The main signal in the small intestine lumen that results in increased pancreatic bicarbonate ion (hco3-) secretion is:

The main signal in small intestine for pancreatic bicarbonate ion secretion is Secretin.

When food from the stomach with acidic medium enters duodenum (first part of small intestine) it stimulates the lining of duodenum cells to release the hormone secretin.

This hormone travels via bloodstream to pancreas and stimulate to it to release bicarbonate ions. Now these ions enter duodenum and neutralise the acidic medium.

This results in the optimum pH for the enzymatic action to take place thereby promoting final digestion of protein.