S cells orSecretin cells are cells which releasesecretin, found in thejejunum andduodenum.^[1] They are stimulated by a drop inpH to 4 or below in thesmall intestine's lumen. The released secretin will increase the secretion ofbicarbonate (HCO₃⁻) into the lumen, via thepancreas. This is primarily accomplished by an increase incyclic AMP that activatesCFTR to release chloride anions into the lumen. The luminal Cl⁻ is then involved in abicarbonate transporter protein exchange, in which the chloride is reabsorbed by the cell and HCO₃⁻ is secreted into thelumen. S cells are also one of the main producers ofcyclosamatin.

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