Characterization of the Oxybutynin Antagonism of Drug-Induced Spasms in Detrusor

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Abstract
Oxybutynin chloride exerted a moderate anticholinergic effect on rabbit detrusor in vitro, which was reversible and competitive in nature (KB [dissociation constant] = 4.7 .times. 10-9), and midway in potency between atropine and papaverine. In addition, oxybutynin strongly antagonized BaCl2-induced spasms of detrusor with a potency equivalent to that of papaverine and 10 times that of atropine. This musculotropic spasmolytic effect was slightly greater in rabbit than human or monkey tissue, and noncompetitive (pD2 [the negative logarithm of the molar concentration of the ED50 = 5.5). This direct relaxant effect, unlike that of papaverine, is not mediated by the inhibition of tissue phosphodiesterase, but probably reflected oxybutynin''s local anesthetic properties and associated effects on Ca2+ fluxes and binding.

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