Molecular events following activation of muscarinic receptors: the role of inositol phospholipids

Muscarinic cholinergic agonists can provoke a wide variety of physiological responses in target tissues. This can be achieved by mobilisation of calcium from intracellular and extracellular stores. The members of the family of Ca²⁺-mobilising receptors to which the muscarinic receptor belongs share the ability to provoke a calcium-independent decrease in the concentration of inositol phospholipids. This response is closely coupled to agonist binding and is thus implicated in the mechanism whereby muscarinic and related receptors bring about elevated levels of Ca²⁺ in cells. Another widespread response to activation of muscarinic receptors is inhibition of adenylate cyclase possibly through direct interaction with a nucleotide regulatory protein. Circumstantial evidence and analogy with α₂-adrenergic receptors suggests that adenylate cylase inhibition may be mediated by a different form of the muscarinic receptor to that involved in calcium mobilisation.