Differences between proximal and distal portions of the male rabbit posterior urethra in the physiological role of muscarinic cholinergic receptors.

1. The aim of the present study was to elucidate functional differences between embryologically different portions of the posterior urethra of male rabbits in response to muscarinic acetylcholine receptor (mAChR) stimulation using in vitro isometric tension experiments and radioligand binding studies. 2. In the in vitro isometric tension experiments, carbachol, produced a dose-dependent contraction of the proximal portion under the resting state, but did not change the basal tone of the distal portion. Contraction of the proximal portion by 10(-5) M noradrenaline (NA) was dose-dependently enhanced by carbachol either in the presence or absence of NG-nitro-L-arginine (NOARG). In contrast, carbachol induced relaxation of the distal portion contracted by 10(-5) M NA, which was reversed to dose-dependent contraction in the presence of NOARG. 3. Both portions of the urethra had a similar number of [3H]-quinuclidinyl benzilate ([3H]-QNB) binding sites (195.3+/-74.1 fmols mg(-1) protein for the proximal portion and 146.5+/-8.5 fmols mg(-1) protein for the distal portion) with similar affinities (115.0+/-45.4 pM for the proximal portion and 79.9+ 2.9 pM for the distal portion). 4. The concentration-response curves to carbachol in both portions were shifted to the right in a parallel manner in the presence of pirenzepine (an M1 antagonist), 11-[[2-[(diethylamino)methyl]-1-piperidinyl] acetyl]-5, 11-dihydro-6H-pyrido-2,3-b)-(1,4)-benzodiazepin-6-one (AFDX-116, an M2 antagonist) and 4-diphenyl-acetoxy-N-methyl-piperidine (4-DAMP, an M1/M3 antagonist). The pA2 values for pirenzepine, AFDX-116 and 4-DAMP were 7.5+/-0.1, 7.2+/-0.02 and 9.3+/-0.1 respectively for the contraction of the proximal portion, and 7.2+/-0.1, 7.1+/-0.2 and 9.1+/-0.2, respectively for the relaxation of the distal portion. 5. In conclusion mAChR subtypes distribute in a similar fashion throughout the length of the male rabbit posterior urethra with the discrepant responses to carbachol attributable to the differential involvement of the NO pathway in mAChR-generated reactions.