Muscarinic Receptor-Mediated Ca 2+ Control, Protein Kinase C Activity, and Proteoglycan Release and Synthesis in Articular Chondrocytes

To determine the presence and role of muscarinic receptors on articular chondrocytes, Scatchard analysis of muscarinic antagonist binding to membrane fragments of rabbit articular chondrocytes, and analysis of the changes in the intracellular calcium ion concentration, protein kinase C (PKC) activity, and the synthesis and release of glucoseaminoglycan (GAG) induced by the muscarinic agonists and antagonists were carried out. Scatchard analysis of the muscarinic antagonist [3H]quinuclidinyl benzilate (QNB) revealed a single class of binding sites with a K D of 3.2 ± 0.12nM and a Bmax of 83.19 ± 2.73fmol/mg of protein. The addition of the muscarinic agonist oxotremorine elevated the intracellular calcium ion concentration and protein kinase C activity, increased the release, and decreased the synthesis of GAG. The specific Ml receptor antagonist inhibited all these stimulative or inhibitory effects of oxotremorine almost completely, although specific M2 and M3 receptor antagonists did not. This is the first study, to our knowledge, documenting the presence and role of muscarinic receptors on articular chondrocytes.