Fig. 4. Relative quantification of the proportion of the D₂ receptor isoforms located at the plasma membrane. After membrane biotinylation (see Materials and Methods), the COS-7 cells transfected by either D_2a or D_2b receptors were lysed and the whole cell lysate was fractionated by centrifugation to provide a nuclear pellet (N; nuclei and cell debris) and a post-nuclear supernatant. This supernatant was submitted to streptavidine chromatography to separate biotinylated membranes from nonbiotinylated membranes. In the three fractions, the amount of D_2a (grey bars) and D_2b (empty bars) receptors were quantified by [³H]-spiperone binding (A) and compared with the total activity of alkaline phosphatase, a marker of plasma and nuclear membrane (B). The histograms correspond to values ± s.e.m (n=3).