M2 receptors in genito-urinary smooth muscle pathology

In vitro bladder contractions in response to cumulative carbachol doses were measured in the presence of selective muscarinic antagonists from rats which had their major pelvic ganglion bilaterally removed (denervation, DEN) or from rats in which the spinal cord was injured (SCI) via compression. DEN induced both hypertrophy (505+/-51 mg bladder weight) and a supersensitivity of the bladders to carbachol (EC50=0.7+/-0.1 uM). Some of the SCI rats regained the ability to void spontaneously (SPV). The bladders of these animals weighed 184+/-17 mg, significantly less than the bladders of non voiding rats (NV, 644+/-92 mg). The potency of carbachol was greater in bladder strips from NV SCI animals (EC50=0.54+/-0.1 uM) than either bladder strips from SPV SCI (EC50=0.93+/-0.3 microM), DEN or control (EC50=1.2+/-0.1 microM) animals. Antagonist affinities in control bladders for antagonism of carbachol induced contractions were consistent with M3 mediated contractions. Antagonist affinities in DEN bladders for 4-diphenlacetoxy-N-methylpiperidine methiodide (4-DAMP, 8.5) and para fluoro hexahydrosilodifenidol (p-F-HHSiD, 6.6); were consistent with M2 mediated contractions, although the methoctramine affinity (6.5) was consistent with M3 mediated contractions. p-F-HHSiD inhibited carbachol induced contraction with an affinity consistent with M2 receptors in bladders from NV SCI (pKb=6.4) animals and M3 receptors in bladders from SPV SCI animals (pKb=7.9). Subtype selective immunoprecipitation of muscarinic receptors revealed an increase in total and an increase in M2 receptor density with no change in M3 receptor density in bladders from DEN and NV SCI animals compared to normal or sham operated controls. M3 receptor density was lower in bladders from SPV SCI animals while the M2 receptor density was not different from control. This increase in M2 receptor density is consistent with the change in affinity of the antagonists for inhibition of carbachol induced contractions and may indicate that M2 receptors or a combination of M2 and M3 receptors directly mediate smooth muscle contraction in bladders from DEN and NV SCI rats.