Gastric acid is a digestive fluid that is produced in the corpus and antrum zones of the stomach. The gastric acid has a pH of 1.5–3.5 and is composed of hydrochloric acid (HCl) and NaCl and KCl salts ~7 mM with only a trace of other electrolytes. The concentration of hydrogen ions is ~1 million times higher (pH ~1) than that of plasma. The function of gastric acid is to denature the organized three-dimensional properties of proteins that were included in a recent meal. The HCl also participates in solubilizing and absorbing Ca2+, Fe2+, and also vitamin B12.

Figure 1 provides a description of how human parietal cells in the stomach are able to secrete a strong concentration of hydrochloric acid (HCl), thereby acidifying the stomach to a pH of 1.5–3.5 which facilitates the digestion of proteins. The model shown in Figure 1A illustrates how the stomach’s D and G cells collaborate to secrete the gastrin hormone and have it available in the capillary bed which supplies the stomach’s fundus where there are gastrin receptors. Panel B of the figure illustrates the parietal cell’s plasma mem brane receptor (CCK2R); it is specific for gastrin.

Fig1. Integrated signal transduction by D cells, G cells, enterochromaffin-like (ECL) cells, and parietal cells in the antrum and fundus of the stomach, which results in the secretion of hydrochloric acid in the stomach so as to facilitate food denaturation. The left side of both panels A and B illustrates the various regions of the stomach. At the bottom of the antrum (left of panel A) and the bottom of the fundus (top left of panel B) there is a small black box. Each box identifies the zone of cells that are enlarged in the center of both panels A and B. In the center panel of this figure a second pair of black boxes identifies the single cells illustrated in the center and right sides of panels A and B, respectively. In the center of both right panels a capillary bed is illustrated in red and blue. In the right side of panel A, the combined partnership of the D and G cells’ gastrin releasing peptide (GRP) regulates the production in the G cell of the hormone gastrin (blue balls) which is released to the capillary bed for transport to the parietal cells in the stomach’s fundus region (see bottom right of panel B). The antrum’s receptor for the G cell’s gastrin releasing protein (GRP; see upper right panel) is present in the plasma membrane of D cells (at the top) and G cells (at the bottom). The GRP receptor binds the GRP hormone (small orange balls) as a ligand in response to a GRP nerve signal (see upper right panel B). As shown in the bottom right panel, the fundus’ oxyntic (acid secreting) parietal cells and the ECL cells receive the hormone gastrin from the capillary bed (small blue balls) which binds to their CCK2R plasma membrane receptor. This ligand-receptor action results in the production and secretion of essential hydrochloric acid. Following is a key to the abbreviations employed in this figure: G cells are a source of the endocrine hormone, gastrin; D cells are a source of the hormone somatostatin; ECL are enterochromaffin-like cells that are a type of neuroendocrine cells found in the stomach’s antrum in the vicinity of parietal cells; parietal cells are a source of hydrochloric acid which is secreted into the stomach; CCK2R is a plasma membrane receptor for gastrin; SSTR2 is a plasma membrane receptor for somastostatin; Ach is messenger produced by a postganglion cholinergic muscarinic nerve; histamine is an endocrine messenger that is derived from the amino acid histidine.

Figure 1B also illustrates three stimulants necessary to orchestrate the production of HCl which is secreted inside the stomach. The stimulants are gastrin, acetyl choline (Ach; a neurocrine messenger), and histamine (a paracrine messenger from an adjacent stomach cell). Histamine (orange balls) (see panel B) is released from the enterochromaffin-like (ECL) cells followed by binding to its HRH2 receptors on the surface of parietal cells and also on the plasma membrane of the D cell. This activates the adenylate kinase that generates the second messenger, cAMP. The GI hormone, gastrin (blue balls) is produced in the G cells (see panel A) and then binds to its CCK2R receptors also associated with the partner D cell (panel A). In addition, the gastrin, after transport through the capillary bed, arrives and enters (see panel B) in both the ECL and parietal cells of the fundus cells. The gastrin then binds to the CCK2R gastrin receptor that is present in the plasma membrane of the parietal cells and also the ECL cells. As a consequence, the parietal cells engage in the production of hydrochloric acid. Separately, the ECL cells initiate a release of histamine (orange balls) that moves to the HRH2 plasma membrane receptor of both the parietal cell and D cells, thereby activating phospholipase C to mediate release of cytosolic Ca2+. Also, acetylcholine (Ach) that has been released from neurons binds to the M3 plasma membrane receptor (see Figure 1B) on both the parietal cells and D cells that are linked to stimulating intracellular Ca2+. Thus in the parietal cells, the combination of calcium-dependent and intracellular cAMP signaling systems stimulates downstream protein kinases that lead to stimulation of H+K+-ATPase proton pumps, resulting in the secretion of HCl into the stomach cavity.

The G cell with microvilli on the luminal surface is a classic gut endocrine cell. The G cell responds to the presence of food within the stomach. The G cell is the principal site of synthesis of the hormone gastrin. A typical gland is shown in the small black box in the upper middle panel of Figure 1A. The antrum and fundus have a receptor for the G cell’s gastrin releasing protein (GRP; see upper right panel) that is present in the plasma membrane of both G cells and D cells. The GRP receptor is located in the plasma membrane where it binds the extracellular GRP hormone (small yellow balls) as a ligand in response to a GRP nerve signal (see upper right panel). The gastrin-releasing peptide (GRP), which is present in the neuronal fibers innervating the antral mucosa, functions to stimulate gastrin secretion by acting as a secretagogue of gastrin.

As shown in the lower right panel the parietal cell can be further stimulated by the genomic expression actions of gastrin on the production of histidine decarboxylase, which has been shown to stimulate the production of the histamine. See the binding of histamine (orange-colored balls) to the parietal cell’s plasma membrane receptor HRH2 for gastrin, which further stimulates the secretion of hydrochloric acid.

The hormone somatostatin is also involved in the D cells of both the antrum (upper right panel) and the oxyntic cells present in the fundus (bottom right panel). Both the antrum’s and fundus’s D cell somatostatin has been shown to modulate the gene transcription of gastrin so as to secrete more or less gastrin by adjacent G cells, depending on the physiological circumstances. As shown in Figure 1A/B, somatostatin (orange circles) is secreted from the D cells and binds to the SSTR2 receptor for somatostatin in the basal lateral membranes of the adjacent G cells (upper right panel) or adjacent parietal cells (lower right panel). In both instances, the presence of somatostatin in the G cells or parietal cells creates an inhibitory effect. In the parietal cells, less HCl is secreted (lower right panel) and in the G cells (upper right panel) less gastrin is produced and available for export by secretion and transport through the capillary bed. The GRP nerve generates in the D cells of both the antrum (upper right panel) and fundus (bottom right panel) the production of the gastrin releasing peptide hormone (GRP; yellow balls), which binds to the plasma membrane receptor for GRP (see 1 yellow ball), thus leading to the immediate secretion of somatostatin, which will inhibit the secretion of gastrin (upper right panel) and inhibit the production of HCl by the parietal cells (bottom right panel).

Gastrin is synthesized as a large precursor molecule of 101 amino acids that is relatively slowly processed to gas trin-34, which then has a much longer half-life of 42 minutes as compared to gastrin-17, which has a half-life of only 5 minutes in the circulatory system; see Figure 2.

Fig2. The circulating half-lives of the blood levels of 15 gastrointestinal peptide hormones are compared by their hormone family. The gut hormones are generally released in response to intake of meals. This figure illustrates the circulating half-life for 15 GI peptide hormones from three GI families (the gastrin-CCK, secretin, and pancreatic polypeptide families) ranging from <1 minute to an average of 3.8 minutes with only the gastric inhibitory peptide (GIP) and Gastrin-34 (Big Gastrin) having a half-life significantly longer than 10 minutes.

For the 15 GI peptides in this figure, only two pep tides had a half-life less than 10 minutes and only gas trin-34 and GIP had half-lives greater than 10 minutes (42 min and 20 min, respectively). Thus, evolutionarily it appears to have been determined that there is an advantage to a short half-life over longer GI peptide half-lives. It has been postulated that the shorter half-life is safer. If gastrin-34 hormone had a long half-life, it could be producing large quantities of HCl even when there was little food intake to process.

اخر الاخبار

اخبار العتبة العباسية المقدسة

جناح العتبة العباسية المقدسة في معرض طهران الدولي يشهد فعاليات متنوِّعة

شعبة مدارس الكفيل تطلق حملتها السنوية لدعم العوائل المتعففة في شهر رمضان المبارك

بأجواء رمضانية مميزة.. مجموعة مشاتل الكفيل تستقبل العوائل العراقية يوميًا

المجمع العلمي: الختمة القرآنية الرمضانية المركزية المرتلة تشهد تنوعًا في القراءات والمشاركات

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Cholecystokinin/Gastrin Family

Problems of Food Processing and Digestion

General Principles of Gastrointestinal Motility

امتصاص نواتج الهضم الميكروبي

الهضم الميكروبي

أمتصاص الاملاح والماء

امتصاص الفيتامينات

هضم وامتصاص الأحماض النووية

هضم وامتصاص الدهون

هضم وامتصاص البروتين

هضم وامتصاص السكريات

هضم المكونات الغذائية