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

Abstract

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.