The long-term objectives of this research program are to elucidate the mechanisms through which calcitonin (CT) regulates its own responses in tissues such as bone and kidney. Evidence has been presented that stimulation of adenylyl cyclase (AC) by CT desensitizes during agonist action. Although there are other mechanisms by which desensitization can occur, a major site of modification causing desensitization is the receptor itself. With other guanine nucleotide-binding regulatory (G) protein-coupled receptors (GPCRs), desensitization has been found to be caused by receptor-G protein uncoupling, receptor internalization and receptor down-regulation. We will test the following hypothesis regarding the mechanisms involved in desensitization of responses to CT. Amino acid residues in the putative carboxyl termini and third intracellular loops of the CT receptor mediate receptor-G protein uncoupling, receptor internalization and receptor down-regulation, which occurs in part by stimulating receptor degradation. To test this hypothesis, we will pursue the following Specific Aims: 1) To design and construct a synthetic gene for the CT receptor. This goal when accomplished will have a major practical impact on research in this field because it will greatly facilitate the study of this receptor by molecular genetic techniques. 2) To establish cell systems with transfected CT receptor cDNAs that show stimulation and desensitization of both AC and phosphoinoitide-specific phospholipase C (PPlase), receptor internalization and receptor down-regulation. 3) To determine the structural features of the CT receptor that interact with a G protein(s) to activate AC and PPlase. 4) To determine the structural features of the CT receptor that mediate receptor-G protein uncoupling, and receptor internalization and down-regulation. Molecular genetics will be used to construct the synthetic genes and to mutate the receptor DNAs. Desensitization (or tachyphylaxis), which limits the magnitude and duration of responses, limits the efficacy of drugs that act through these receptors. Therefore, one important reason to understand the mechanism of desensitization is to be able to design non-peptide drugs, which would not require parenteral nor intranasal administration, that may circumvent desensitization. These drugs could enhance the efficacy of receptor agonists by inhibiting desensitization or be agonists themselves of the CT-R. These agents could be used to treat osteoporosis.