ATROPINE RESISTANCE AND MUSCARINIC RECEPTORS IN THE RAT URINARY BLADDER

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Abstract
The action of an anticholinesterase and an antimuscarinic drug upon nerve‐induced contractions of the rat urinary bladder were examined during transmural stimulation at 20 Hz. Responses were graded in magnitude by limiting the duration of the stimulus trains. Responses of low magnitude produced by short stimulus trains were unchanged by atropine; however, maximal responses resulting from long stimulus trains were diminished in magnitude and shortened in duration. Responses of small magnitude elicited by short stimulus trains involve muscarinic receptors in close proximity to the neuroeffector junction and are resistant to atropine. Maximal responses elicited by long stimulus trains involve ‘junctional’ muscarinic receptors as well as receptors located at the periphery of the junction; the ‘extrajunctional’ receptors are blocked by atropine. Responses of low magnitude produced by short stimulus trains were unaffected by echothiophate; however, the duration of maximal responses resulting from the long stimulus trains was extended. The inhibition of cholinesterase did not increase the occupation of muscarinic receptors by the transmitter; however, after large quantities of transmitter were released by the long stimulus trains the association between the receptors and acetylcholine was prolonged.