Muscarinic acetylcholine receptors in the sino-atrial node and right atrium of bovine heart.
Muscarinic acetylcholine receptors were identified by the specific binding of [H](-)quinuclidinylbenzilate [( 3H](-)QNB) and [3H]oxotremorine-M [( 3H]Oxo-M), to membranes isolated from the sino-atrial (SA) node and right atrium (RA) of bovine heart. The density of [3H](-)QNB binding sites was greater in the SA node compared to the RA. Specific [3H](-)QNB binding was saturable and occurred to a single population of binding sites in both regions. The binding of antagonists, as assessed by competition with [3H](-)QNB, also occurred to a single population of sites; the binding affinities of all antagonists were similar in either region. Agonist competition curves, except for McN-A-343, were complex and computer analyses indicated that agonists bound to at least two populations of binding sites that differed in affinity. The proportion of high-affinity agonist binding sites was consistently greater in the SA nodal, relative to the RA membranes, while the affinity of the high-affinity agonist binding sites to a given agonist was essentially similar in either region. The high-affinity binding of [3H]Oxo-M was saturable and occurred to a single population of sites. The maximal binding of [3H]Oxo-M in the SA nodal membranes was higher than in the RA membranes. Guanine nucleotides and N-ethylmaleimide (NEM) markedly decreased [3H]Oxo-M binding; NEM did not appear to influence guanine nucleotide-dependent decrease in [3H]Oxo-M binding. Phospholipase A2 decreased both [3H](-)QNB and [3H]Oxo-M specific binding, the latter being affected to a greater extent. Phospholipase C also decreased [3H](-)QNB and [3H]Oxo-M binding, although to a lesser degree compared to phospholipase A2. Either lipase, however, increased the guanine nucleotide-sensitive agonist binding. Analysis of [3H](-)QNB binding to microsomal subfractions showed that binding sites were enriched in the light plasma membrane fractions that were also enriched in pertussis toxin sensitive guanine nucleotide binding proteins.
Braun
A P
AP
Department of Physiology, College of Medicine, University of Saskatchewan, Saskatoon, Canada.
Phan
T D
TD
Sulakhe
P V
PV
D000818:Animals; D002417:Cattle; D002499:Centrifugation, Density Gradient; D004247:DNA / Q000378:metabolism; D005033:Ethylmaleimide / Q000494:pharmacology; D006150:Guanine Nucleotides / Q000378:metabolism; D006325:Heart Atria / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D008861:Microsomes / Q000378:metabolism; D010276:Parasympatholytics / Q000378:metabolism / Q000494:pharmacology; D010277:Parasympathomimetics / Q000378:metabolism / Q000494:pharmacology; D010740:Phospholipases / Q000494:pharmacology; D011813:Quinuclidinyl Benzilate; D011950:Receptors, Cholinergic / Q000378:metabolism*; D012849:Sinoatrial Node / Q000378:metabolism*
D006150:Guanine Nucleotides; D008024:Ligands; D010276:Parasympatholytics; D010277:Parasympathomimetics; D011950:Receptors, Cholinergic; D011813:Quinuclidinyl Benzilate; D004247:DNA; D010740:Phospholipases; D005033:Ethylmaleimide