Effect of pertussis toxin on the heart acetylcholine muscarinic receptor affinity

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.