α 2B-Adrenoceptors couple to Ca 2+ increase in both endogenous and recombinant expression systems

Abstract

The ability of cloned human α 2B-adrenoceptors heterologously expressed in Sf9 cells and endogenous α 2B-adrenoceptors in NG 108-15 neuroblastoma×glioma cells to couple to increase of intracellular Ca 2+ was studied. Ca 2+ increases in NG 108-15 cells were detectable but slight, whereas those in α 2B-adrenoceptor-expressing Sf9 cells were greater. In the latter, the maximum Ca 2+ increase correlated positively, and the EC 50-value of noradrenaline negatively, with the receptor expression density. The order of potency of the agonists was d-medetomidine ([ d]-4-[5]-[1-(2,3-dimethylphenyl)ethyl]-1 H-imidazole) >noradrenaline≈clonidine>oxymetazoline, with clonidine and UK14,304 (5-bromo- N-[4,5-dihydro-1 H-imidazole-2-yl]-6-quinoxalinamine) being weak partial agonists. In Sf9 cells Ca 2+ increases consisted of concomitant mobilization from an intracellular store and influx of extracellular Ca 2+. In these cells α 2B-adrenoceptor stimulation also increased the inositol 1,4,5-trisphosphate mass. We conclude that α 2B-adrenoceptors can couple to intracellular Ca 2+ increases which may involve prior activation of phospholipase C.