Two distinct, divalent cation-sensitive, antagonist binding states of heart muscarinic receptors: differential modulation by guanine nucleotide.
1. The binding of [3H]quinuclidinylbenzilate (QNB), a muscarinic antagonist, to cardiac muscarinic receptors was investigated in two subcellular fractions (washed particles and microsomes) isolated from rat heart atria and ventricles. 2. 5'-guanylylimidodiphosphate (Gpp(NH)p, 0.1 mM), increased (2-3-fold) the binding to washed particles, but not to microsomes, whereas Mg2+ (1-20 mM) increased (up to 5-fold) the binding to microsomes, but not to washed particles. Gpp(NH)p modestly increased the affinity while Mg2+ decreased the affinity towards the radiolabelled antagonist. 3. Treatment with N-ethylmaleimide (NEM, 2 mM) increased the antagonist binding to either fraction. The stimulatory effect of Gpp(NH)p was not evident while that of Mg2+ survived in the NEM-treated fractions. 4. The treatment of fractions with divalent cations chelators (EDTA, EGTA; 10 mM) augmented the stimulatory effect of Mg2+ on [3H]QNB binding to microsomes while that of Gpp(NH)p on the washed particle [3H]QNB binding was decreased. Such treatment further revealed an inhibitory action (about 40%) of Mg2+ on the washed particle binding.
Sulakhe
P V
PV
Department of Physiology, College of Medicine, University of Saskatchewan, Saskatoon, Canada.
Jagadeesh
G
G
Phan
N T
NT
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D004492:Edetic Acid / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D005033:Ethylmaleimide / Q000494:pharmacology; D006150:Guanine Nucleotides / Q000502:physiology*; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D010276:Parasympatholytics / Q000494:pharmacology*; D011813:Quinuclidinyl Benzilate / Q000494:pharmacology; D051381:Rats; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D013347:Subcellular Fractions / Q000187:drug effects
D002413:Cations, Divalent; D006150:Guanine Nucleotides; D010276:Parasympatholytics; D011976:Receptors, Muscarinic; D006165:Guanylyl Imidodiphosphate; D004533:Egtazic Acid; D011813:Quinuclidinyl Benzilate; D004492:Edetic Acid; D008274:Magnesium; D005033:Ethylmaleimide; D002118:Calcium