Identification and Characterization of Muscarinic Cholinergic Receptors in the Isolated Plasma Membranes and Intact Tissue of the Urinary Bladder

Characterization of muscarinic receptors in dog tracheal smooth muscle cells

1. The tritiated muscarinic antagonist N-methyl scopolamine, [3H]-NMS, was used to characterize the muscarinic receptors associated with the intact dog tracheal smooth muscle cells. Based on receptor binding assays, the intact tracheal smooth muscle cells had specific, saturable, high-affinity binding sites for [3H]-NMS. 2. Specific binding was cell concentration- and time-dependent. The specific binding of [3H]-NMS was increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-NMS with the muscarinic receptors was attained within 30 min at 37 degrees C. 3. Binding was saturable with respect to [3H]-NMS concentrations. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 320 +/- 20 pM and a maximum receptor density (Bmax) of 13.7 +/- 1.4 fmole per 5 x 10(4) cells. The Hill coefficient for [3H]-NMS binding was 1.00 +/- 0.01. The association (K1) and dissociation (K-1) rate constants were determined to be (1.19 +/- 0.23) x 10(8) M-1 min-1 and 0.034 +/- 0.09 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 286 +/- 65 pM; this value is close to the value of KD calculated from Scatchard plots of binding isotherms. 4. The non-selective muscarinic antagonist atropine and M1 selective antagonist pirenzepine did not reveal any selectivity of these muscarinic receptors. Pirenzepine competed with [3H]-NMS for a single binding site with a Ki value of (6.02 +/- 0.69) x 10(-7) M which is close to the value of M2 or M3 receptors, indicating that the M1 receptor subtype did not exist in the intact tracheal smooth muscle cells. 5. Competition with cardioselective antagonist (M2), methoctramine; smooth muscle selective antagonists (M3), hexahydrodifenidol and hexahydrosiladifenidol; as well as carbachol, were best fit by a two-binding site model. The results suggest that both M2 and M3 receptor subtypes exist at the cell surface of tracheal smooth muscle cells.

Characterization of muscarinic cholinergic receptors in membrane preparations from rat prostatic adenocarcinoma

The binding characteristics of 3H-quinuclidinyl benzilate (QNB) to muscarinic sites in isolated plasma membrane fractions from R-3327 Dunning tumors (H and AT-1 sublines); ventral, dorsolateral prostate; and urinary bladder of the rat were studied. QNB binding to all preparations, except from AT-1 tumors, was specific, saturable, and of high affinity. The AT-1 tumors completely lacked specific QNB binding. The muscarinic receptor density in H tumors was twofold and twentyfold higher than that in the ventral prostate and dorsolateral prostate respectively. The receptor density in the urinary bladder was approximately twofold higher than that in H tumors. The Kd values in H tumors and ventral prostate were very similar and significantly higher than that in dorsolateral prostate or the urinary bladder. QNB binding in H tumors was strongly inhibited by classical muscarinic receptor antagonists atropine and scopolamine, but poorly by the agonists carbacholine and pilocarpine. In contrast to scopolamine or atropine, inhibition by pirenzepine and AF-DX116 was relatively low. These data indicate that the muscarinic receptor in Dunning H tumors is of M3 type.

Identification and characterization of muscarinic cholinergic receptors in the human urinary bladder and parotid gland

The binding characteristics of [3H]quinuclidinyl benzilate (QNB) to muscarinic sites in isolated plasma membrane fractions of the human urinary bladder and parotid gland were studied. QNB binding to both preparations was of high affinity and low capacity. Mean values for the apparent dissociation constants (Kd) for binding to membrane preparations from the urinary bladder and parotid glands were 22 and 34 pM and the Bmax values 234 and 456 fmol/mg protein, respectively. Significance of difference between Kd and Bmax values from the two tissues was at the level of P less than 0.005 and P less than 0.05, respectively. QNB binding was inhibited by muscarinic receptor antagonists with varying degree of effectiveness. The mean values for the inhibition constant (Ki) were significantly lower for oxybutynin, amitriptyline, and pirenzepine but higher for secoverine in preparations of the urinary bladder than of the parotid gland. The mean Ki values for quinidine and verapamil were lower in the urinary bladder than that in the parotid gland. Carbachol exhibited a marked selectivity for the urinary bladder (about 30-fold) compared with the parotid gland. The present data obtained in two human tissues that are highly cholinergic in their innervation give support to the argument for heterogeneity of the muscarinic cholinergic receptors.

Comparison of muscarinic acetylcholine binding in the urinary bladder and submandibular gland of the rabbit

In order to explore the possibility of heterogeneity in peripheral muscarine receptors, receptors were characterized in membrane fractions isolated from rabbit urinary bladder and submandibular gland. With [3H]QNB as radioligand, specific binding with very high affinity was found in both preparations. Although the BmaxS for binding in the two preparations were very similar, the mean KD value in the submandibular gland was significantly higher (P less than 0.005) than that in the bladder. Among the anticholinergic drugs, oxybutynin had a significantly lower value for the inhibition constant (Ki) in the submandibular gland whereas Ki for both secoverine and pirenzepine was significantly higher in this tissue than in urinary bladder. The Ki for carbacholine was about 7-fold higher in submandibular gland than in the bladder. Although quinidine and verapamil showed relatively weak binding to the muscarine receptor site, their Ki in the submandibular gland was significantly higher than that in the bladder. The results indicate that although there is a considerable similarity between muscarinic receptors in urinary bladder and submandibular gland, the differences in Ki values for different compounds in the two tissues support the argument favouring heterogeneity of muscarinic acetylcholine receptors in peripheral effector organs.