ABSTRACT
A radioactive tag is attached to the ligand, which is then incubated with tissue slices, isolated cells, purified biological membranes or solubilized preparations under appropriate conditions. Bound ligand is separated from unbound ligand by centrifugation, filtration, simple washing or column chromatography, and radioactivity of the bound ligand is enumerated in a counter. Receptors are macromolecules on cell membranes or inside the cell that, upon binding the agonist, initiate a chain of events culminating in the biological response. Substance P (SP) has received much attention due to a number of biological actions and its presence in both central and peripheral neurons. SP is one member of a family of structurally related neuropeptides, the neurokinins. Vasoactive intestinal polypeptide (VIP) is a widely distributed neuropeptide and VIP-immunoreactive nerves have been localized throughout the gastrointestinal tract. These nerves supply the circular smooth muscle and also the myenteric and submucous plexuses.
