Muscarinic acetylcholine receptor regulates phosphatidylcholine phospholipase D in canine brain

The hydrolytic activity of phosphatidylcholine phospholipase D in the synaptosomes from canine brain was examined using a radiochemical assay with 1,2-dipalmitoyl-sn-glycerol-3-phosphoryl[³H]choline as the exogenous substrate. The involvement of G protein(s) in regulation of this enzyme was demonstated by a 2- to 3-fold stimulation of the basal activity (4.81 ± 0.44 nmol choline released/mg protein/h) with guanosine 5'-(3-O-thiol)triphosphate (GTPγS), guanyl-5'-yl-(β,γ-methylene)diphosphonate, aluminum fluoride, or cholera toxin. The stimulation of phospholipase D hydrolytic activity by GTPγS was inhibited by 2 mM guanosine 5'-(2-O-thiol)diphosphate. GTPγS at the maximum stimulatory concentration (10 μM) had an additive effect on the maximum cholera toxin stimulation of phospholipase D activity. However, the reverse was not true, thus indicating the possibility that more than one G protein may be involved. Furthermore, cholinergic agonists, including acetylcholine, carbachol, and muscarine, were able to increase the phospholipase D hydrolytic activity at low but not maximally stimulatory concentrations of guanine nucleotide. These cholinergic stimulations were antagonized by atropine, a muscarinic blocker. In addition, O-tetradecanoylphorbol 13-acetate, a protein kinase C activator, was able to stimulate the hydrolytic activity of phospholipase D more than 300% in the presence of 0.2 μM GTPγS. However, in the absence of GTPγS, stimulation was less than 60%. Our results not only indicate that the receptor-G protein-regulated phospholipase D may be directly responsible for the rapid accumulation of choline and phosphatidic acid in the central nervous system but also reveal that muscarinic acetylcholine receptor-G protein-regulated phospholipase D is a novel signal transduction process coupling the neuronal muscarinic receptor to cellular responses.