Binding of gold-protein ligand complexes to neurokinin receptors - Visualization with the electron microscope on pig coronary artery and rat portal vein

Active neurokinin (NK) receptors were visualized with 5-nm colloidal gold-p rotein-substance P (GPSP) and -senktide (GPSenk) complexes on vascular tiss ues. Electron micrographs of pig coronary strips incubated with GPSP showed gold particles either bound to the plasmalemma or inside intracytoplasmic vesicles of endothelial cells. Preincubation with SP or the NK1 receptor an tagonist L-703606 prevented GPSP marking, No gold particles were seen after incubation with GPSenk. On coronary strips in vitro, which had been precon tracted with U46619, GPSP induced relaxations similar to those produced by equimolar concentrations of SP, both relaxations being inhibited by L-70360 6. Analogous to senktide, GPSenk was totally inactive on arterial strips. I ncubation of rat portal veins with GPSenk showed gold particles bound to th e plasmalemma or inside intracytoplasmic vesicles of smooth muscle cells. P reincubation with senktide or the NK3 receptor antagonist R-820 prevented G PSenk marking. On portal veins in vitro GPSenk induced contractions similar to those induced by equimolar concentrations of senktide or NKB; these eff ects were inhibited by R-820, Our results show that colloidal gold-protein complexes present biological activity and selectivity similar to those of t heir respective native ligand and detect the presence of active receptors; in addition, they suggest the presence of NK-receptor-mediated endocytosis in endothelial cells of coronary artery and in smooth muscle cells of the p ortal vein.