Muscarinic cholinergic receptors on cultured thyroid cells. II. Carbachol-induced desensitization.

Muscarinic cholinergic receptors were previously characterized on cultured porcine thyroid cells. The receptor number was increased by chronic thyrotropin or prostaglandin E2 treatments [Champion, S., and J. Mauchamp. Mol. Pharmacol. 21:66-72 (1982)]. The long-term effect of carbachol was studied. After chronic treatment with carbachol, cells were completely desensitized to acute carbachol stimulation. This process was blocked by muscarinic antagonists. A complete desensitization was obtained after 6 hr of treatment with 100 microM agonist. Under these conditions the quinuclidinyl benzilate binding capacity of cell homogenates was reduced by 50%. Withdrawal of carbachol allowed the complete restoration of the sensitivity of cells within 6 hr with only a partial recovery of the binding capacity (25%). The complete complement of receptors was obtained after 24 hr of recovery. Desensitization and agonist-induced decrease in receptor number were not affected by cycloheximide, whereas the recovery of both effect and binding sites required active protein synthesis.