Abstract

We investigated the ligand-binding properties and selectivities of rat substance P, substance K and neuromedin K receptors by transfection and functional expressions of the cDNAs for these receptor subtypes in monkey kidney COS cells. Selective radioligand binding analysis of both substance P and substance K receptors revealed the presence of high-affinity and low-affinity components which are governed primarily by the difference in rates of dissociation of the ligand-receptor complex. The two affinity components were interconvertible by the presence and absence of a guanine nucleotide, suggesting the involvement of a G protein in the two affinity states. The ligand bindings of the three receptors were inhibited in different potencies by naturally occurring tachykinins and a series of carboxyl-terminal fragments containing a common tachykinin sequence, and this comprehensive analysis indicated that a high and selective affinity of each of the tachykinin receptors is governed by interaction with several key amino acids under recognition of the fundamental core sequence of the tachykinin peptides. The potencies and selectivities of several synthetic agonists and an antagonist for the three receptors were also examined, and senktide was found to be a highly selective and potent agonist for neuromedin K receptor. This investigation thus indicates the detailed properties and binding selectivities characteristic of the three tachykinin receptors and also the usefulness of this receptor expression system for examination and development of a new receptor agonist or antagonist.