M-2 receptors in genito-urinary smooth muscle pathology

In vitro bladder contractions in response to cumulative carbachol doses wer e measured in the presence of selective muscarinic antagonists from rats wh ich had their major pelvic ganglion bilaterally removed (denervation, DEN) or from rats in which the spinal cord was injured (SCI) via compression. DE N induced both hypertrophy (505+/-51 mg bladder weight) and a supersensitiv ity 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 ani mals weighed 184+/-17 mg, significantly less than the bladders of non voidi ng rats (NV, 644+/-92 mg). The potency of carbachol was greater in bladder strips from NV SCI animals (EC50=0.54=/-0.1 mu than either bladder strips f rom SPV SCI (EC50,=0.93+/-0.3 mu M), DEN or control (EC50=1.2+/-0.1 mu M) a nimals. Antagonist affinities in control bladders for antagonism of carbach ol induced contractions were consistent with M-3, mediated contractions. An tagonist affinities in DEN bladders for 4-diphenlacetoxy-N-methylpiperidine methiodide (4-DAMP, 8.5) and para flouro hexahydrosilodifenidol (p-F-HHSiD , 6.6); were consistent with M-2, mediated contractions, although the metho ctramine affinity (6.5) was consistent with M-3,mediated contractions, p-F- HHSiD inhibited carbachol induced contraction with an affinity consistent w ith M-2,receptors in bladders from NV SCI pKb=6.4) animals and M-3, recepto rs in bladders from SPV SCI animals (pKb=7.9). Subtype selective immunoprec ipitation of muscarinic receptors revealed an increase in total and an incr ease in M-2, receptor density with no change in M-3, receptor density in bl adders from DEN and NV SCI animals compared to normal or sham operated cont rols. M-3 receptor density was lower in bladders from SPV SCI animals while the M-2 receptor density was not different from control. This increase in M-2, receptor density is consistent with the change in affinity of the anta gonists for inhibition of carbachol induced contractions and may indicate t hat M-2, receptors or a combination of M-2, and M-3, receptors directly med iate smooth muscle contraction in bladders from DEN and NV SCI rats.