Novel ion exchangers/transporters for constipation in lower functional gastrointestinal disorders (FGIDs). DRA, downregulated in adenoma; ENaC, epithelial Na+ channel; NHE3, Na+/H+ exchanger 3; PAT-1, putative anion transporter-1; SAACTs, sodium-amino acid cotransporters; SGLTs, sodium-glucose cotransporters; SLC26A3, solute carrier family 26 member 3.