Effect of alpha 1-adrenergic blockade on canine ileal water, electrolyte, and glucose absorption

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.

Animal and human models for studying effects of drugs on intestinal fluid transport in vivo

The understanding of the physiology and pathophysiology of intestinal fluid transport has been derived from animal and human models of normal and perturbed intestines. This understanding helped in designing drugs and changing the composition of oral rehydration solutions in a targeted manner to affect intestinal fluid absorption/secretion that was tested both in vitro and in vivo before embarking on clinical trials. In this review, in vivo techniques used to study water transport in both animal and human models are described. In particular, steady state intestinal perfusion techniques, closed segment techniques, fistulous animal models, balance study models, enteropooling models, and isotope tracer models are reviewed. Advantages and drawbacks of each technique and examples where drug effects have been studied in a particular model are provided.