tokens: In both types , purinoceptor desensitization with alpha,beta-methylene adenosine-5'-triphosphate ( alpha,beta-meATP ) caused further reductions at low frequencies (< 10 Hz ). The muscarinic receptor antagonists atropine , 4-diphenylacetoxy-N-methylpiperidine ( 4-DAMP )(' M ( 1 )/ M ( 3 )/ M ( 5 )- selective '), methoctramine (' M ( 2 )- selective ') and pirenzepine (' M ( 1 )- selective ') antagonized the tonic component of the electrical field stimulation - evoked contractile response more potently than the phasic component .