestive enzymes and then absorption are regulated by the neuro-endocrine system. Luminal glucose enhances its own absorption through a neuronal reflex that involves capsaicin sensitive primary afferent (CSPA) fibres. Absorbed glucose stimulates insulin release that activates hepatoenteric neural pathways leading to an increase in the expression of glucose transporters. Adrenergic innervation increases glucose absorption through α1 and β receptors and decreases absorption through activation of α2 receptors. The vagus nerve plays an important role in the regulation of diurnal variation in transporter expression and in anticipation to food intake. Vagal CSPAs exert tonic inhibitory effects on amino acid absorption. It also plays an important role in the mediation of the inhibitory effect of intestinal amino acids on their own absorption at the level of proximal or distal segment. However, chronic extrinsic denervation leads to a decrease in intestinal amino acid absorption. Conversely, adrenergic agonists as well as activation of CSPA fibres enhance peptides uptake through the peptide transporter PEPT1. Finally, intestinal innervation plays a minimal role in the absorption of fat digestion products. Intestinal absorption of nutrients is a basic vital mechanism that depends essentially on the function of intestinal mucosa. However, intrinsic and extrinsic neural mechanisms that rely on several redundant loops are involved in immediate and long-term control of the outcome of intestinal function.

Epithelial distribution of neural receptors in the guinea pig small intestine

Neural and paracrine agents, such as dopamine, epinephrine, and histamine, affect intestinal epithelial function, but it is unclear if these agents act on receptors directly at the enterocyte level. The cellular localization and villus-crypt distribution of adrenergic, dopamine, and histamine receptors within the intestinal epithelium is obscure and needs to be identified. Single cell populations of villus or crypt epithelial cells were isolated from the jejunum of adult guinea pigs. Enterocytes were separated from intraepithelial lymphocytes by flow cytometry and specific binding was determined using fluorescent probes. α1-adrenergic receptors were located on villus and crypt intraepithelial lymphocytes and enterocytes. β-adrenergic receptors were found on villus and crypt enterocytes. Dopamine receptors were found on all cell types examined, whereas histamine receptors were not detected (<10% for each cell population). These studies demonstrated that (1) receptors for epinephrine and dopamine exist on epithelial cells of the guinea pig jejunum, (2) β-adrenergic receptors are found primarily on villus and crypt enterocytes and (3) intraepithelial lymphocytes contain α1-adrenergic, but have few β-adrenergic, receptors. The presence of neural receptors suggests that these agents are acting, at least in part, at the enterocyte or intraepithelial lymphocyte levels to modulate intestinal and immune function.Key words: enterocyte, receptor, intestine, epithelium.

Cytoskeleton involvement on intestinal absorption processes

It has been recently demonstrated in the laboratory that the cytoskeletal inhibitor cytochalasin E has an indirect inhibitory effect on the function of the intestinal Na+-sugar cotransporter (SGLT1). The present work confirms that cytochalasin E inhibits SGLT1 activity through cytoskeleton disruption, showing that in anaerobic conditions (N2 bubbling), which implies low cytosolic ATP levels, the inhibition is not observed. As it occurs in sugar transport, the Na+-dependent intestinal transport of phenylalanine decreases if cytochalasin E is present in the incubation medium. However, the activity of the brush border enzymes sucrase, amino peptidase N and gamma-glutamyl transferase is not affected by the inhibitor. These enzymes only have one transmembrane domain and the active center is projected to the intestinal lumen. Therefore, cytoskeleton changes that could modify the transmembrane enzyme segment do not alter the activity of these enzymes. Examination of the intestine morphology after 30 min incubation with cytochalasin E shows only light modifications which do not seem to explain the inhibitory effects of the toxin on Na+-sugar or Na+-phenylalanine cotransporters function. On the whole, these results indicate that the inhibition of cytochalasin E on galactose and phenylalanine intestinal transport is secondary to its action on cytoskeleton through protein structure modifications.