Luminal Amino Acid-Sensing Cells in Gastric Mucosa

Abstract

Chemosensing of nutrients in the gastrointestinal tract plays physiologically important roles in the regulation of food intake behaviors, including digestion, absorption, metabolism and other subsequently occurring body functions via brain activation. Free amino acids, liberated from ingested foods, are of course essential nutrients which compose the body proteins and sometimes determine the taste of the food. Glutamate, one of the most abundant amino acids in the foods and the liberated free form, critically contributes to the ‘umami’ taste perception. Recently, it has been revealed that dietary glutamate has many beneficial functions in the gastrointestinal tract. However, the precise mechanism of glutamate sensing still remains unclear. Using primary rat gastric mucosal cell cultures, we demonstrated that somatostatin-secreting D cells are candidate cells for glutamate sensing in the stomach through inhibition of somatostatin release. Considering that somatostatin is one of the major negative regulators of gastric functions, it is suggested that some parts of glutamate’s beneficial effects could be explained by suppression of the inhibitory somatostatin effects, i.e. stimulation, by glutamate.

The Macronutrients, Appetite, and Energy Intake

“…Subsequent studies in humans supported these findings (36, 220) In addition to decreased energy intake, self-reports of hunger and desire to eat were significantly lower with high protein compared to a high carbohydrate meals (123). Hypotheses for the increased satiety due to protein included circulating AAs (related to Mellinkoff’s aminostatic theory discussed previously) (184), increased thermogenesis with protein intake (75), sensory specific satiety (264) and stimulation of gut hormone release (192). However, the associations between protein exposure and gut peptide release are weak and inconsistent.…”

Obesity and Diabetes

“…L-glutamate also potentiates gastric secretion via the vagus nerve after binding to glutamate receptors (T1R1/T1R3 and mGluRs) located in gastric glands (Khropycheva et al 2011;Nakamura et al 2011;San Gabriel et al 2007;Uneyama et al 2006). As a result, single AAs from the diet not only add flavor but may also increase protein digestion efficiency.…”

“…I-and L-cells express AA taste receptors, are on direct contact with the intestinal content, and secrete satiating gut peptides like CCK, GLP-1, and PYY. Adapted from Nakamura et al (2011) and Depoortere (2014) depending of meal volume, but also seem to be selective for certain nutrients such as L-glutamate and IMP (San Gabriel and Uneyama 2013;Uneyama et al 2006). Other AAs, glucose, or isotonic saline solutions do not have the ability to trigger vagal afferents from the gastric lumen.…”

Gastric secretion

“…Dietary glutamate stimulates acid secretion. The mechanism may involve inhibition of somatostatin secretion and/ or activation of vagal afferents [30,31].…”