Cholinergic activation of phasic activity in the isolated bladder: possible evidence for M 3 ‐ and M 2 ‐dependent components of a motor/sensory system

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Abstract
OBJECTIVES: To analyse pressure changes induced by muscarinic agonists on the isolated bladder in order to examine whether there are different responses representing different components of a motor/sensory system within the bladder wall. MATERIALS AND METHODS: Whole isolated bladders from 19 female guinea‐pigs (280–400 g) were used. A cannula was inserted into the urethra to monitor intravesical pressure and the bladder was suspended in a heated chamber containing carboxygenated physiological solution at 33–36 °C. Initially, the responses to the cholinergic agonists, arecaidine but‐2‐ynyl ester tosylate and carbachol were assessed. Then, in an attempt to identify the muscarinic receptor subtypes involved, the effects of selective muscarinic antagonists on the arecaidine‐induced bladder responses were assessed. The antagonists used were the relatively M 3 ‐selective 4‐diphenylacetoxy‐N‐methylpiperidine methobromide (4‐DAMP) and darifenicin, and relatively M 2 ‐selective AFDX‐116. All drugs were added to the solution bathing the ablumenal surface of the bladder. RESULTS: The whole bladders exposed to cholinergic agonists respond with complex changes in intravesical pressure. Immediately after application of the agonist there was a burst of high frequency transient contractions. During continued application of agonist the frequency of the transients decreased and their amplitude increased. Thus, there appear to be two components to the response: an initial fast phase and a later slow component. The maximum frequency of the initial burst increased with increasing concentrations of agonist. By contrast, the frequency of the transients in the steady state showed little dependence on agonist concentration. There were quantitative differences between the responses to arecaidine and carbachol. Arecaidine was less effective in generating the initial burst of high‐frequency activity and the transients were significantly larger. At low dose, arecaidine was more effective in producing the large transients in the steady state. Pre‐exposure of the bladder to 4‐DAMP (0.1–10 n m ) or darifenicin (0.1–10 n m ) significantly reduced the frequency of the initial burst of activity; 0.3 n m 4‐DAMP reduced the frequency by half. In this concentration range, 4‐DAMP reduced the amplitude of the initial transients but did not affect the frequency of the transients in the steady state. There were similar results with darifenicin. However, darifenicin was less effective in reducing the amplitude of the initial transients. By contrast, ADFX‐116 had little effect on the frequency of the initial transients but did reduce amplitude; 300 n m AFDX‐116 was needed to reduce the frequency of the initial burst by half. CONCLUSIONS: This analysis suggests that there are different but interrelated mechanisms in the isolated bladder contributing to complex contractile activity. Three components can be identified: a mechanism operating during voiding to produce a global contraction of the whole bladder and two mechanisms, pacemaker and conductive, involved in generating and propagating local contractions in the bladder wall. The pacemaker component is more sensitive to darifenicin and 4‐DAMP than to AFDX‐116 suggesting that the underlying processes rely predominantly on M 3 receptors and less so on M 2 (M 3 > M 2 ). The phasic activity in the later stages is less affected by M 3 antagonists and might therefore involve predominantly M 2 receptors (M 2 > M 3 ). The potential importance of these results in terms of the general physiology and pharmacology of the bladder is discussed.

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