Microtubule-dependent regulation of alpha(2B) adrenergic receptors in polarized MDCKII cells requires the third intracellular loop but not G protein coupling

Previous studies in cultured, polarized Madin-Darby canine kidney II (MDCKI I) renal epithelial cells have demonstrated that the apical steady-state lo calization and delivery of the A(1) adenosine receptor is modified by disru ption of the microtubule network with colchicine, whereas the basolateral l ocalization and trafficking of the alpha(2)-adrenergic receptors (alpha(2)A R) are not; instead, the binding capacity of the alpha(2B)AR, but not alpha (2A)AR or alpha(2C)AR subtypes, is increased in a time-dependent fashion. T he present studies explore the molecular basis for this alpha(2B)AR subtype -selective phenomenon. Colchicine selectively increased alpha(2B)AR density at the cell surface, as determined by confocal microscopy, receptor bindin g, and surface biotinylation studies. The colchicine-induced increase in th e functional density of the alpha(2B)AR requires the third intracellular lo op because the alpha(2B)AR loop deletion (alpha(2B)AR Delta i3) mutant did not show an increased receptor density after colchicine treatment. Furtherm ore, the colchicine-mediated increase in alpha(2B)AR density is manifest on ly in polarized cells because colchicine treatment of nonpolarized MDCKII r enal epithelial cells as well as simian kidney COSM6 and human embryonic ki dney HEK293 cells did not effect an increase in alpha(2B)AR density. Colchi cine-dependent increases in alpha(2B)AR density did not depend on functiona l coupling to G proteins, however, because pretreatment with pertussis toxi n did not eliminate the effect of colchicine. These data indicate that micr otubule-dependent regulation of alpha(2B)AR density at the basolateral surf ace of polarized MDCKII cells requires the third intracellular loop of alph a(2B)AR but not functional alpha(2B)AR-G protein coupling.