Complex dose-response curves of atropine in man explained by different functions of M1- and M2-cholinoceptors

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Abstract
Summary In the present study we set out to explain the complex atropine dose-response curves in man in relation to M-cholinoceptor subtype occupancy. In healthy volunteers the effects of atropine on heart rate and salivary flow were quantified. M-cholinoceptor subtype occupancy by antagonist present in plasma samples was detected in an in vitro radioreceptor assay. Atropine effects were studied without and after propranolol (240 mg oral dose) and without and after pirenzepine (1.1 mg i. v.) to differentiate β-adrenoceptor and M-cholinoceptor subtype mediated effects. 1. In receptor binding studies, M-cholinoceptors in bovine cerebral cortex membranes were labelled with 3H-pirenzepine (pK d = 8.05), M-cholinoceptors in rat salivary gland membranes with 3H-N-methylscopolamine (pK d = 9.02). Atropine competed for binding of these ligands with a small (2.1-fold) preferential selectivity via the cerebral in comparison to the glandular receptors (pK i = 9.18 versus 8.86). Pirenzepine showed a marked selectivity (40-fold) in this respect with pK i-values of 8.05 (M1: cerebral cortex) and 6.45 (M2: salivary glands). 2. At heart rate and at salivary flow, bivalent dose-response curves of atropine were observed with opposite effect vectors. The typical antagonist effects at M-cholinoceptors (i. e. an increase of heart rate and an inhibition of salivary flow) were observed at doses > 1 μg/kg, whereas “paradoxical” cholinomimetic effects of atropine became apparent at lower doses. From a superposition of two isotherms with opposite effect vectors ED50-values were calculated, which were in the range of half-maximal M-cholinoceptor occupancy in the in vitro radioreceptor assay of plasma samples. 3. The time-dependent decline of atropine effects after the highest dose (40 μg/kg) and the respective in vitro M-cholinoceptor occupancy were in good agreement with the dose-response data. 4. β-Adrenoceptor blockade did not influence the pattern of the atropine dose-response relation. After pirenzepine (∼ 70% of M1-cholinoceptor occupancy), the cholinomimetic effects of atropine were abolished and only monovalent atropine dose-response curves were observed. Maximal effects of atropine were not different in comparison to the respective controls without pirenzepine. 5. The standard deviation of the RR-intervals in the ECG as a measure of postsynaptic M-cholinoceptor stimulation was increased after pirenzepine by 58% and decreased after the maximum atropine dose by 85%. It is concluded, that the cholinomimetic effects observed after atropine result from its antagonism at peripheral M1-cholinoceptors. A negative feedback mechanism of acetylcholine release via M1-autoreceptors is discussed as a possible mechanism involved.