title: Effect of pertussis toxin on the heart acetylcholine muscarinic receptor affinity.
abstract: The effect of pertussis toxin on the affinity for agonists and antagonists of the heart muscarine acetylcholine receptor was studied using the radiolabeled antagonist [3H]quinuclidinyl benzylate ([3H]QNB). In cardiac membranes from control rats the displacement of [3H]QNB by carbachol was consistent with two classes of binding sites, kDH 25 +/- 10 nM and kDL 3,006 +/- 869 nM. The proportion of sites in the high and low affinity state for agonists was 55 and 45% respectively. In the presence of 100 microM guanyl-5'-yl imidodiphosphate (Gpp(NH)p), only the low affinity state for agonists was observed (kDL 3,804 +/- 759 nM). In cardiac membranes from pertussis toxin-treated rats, two classes of binding sites with affinities similar to those seen in the controls were also observed in the absence of guanine nucleotide (kDs 39 +/- 12 and 3,315 +/- 845 nM) but the proportion of sites were 20 and 80% for high and low affinity respectively. Gpp(NH)p shifted the remaining 20% of sites from the high affinity to the low affinity state (KD 4,093 +/- 744 nM). The receptor KD for antagonists was decreased by pretreatment with pertussis toxin from 83 +/- 7 to 56 +/- 5 pM (P less than 0.01); Gpp(NH)p induced a further change in the affinity for the antagonist in membranes from both control and pertussis toxin-treated rats. The change suggested positive cooperativity. The total number of sites was not modified significantly by either pertussis toxin treatment or guanine nucleotides. These results are consistent with a possible reciprocal modulation of the affinity for agonists and antagonists of the cardiac muscarine receptor.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002217:Carbachol / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D037342:Pertussis Toxin; D011813:Quinuclidinyl Benzilate / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000187:drug effects* / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
chemical_list: D006150:Guanine Nucleotides; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D011813:Quinuclidinyl Benzilate; D002217:Carbachol; D037342:Pertussis Toxin