Effects of deoxyribonuclease I and neuraminidase treatments on the specific binding of 3H-prazosin and 3H-quinuclidinyl benzilate (3H-QNB) to alpha-adrenergic and muscarinic receptors in rat myocardial membranes

Tamsulosin: assessment of affinity of 3H-prazosin bindings to two alpha1-adrenoceptor subtypes (alpha1H and alpha1L) in bovine prostate and rat heart and brain

1. The present study was designed to assess the displacement potencies of tamsulosin to 3H-prazosin bindings in two alpha1-adrenoceptor (AR) subtypes (alpha1H and alpha1L) in bovine prostate, rat heart and brain compared with those of amosulalol, labetalol, ketanserin, clonidine and propranolol. 2. The pKi values of tamsulosin to alpha1H and alpha1L subtypes in bovine prostate were 9.13 and 8.99 and these values were almost the same as those of prazosin. On the other hand, low pKi binding values of amosulalol, labetalol, ketanserin, clonidine and propranolol to these subtypes were observed. 3. Low pKi values of tamsulosin to alpha2- and beta-ARs and muscarinic and 5HT2 receptors in the rat brain were observed. 4. These results suggest that tamsulosin has high affinities to alpha1L-AR subtypes in bovine prostate and rat hearts as well as alpha1H-AR subtypes, implying an inhibitory effect of this drug on the contraction of the prostate.

Sialic acid is selectively involved in the interaction of agonists with M2 muscarinic acetylcholine receptors

Neuraminidase and slight acid hydrolysis were used to investigate the role of sialic acid residues in the binding of muscarinic agonists and antagonists to membranes from tissues rich in M1 and M2 receptors. Membranes were pretreated with neuraminidase at pH 5 and the binding parameters were determined from competitive experiments with (3H)-quinuclidinylbenzylate. The removal of sialic acid residues reduced the affinity of muscarinic agonists for cerebellum, heart and lung membranes (M2), in contrast to striatum (M1). The affinity of antagonists was not affected. Thus, sialic acid is selectively involved in the interaction of agonists with M2 muscarinic receptors.