GIP and pancreatic insulinoma: Although GIP(1-30)NH2 is as potent as GIP(1-42) in GIPR binding and cAMP production [228], and to stimulate insulin secretion in either the perfused mouse pancreas [109], rat islet insulinoma RIN1046-38 cells [229], mouse pancreatic βTC-3 cells [229], and healthy human subjects [230], its circulating levels are only ∽1–3 pmol/l, which is ∽15 fold lower than the fasting GIP(1-42) serum concentration [226,231].