Pharmacological evaluation of alpha and beta human tachykinin NK(2) receptor splice variants expressed in CHO cells

In the present study, we have investigated, by binding and functional experiments, the pharmacological profile of a new human tachykinin NK(2) receptor splice variant named beta isoform. Neurokinin A, nepadutant, SR48968 [(S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl) butyl]benzamide] and substance P have been tested for binding on the receptor expressed in whole CHO transfected cells. Only SR48968 binds, but with an affinity about sixfold lower in respect to the alpha isoform. Moreover, neurokinin A was unable to inhibit the [(3)H]SR48968 binding to the beta isoform up to microM concentrations. In cells expressing the human tachykinin NK(2) receptor beta isoform, contrary to those expressing the alpha isoform, natural or selective tachykinin receptor agonists (1 microM) were unable to produce a significant activation of inositol phosphate (IP) production or increase of intracellular calcium concentration [Ca(2+)](i). The recently discovered tachykinins, endokinins C and D, did not activate IP production or [Ca(2+)](i) increase in cells expressing the alpha or beta isoform of the human tachykinin NK(2) receptor. The present data indicate that the human tachykinin NK(2) receptor beta isoform is poorly or not expressed on the cell membrane surface and that it may possibly act as a regulator of tachykinin NK(2) receptor function. We cannot exclude the possibility that this receptor could interact with other presently unknown ligands.