Stomach Secretions (Contd..)
  • When stimulated, parietal cells secrete HCl at a concentration of roughly 160 mM (equivalent to a pH of 0.8).
  • The acid is secreted into large cannaliculi, deep invaginations of the plasma membrane which are continuous with the lumen of the stomach.
  • Hydrogen ion concentration 3,000,000 fold higher than in blood.
  • Ability of the partietal cell to secrete acid is dependent on active transport.
  • Key player is a H+/K+ ATPase or "proton pump" located in the cannalicular membrane.

 

  • The binding of acetylcholine, gastrin and histamine to the receptors, is a very strong stimulus for acid production. Acetylcholine and gastrin increase the intracellular calcium concentration - calcium is neccessary for acid production. Histamine, activates the enyme adenyl cyclase that in turn increases the production of cyclic AMP. Single stimulation by low levels of the 3 compounds, result in a slight increase in acid production, but if the 3 compounds are present concurrently even in low amounts, the acid production increases substantially.

  • Within the parietal cell, water reacts with carbon dioxide to form hydrogen and bicarbonate ions. This reaction is cataylzed by carbonic anhydrase.

  • Bicarbonate is transported out of the basolateral membrane in exchange for chloride.

  • Potassium and chloride ions diffuse into the lumen.
    A hydrogen ion binds on the internal side of the hydrogen/potassium pump, whilst a potassium ion binds to the external side. In the presence of ATP, the pump undergoes a conformational change, and potassium is released inside while hydrogen is released in the lumen.