Mechanism of agonist-induced degradation of muscarinic cholinergic receptor in cultured vas deferens of guinea-pig.

The mechanism of agonist-induced degradation of muscarinic cholinergic receptors (mAChRs) was examined by means of [3H]QNB binding by organ-cultured guinea-pig vas deferens. Long-term exposure to ACh decreased the surface mAChR. This so-called 'down regulation' of mAChR was significantly inhibited by anti-endocytotic drugs. Anti-microtubular agents also strongly inhibited the decrease of mAChR remaining on the cell surface. Ammonium chloride and protease inhibitors, which are known to inhibit lysosomal enzymes, had little effect on the decrease and no increase in intracellular mAChR could be detected under these conditions. Chloroquine blocked the decrease in mAChR remaining on the cell surface. Based on these findings, the involvement of clustering and endocytosis in mAChR degradation were considered as possibilities. Furthermore, contraction of the smooth muscle to ACh in long-term desensitization was also examined in relation to the number and nature of the receptors. When the muscles were cultured with ACh and chloroquine or vinblastine, there were indications that the surface mAChR and the contractile system were uncoupled.