Muscarinic cholinoceptor subtypes in the rat prostatic membrane were characterized by using [
3H]-methyl-quinuclidinyl benzilate (QNB)in ligand binding studies. [
3H]-Methyl-QNB saturation experiments showed the existence of a homogeneous population of binding sites with a high affinity (K
D value)of 0.24 ± 0.04 nM and a maximum binding site number (B
max)of 219 ± 65 fmol/mg protein. Inhibition of [
3H]-methyl-QNB binding by nonlabelled compounds was in the following order of effectiveness in rat prostate: atropine > 4-diphenylacetoxy-
N-methylpiperidine methiodide (4-DAMP)> hexahydro-sila-difenidol hydrochloride,
p-fluoroanalog (p-F-HHSiD)> pirenzepine > methoctramine > [1 l-((2-((dimethylamino)methyl)-1-piperidinyl)acetyl)-5, 11-dihydro-6
H-pyrido(2, 3-
b)(1, 4)benzodiazepine-6-one] (AF-DX 116). This ranking order was similar to that for the salivary gland (M
3 subtype), but not for the brain (M
1 subtype)or the heart (M
2 subtype). These results indicate that the muscarinic cholinoceptors in the rat prostate belong mainly to the M
3 subtype. Furthermore, B
max values for muscarinic cholinoceptors in the aged rat prostate (approximately 1-year-old)were smaller than those in the young rat prostate (6 to 8-week-old)(87 ± 13 vs. 183 ± 32 fmol/mg protein). However, K
D values for muscarinic cholinoceptors, and B
max and K
D values for β-adrenoceptors showed no change. These results suggest that the number of prostatic muscarinic cholinoceptors decreases with aging.
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