S cell

[1] They are stimulated by a drop in pH to 4 or below in the small intestine's lumen.

The released secretin will increase the secretion of bicarbonate (HCO3−) into the lumen, via the pancreas.

This is primarily accomplished by an increase in cyclic AMP that activates CFTR to release chloride anions into the lumen.

The luminal Cl− is then involved in a bicarbonate transporter protein exchange, in which the chloride is reabsorbed by the cell and HCO3− is secreted into the lumen.

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