Pepsin and Hydrochloric Acid Secretion

The parietal cells secrete H+, at a pH as low as 0.8, into the gastric lumen by primary active transport (involving carriers that function as an ATPase). These carriers, known as H+/K+ ATPase pumps, transport H+ uphill against a million-to-one concentration gradient into the lumen of the stomach while they transport K+ in the opposite direction (fig. 18.8).

At the same time, the parietal cell's basolateral membrane (facing the blood in capillaries of the lamina propria) take in Cl-against its electrochemical gradient by coupling its transport to

Chief Cells Parietal Cells

■ Figure 18.7 Gastric pits and gastric glands of the mucosa.

(a) Gastric pits are the openings of the gastric glands. (b) Gastric glands consist of several types of cells (including mucous cells, chief cells, and parietal cells), each of which produces a specific secretion.

the downhill movement of bicarbonate (HCO3-). The bicarbonate ion is produced within the parietal cell by the dissociation of carbonic acid, formed from CO2 and H2O by the enzyme carbonic anhydrase. Therefore, the parietal cell can secrete Cl- (by facilitative diffusion) as well as H+ into the gastric juice while it secretes bicarbonate into the blood (fig. 18.8).

The secretion of HCl by the parietal cells is stimulated by a variety of factors, including the hormone gastrin, secreted by the G cells, and acetylcholine (ACh), released by axons of the vagus nerve. Most of the effects of gastrin and ACh on acid secretion, however, are currently believed to be indirect. Gastrin and ACh from vagal axons stimulate the release of histamine from the ECL cells of the gastric mucosa, and hista-mine, in turn, acts as a paracrine regulator to stimulate the parietal cells to secrete HCl (see fig. 18.30). The endocrine regulation of the digestive system is discussed in detail later in this chapter.

People with gastroesophageal reflux disease, a common disorder involving the reflux of acidic gastric juice into the esophagus, are often treated with specific drugs (including Prilosec and Prevacid) that inhibit the K+/H+ pumps in the gastric mucosa. Since gastric acid secretion is stimulated by histamine released from the ECL cells, people with peptic ulcers can be treated with drugs that block histamine action. Drugs in this category, such as Tagamet and Zantac, specifically block the H2 histamine receptors in the gastric mucosa. This is a different receptor subtype than that blocked by antihistamines commonly used to treat cold and allergy symptoms.

The only stomach function that appears to be essential ■J for life is the secretion of intrinsic factor. This polypep-

^___ 1 ^ tide is needed for the absorption of vitamin Bu in the terminal portion of the ileum in the small intestine, and vitamin B|2 is required for maturation of red blood cells in the bone marrow. Following surgical removal of the stomach (gastrectomy), a patient has to receive B|2 injections or take B|2 orally, together with intrinsic factor. Without vitamin B|2, pernicious anemia will develop.

© The McGraw-Hill Companies, 2003

The Digestive System

Facilitative diffusion

Lumen of stomach

Facilitative diffusion

Primary active transport

(ATPase carrier)

Primary active transport

(ATPase carrier)

Parietal cell -

H2CO

Secondary active transport HCO

Blood capillaries

■ Figure 18.8 Secretion of gastric acid by parietal cells. The apical membrane (facing the lumen) secretes H+ in exchange for K+ using a primary active transport carrier that is powered by the hydrolysis of ATP. The basolateral membrane (facing the blood) secretes bicarbonate (HCO3-) in exchange for Cl-. The Cl- moves into the cell against its electrochemical gradient, powered by the downhill movement of HCO3- out of the cell. This HCO3- is produced by the dissociation of carbonic acid (H2CO3), which is formed from CO2 and H2O by the action of the enzyme carbonic anhydrase (abbreviated CA). The Cl- then leaves the apical portion of the membrane by diffusion through a membrane channel. The parietal cells thus secrete HCl into the stomach lumen as they secrete HCO3- into the blood.

Lumen of stomach

Lumen of stomach

Lumen Stomach Pepsin

■ Figure 18.9 The activation of pepsin. The gastric mucosa secretes the inactive enzyme pepsinogen and hydrochloric acid (HCl). In the presence of HCl, the active enzyme pepsin is produced. Pepsin digests proteins into shorter polypeptides.

The high concentration of HCl from the parietal cells makes gastric juice very acidic, with a pH of less than 2. This strong acidity serves three functions:

1. Ingested proteins are denatured at low pH—that is, their tertiary structure (chapter 2) is altered so that they become more digestible.

2. Under acidic conditions, weak pepsinogen enzymes partially digest each other—this frees the fully active pepsin enzyme as small inhibitory fragments are removed (fig. 18.9).

3. Pepsin is more active under acidic conditions—it has a pH optimum (chapter 4) of about 2.0.

As a result of the activation of pepsin under acidic conditions, the fully active pepsin is able to catalyze the hydrolysis of peptide bonds in the ingested protein. Thus, the cooperative activities of pepsin and HCl permit the partial digestion of food protein in the stomach.

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Responses

  • Morgan Bruce
    Which stomach cells produce HCl and which produce enzymes?
    8 years ago
  • semrawit
    How is cl transported to the gastric pit?
    8 years ago
  • john hall
    Which is more acidic, hcl or pepsin?
    8 years ago
  • JAMES
    Why is pepsin able to function in a ph?
    8 years ago
  • emaan burns
    How is the secretion of hydrochloric acid and pepsin regulated?
    8 years ago
  • thomas
    Is pepsin inactive in the presence of hydrochloric acid?
    7 years ago
  • shishay
    Why pepsin digests HCl?
    8 years ago
  • lukas
    How do the parietal cells secrete hco3 into stomach lumen?
    8 years ago
  • Tombur
    How gastrin stimulate parietal cell to secrete hydrochloric acid?
    8 years ago
  • aleisha
    What secretes pepsin and hydrochloric acid?
    8 years ago
  • linn adler
    What cells secrete hco3 and under what pH conditions?
    8 years ago
  • mungo
    Which cells in human stomack secrete hydrocholoric acid?
    8 years ago
  • Asmeret
    Why is pepsin secreted at the same time as hcl?
    8 years ago
  • Robert Villalobos
    Why is pepsin active in acidic conditions?
    7 years ago
  • conner alexander
    How is pepsin and hco3 related in the stomach?
    7 years ago
  • tanta
    Where is pepsinogen secreted pepsinogen?
    7 years ago
  • LEMLEM
    Where are HCl and HCO3 secreted?
    7 years ago
  • RITA
    What cells secrete pepsin?
    7 years ago
  • Settimo
    Which cells produce HCI and Pepin?
    7 years ago
  • germana
    Does gastrin stimulate chief cells to secrete pepsin?
    7 years ago
  • sarama
    Which are the pepsin and hyrochloric located in the stomach?
    7 years ago
  • Anne Bayer
    Does prevacid block pepsin?
    7 years ago
  • matti
    What drug suppresses secretion of HCL in the lumen of the stomach?
    7 years ago
  • rahel
    Which cell scretes HCl acid?
    7 years ago
  • Amanda
    Which cells in the mucosa secrete pepsinogen?
    7 years ago
  • ciara mcdonald
    How does gastrin, acetycholine and histamin regulate HCL formation in the gastric pits?
    7 years ago
  • Andreas
    Do the cells in the mucosa of the stomach produce pepsinogen?
    7 years ago
  • consolata
    What cells produce pepsin acid?
    7 years ago
  • Lemlem
    Do melanocytes secrete hydochloric acid?
    7 years ago
  • Segan
    What cells secrete pepsinogen into the lumen of the stomach?
    7 years ago
  • sarah
    What duodenal cell secretes hco3?
    7 years ago
  • andrew christen
    What type of cell in the stomach mucosa that produces pepsin is the?
    6 years ago
  • saimi
    How much bicarbonate is produced by parietal cells?
    6 years ago
  • Jason
    What hormone produces hydrochloric acid and pepsin?
    6 years ago
  • Anna
    What stomach cells produce pepsin?
    6 years ago
  • simone
    Which stomach cell secretes pepsin?
    6 years ago
  • connie
    When is hydrochloric acid secreted to activate pepsin?
    6 years ago
  • HEIKE
    Why does pepsin remain inactive in lumen?
    6 years ago
  • Michael
    What inhibitits its secreation of acid by parietal cell?
    6 years ago
  • victor
    Why do chief cells secret pepsinogen instead of pepsin?
    5 years ago
  • serena
    How is pepsin and acid secretion regulated in digestion?
    5 years ago
  • christin theiss
    How do hydrocloric acid, pepsin and bicarbonate relate in the stomachj of humans?
    5 years ago
  • arcangelo
    Why is pepsin secreted as pepsinogen?
    5 years ago
  • sebastian mayer
    Where sructures are hydrochloric acid and pepsin released through?
    4 years ago
  • Berenice
    Does hydrochloric acid stimulate the activation of pepsin?
    4 years ago
  • louie
    How does pepsin secrete its secretion?
    4 years ago
  • Caio Siciliani
    Does pepsin stimulate the secretion of hydrochloric acid?
    4 years ago
  • Emma
    Does pepsin secrete acid?
    4 years ago
  • Roope
    Which cell secrete hcl and pepcin?
    4 years ago
  • Jari
    What hormes does hcl acid activate?
    4 years ago
  • KIRSTI BERGLUND
    Which of the following stimulates the secretion of hydrochloric acid (hcl) by the parietal cells?
    3 years ago
  • Marie
    Why hcl is indispensible in pepsin?
    3 years ago
  • maximilian kaufmann
    What factors control the release of pepsin and hcl in the stomach?
    3 years ago
  • LUCREZIA
    Why are pepsinogen and hydrochloric acid secreted by the same structure in the body?
    3 years ago
  • Alem
    Why is pepsin released as pepsinogen?
    3 years ago
  • Robinia Brown
    Which cells secerete hcl acid in human body?
    3 years ago
  • anja
    What are the factors that stimulate hydrochloric acid stimulation?
    3 years ago
  • Yonas
    How to activate pepsin through drugs?
    3 years ago
  • Ulrike
    Which cell in the stomach secretehydrogen chloride and pepsinogen?
    3 years ago
  • courtney
    What are the factors that stimulate the secretion of hydrochloric acid in the stomach?
    3 years ago
  • aristide
    Who is the secrate Pepsine?
    3 years ago
  • margaret
    Which cell in the stomach secrete hcl and pepsinogen?
    3 years ago
  • bucca tunnelly
    What is the point of pepsin and hcl secretion in the body?
    3 years ago
  • krystal
    How does pepsin contribute in acid secretion?
    3 years ago
  • Lora
    Why is the significance of secreting inactive pepsinogen from mucosal cell insted of pepsin?
    3 years ago
  • peter yates
    How is secretion of hydrochloric acid and pepsin regualted?
    3 years ago
  • abelardo
    Which chemical secrets pepsin and hcl in stomach?
    2 years ago
  • mungo
    WHAT hormone stimulates the secretion of hydrochloric acid?
    2 years ago
  • Larissa
    How much hcl is needed to activate pepsin?
    2 years ago
  • Tytti Harila
    Which hormone secret HCl in stomach?
    2 years ago
  • savanna
    What is the category of drug by hydrochloric acid secretion is blocked?
    2 years ago
  • durante
    What is the gland which excretes hormone known as Pepsine?
    2 years ago
  • menegilda greenhand
    What is physiology of acid pepsin secretion?
    2 years ago
  • hope
    Which organ secretes hydrochloric acid and pepsin?
    2 years ago
  • amaranth mugwort
    How HCI and pepsin produced in stomach?
    2 years ago
  • Sayid
    What organ secretes pepsinogen?
    2 years ago
  • christina
    Why is pepsin produced as pepsinogen?
    2 years ago
  • Letizia
    How the inactive pepsinogen active in presence of HCl?
    2 years ago
  • Amina Afwerki
    How histamine stimulates HCL secrtion?
    2 years ago
  • Damian Hay
    What organ in the human secretes pepsin?
    2 years ago
  • maxima bracegirdle
    Which gland extracted HCl in stomach?
    2 years ago
  • ALINA
    What is the one cell theory of gastric acid secretion in the body?
    2 years ago
  • Allan McLean
    Where is hydrochloric acid produced?
    2 years ago
  • birikti
    What stimulates pepsin production?
    2 years ago
  • CHRISTOPHER
    Which gastric cell secretes pepsin & HCl?
    2 years ago
  • Tolman
    Do parietal cells of the stomach mucosa secrete pepsin?
    2 years ago
  • Donnamira
    Which cell secretes the hormone that promotes production of hydrochloric acid?
    2 years ago
  • Mary
    How Hcl is regulated in stomach?
    2 years ago
  • brogan
    What are the factors stimulating and inhibiting the secretion of hydrochloric?
    2 years ago
  • Kisanet
    What is the factor that affect hcl secretion?
    2 years ago
  • ortensia
    What controls release of hcl?
    2 years ago
  • arianna
    Which hormone controls the production of hydrochloric acid and the release of pepsin?
    2 years ago
  • ulrich
    Which gland is responsible for secretion of pepsin?
    2 years ago
  • Yonatan
    Does liver secrete pepsin?
    2 years ago
  • Negisti
    Where and what releases pepsin?
    2 years ago
  • berylla
    How potential cell secrete hcl?
    2 years ago
  • gorbadoc
    Which gland secretes pepsin enzyme?
    1 year ago
  • paavo
    What is secreted from the gastric pits?
    1 year ago
  • ellena
    What stimulates the secretion of hydrochloric acid (hcl) in the stomach?
    1 year ago
  • Luke
    What secretes pespin in the stomach?
    1 year ago
  • Savanna
    Which cell produce hcl propepsin?
    1 year ago
  • Iggi
    What cells in the gastric pits will secrete hydrochloric acid?
    1 year ago
  • juliane
    What cells produce mucus and hydrochloric acid?
    1 year ago
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    How parietal cell secretes acid quizlet?
    1 year ago
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    Why does the prostate contain pepsinogen?
    1 year ago
  • cody
    How is pepesin produced and regulated?
    1 year ago
  • philipp
    Do parietal cells release Hco3 into ileum?
    1 year ago
  • tewolde
    What are the glands responsible for HCL and Pepsin?
    1 year ago
  • Landro Milani
    Where are secret hcl in body?
    1 year ago
  • pamphila
    What influences hydrochloric acid secretion?
    1 year ago
  • selam
    Does histamine stimulates mucus cells to secrete?
    11 months ago
  • osman
    How does the organelles of a peptic cell in the stomach cooperate to produce and secrete pepsinogen?
    11 months ago
  • rosalia
    How hclsecretions are controlled and whichsubstances stimulate hcl secretions?
    11 months ago
  • ETTORE
    When does the stomach secrete hydrocloric acid and pepsin?
    11 months ago
  • Rhoda
    What halts pepsin secretion?
    11 months ago
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    What gland/cell secretes hcl acid?
    9 months ago
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    When does maximum secretion of hydrochloric acid and pepsinogen in the stomach occur?
    8 months ago
  • Haiduc
    What the effects of low pepsin secretion in human body?
    8 months ago
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    Why do parietal cells release hydrochloric acid?
    8 months ago
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    How much pepsinogen does the human body produce?
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    Which of the following organs secretes gastrin?
    7 months ago
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    How does histamine stimulate the release of hcl in stomach?
    7 months ago
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    How gastrin stimulates the hcl and pepsin secretion?
    7 months ago
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    How hcl secretion regulated by calcium?
    6 months ago
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    5 months ago
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    4 months ago
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    4 months ago
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    3 months ago
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    3 months ago
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    What does the pepsin and hydrochloric acid produce?
    2 months ago
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    What hormone stimulates the parietal cells to release HCl?
    1 month ago
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    Which of the following is the hormone that stimulates secretion of gastric juice and HCL?
    14 days ago
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    Where are gastric acid and pepsin secreted from?
    6 days ago

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