Regulation of muscarinic acetylcholine receptor function in cardiac cells and in cells expressing cloned receptor genes.

The regulation of the number and function of the muscarinic receptors has been investigated in cultured chick cardiac cells and in cells expressing cloned genes encoding mammalian, Drosophila, and chick muscarinic receptors. A serum-free defined medium for the culture of chick embryonic heart cells has been used to study the regulation of mAChR number and function by serum lipoproteins. Addition of rooster high density lipoprotein to the culture medium results in an attenuation of muscarinic receptor-mediated inhibition of cAMP accumulation without a change in the number of receptors or inhibitory G proteins. Clones encoding the mouse m1 receptor and a homologous receptor from Drosophila have been isolated. When expressed in Y1 adrenal cells, both receptors stimulate phosphoinositide hydrolysis but do not inhibit cAMP accumulation. Deletion of 123 out of the 156 amino acids in the third cytoplasmic loop of the mouse m1 receptor does not impair its ability to stimulate phosphoinositide hydrolysis. A genomic clone encoding a muscarinic receptor expressed in chick heart has been isolated. When expressed in Y1 cells, it causes inhibition of cAMP accumulation but does not stimulate phosphoinositide hydrolysis.