Bladder muscle specimens from seven patients with neurogenic bladder dysfunction were analyzed to determine whether the muscarinic receptor subtype mediating contraction shifts from M₃ to the M₂ subtype as found in the denervated, hypertrophied rat bladder. Seven bladder specimens were analyzed from six female and one male patients. Six of the patients had traumatic cervical spinal cord injuries (C₄-C₇), and the other patient had an L₁ congenital myelomeningocele. This was compared with results from bladder specimens obtained from eight organ transplant donors. The affinities of three subtype-selective muscarinic receptor antagonists for inhibition of carbachol-induced contractions were determined. The affinity of the M₃ selective antagonists darifenacin or p-fluoro-hexahydrosiladifenadol (p-F-HHSiD) was determined in six of the seven spinal injury patient specimens. The affinity was consistent with M₂-mediated contractions in four of these six specimens, intermediate between M₂ and M₃ in one specimen, and within the M₃ range in one specimen. The other specimen, tested only with the M₂ selective antagonist methoctramine, showed an M₃ affinity. In the organ donors, the affinity of p-F-HHSiD was within the M₂ range for six of seven specimens, whereas the affinity of darifenacin was within the M₃ range for five of six and intermediate between M₂ and M₃ for the other specimen tested. The affinity of methoctramine in both organ donor specimens tested was within the M₃ range. Whereas normal detrusor contractions are mediated by the M₃ receptor subtype, in patients with neurogenic bladder dysfunction as well as certain organ transplant donors, contractions can be mediated by the M₂ muscarinic receptor subtype.

Major pelvic ganglion electrocautery (MPGE) and spinal cord injury in the rat induce bladder hypertrophy and a change in muscarinic receptor subtypes mediating bladder contraction from predominantly M₃to a combination of M₂and M₃. To determine whether this is a result of bladder hypertrophy or denervation, we studied the following groups: sham-operated controls, urinary diversion (DIV), MPGE together with urinary diversion (DIV-DEN), bilateral MPGE (DEN), bladder outlet obstruction (BOO), and MPG decentralization (MPGDEC). The degree of bladder denervation was determined by the maximal carbachol response normalized to the response to electric field stimulation. Receptor subtype density was determined by immunoprecipitation. The affinity of subtype-selective muscarinic antagonists for inhibition of carbachol-induced contractions was used to determine the subtype-mediating contraction. DEN, MPG-DEC, and BOO bladders were hypertrophic whereas DIV bladders were atrophic compared with sham operated. Bladder contraction in sham-operated, DIV, and DIV-DEN was mediated by the M₃receptor subtype, whereas the M₂subtype participated in contraction in the DEN, MPG-DEC, and BOO groups. The hypertrophied bladders had an increase in total and M₂receptor density while all experimental groups showed a reduction in M₃receptor density. Thus bladder hypertrophy, independent from bladder denervation, causes a shift in the muscarinic receptor subtype mediating bladder contraction from M₃toward M₂.

In vitro bladder contractions in response to cumulative carbachol doses were measured in the presence of selective muscarinic antagonists from rats that had their major pelvic ganglion bilaterally removed. Denervation induced both hypertrophy and a supersensitivity of the bladders to agonist. The affinities in control bladders for antagonism of carbachol-induced contractions were consistent with M₃-mediated contractions. Affinities in denervated bladders for 4-diphenlacetoxy- N-methylpiperidine methiodide (8.5) and p-fluoro hexahydrosilodifenidol (6.6) were consistent with M₂-mediated contractions, although the methoctramine affinity (6.5) was consistent with M₃-mediated contractions. Subtype-selective immunoprecipitation of muscarinic receptors revealed a 50% increase in total and a 60% increase in M₂receptor density with no change in M₃receptor density in denervated bladders compared with normal or sham-operated controls. This increase in M₂receptor density is consistent with the change in affinity of the antagonists for inhibition of carbachol-induced contractions and may indicate that M₂receptors or a combination of M₂and M₃receptors directly mediates smooth muscle contraction in the denervated bladder.

Purpose: We provide an overview of the medical literature supporting the combined use of muscarinic and α-adrenergic antagonist therapy for the treatment of voiding dysfunction.Materials an...