Down-regulation of cardiac muscarinic receptors induced by di-isopropylfluorophosphate.

Unlabelled: The feasability of PET determination of myocardial muscarinic acetylcholine receptor (mAChR) density has been demonstrated in dogs and humans. The results of the PET method, however, were not validated by a direct comparison with the in vitro determination of mAChR density.
Methods: Left ventricular mAChR concentrations were studied in beagle dogs at baseline and after a 5- or a 11-day treatment with the irreversible acetylcholinesterase inhibitor di-isopropylfluorophosphate (DFP). The determination of mAChR densities were performed in vivo using PET, 11C-MQNB, the three-injection protocol and the compartmental model previously described. In a parallel group of dogs, determination of mAChR density was performed in vitro using 3H-(-)-MQNB.
Results: In control dogs (n = 4), PET left ventricular density of mAChR was 61.1 +/- 8.1 pmol/ml tissue. In the 5-day DFP-treated animals (n = 3), Bmax decreased to 38.2 +/- 8.3 pmol/ml tissue (-38%; p = 0.005 versus control). In the 11-day DFP-treated animals (n = 3), Bmax was 34.7 +/- 5.5 pmol/ml tissue (-43%; p = 0.003). There was no change in the affinity constant either at 5 or 11 days. In control dogs, Bmax, measured in vitro, was 9.53 +/- 0.93 pmol/g tissue. In the 5-day DFP-treated animals, Bmax decreased to 6.2 +/- 0.9 pmol/g tissue (-35%; p = 0.003). In the 11-day DFP-treated animals, Bmax was 5.1 +/- 0.6 pmol/g tissue (-47%; p = 0.003 versus control). At that time, there was no change in affinity constant. On the fifth and 11th days, myocardial acetylcholinesterase activity was reduced by 88% and 90%, respectively.
Conclusion: The in vivo and in vitro methods showed a similar decrease in mAChR density while for both methods affinity constant remained unchanged. This study validates the ability of PET and of the compartmental model to in vivo quantify changes in mAChR density.