Why is amitriptyline much weaker than desipramine at decreasing beta-adrenoceptor numbers?

Desipramine is consistently more effective than amitriptyline at causing beta-adrenoceptor down-regulation. Atropine, mepyramine, ketanserin, cyproheptadine and citalopram did not modify this action of desipramine in rats. Therefore inhibition of either muscarinic, histamine-H1, and 5-HT receptors or 5-HT uptake produced by amitriptyline is unlikely to account for its weaker effect on beta-adrenoceptors. A more likely explanation implicates noradrenaline uptake inhibition in vivo since amitriptyline was much weaker than desipramine and only effective after repeated dosing.