The C-terminal half of the α2C-adrenoceptor determines the receptor's membrane expression level and drug selectivity

In tissues as well as in transfected cells, alpha(2C)-adrenoceptors show poorer expression levels compared to alpha(2A)-adrenoceptors. In order to characterize which regions of the alpha(2C)-adrenoceptor are involved in regulating the expression of binding-competent receptors at the plasma membrane, six chimeric alpha(2A)-/alpha(2C)-adrenoceptors were constructed. The wild-type alpha(2A)- and alpha(2C)-adrenoceptors and the six chimeric alpha(2A)-/alpha(2C)-adrenoceptors were transiently transfected into human embryonic kidney 293 (HEK293) cells, and the expression levels were investigated by radioligand binding. The results show that the C-terminal half of the alpha(2C)-adrenoceptor, ranging from the second extracellular loop to the C-terminus, is the main determinant of the low expression level of binding-competent alpha(2C)-adrenoceptors in HEK293 cell membranes. The so-called retention signal in the N-terminus of the alpha(2C)-adrenoceptor had a less profound effect on the expression levels of the chimeric receptors. For seven drugs competing for [H-3]-RX821002 binding, the K (i) values were determined at the wild-type alpha(2A)- and alpha(2C)-adrenoceptors and at four of the chimeric alpha(2A)-/alpha(2C)-adrenoceptors. The results show that the alpha(2C)- over alpha(2A)-selectivity of spiroxatrine, spiperone, clozapine, MK912, and chlorpromazine, as well as the alpha(2A)- over alpha(2C)-selectivity of BRL44408, resides mainly in the C-terminal half of the receptors. To some extent, the region comprising the N-terminal half of the receptors contributed to the alpha(2C)-selectivity of spiperone, clozapine, and chlorpromazine.