Alpha-1 adrenoceptor subtypes in canine aorta

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.

Vascular alpha-1 adrenergic receptor subtypes in the regulation of arterial pressure

Alpha 1 (alpha 1)-adrenoceptors can be found at numerous end organs in the autonomic nervous system, especially vascular smooth muscle. The tonic sympathetic activation of vascular alpha 1-adrenoceptors maintains vascular resistance and is vital to the regulation of arterial pressure. Recent evidence clearly demonstrates that alpha 1-adrenoceptors are a heterogenous class of receptors and that each subtype may subserve specific cardiovascular functions. Elucidation of the physiological role of each subtype in the regulation of vascular resistance and arterial pressure will enhance our understanding of the cardiovascular system and may facilitate the development of therapeutics with improved efficacy and tolerability.

Alpha-1 and beta-adrenergic receptor blocking potencies of bopindolol and its two metabolites (18-502 and 20-785) as assessed by radioligand binding assay methods

1. The pKi value of bopindolol for alpha 1High-subtypes in canine aorta, rat hearts and rat brain was 5.71, 5.52 and 6.56, respectively. In addition, the pKi values of these drugs in canine aorta, rat hearts and rat brain for the alpha 1Low-subtype was very low. 2. The phenylephrine induced-contractions of aortae of guinea pigs and rats were not inhibited by these agents. 3. Both bovine hearts and tracheal smooth muscles indicated that 18-502 had the highest pKi value to beta 1- and beta 2-adrenoceptor subtypes and the rank order of these beta-blocking potencies were 18-502 > bopindolol > 20-785.

Vascular alpha-adrenoceptors: from the gene to the human

Adrenoceptors can be subdivided into three major types, the alpha 1-, alpha 2-, and beta-adrenoceptors. Each of these types can be further subdivided into three subtypes, based on pharmacological characteristics. Molecular cloning techniques have supported this subclassification. Recent data now suggest that alpha-adrenoceptor subtypes identified by pharmacological and molecular techniques correspond well, although species orthologs of several adrenoceptor subtypes have been identified. The secondary structure of the adrenoceptors has been elucidated and correlated with their interaction with second messenger molecules. alpha 1-Adrenoceptors, beta-adrenoceptors, and alpha 2-adrenoceptors mediate their actions through stimulation of inositol phosphate release, stimulation of adenylate cyclase, and inhibition of adenylate cyclase, respectively. Site-directed mutagenesis and the preparation of chimeric receptors have located the site of receptor--second messenger interaction to the third intracellular loop for each of these adrenoceptors. While subtypes of each of these classes all interact with the same second messenger, studies with recombinant alpha 2-adrenoceptors show subtype-related differences in receptor--second messenger interaction. Multiple alpha-adrenoceptor subtypes are expressed in vascular smooth muscle and are involved in various aspects of blood vessel function, including contraction, cellular growth, and proliferation. Various physiological factors can selectively influence responses to a particular subtype, and the relative roles of each subtype can vary between vascular beds and along an individual blood vessel as its caliber changes. Functional studies in blood vessels suggest the presence of additional alpha-adrenoceptor subtypes not yet identified via molecular techniques. Optimization of the therapeutic profile of an alpha-adrenoceptor antagonist may be possible via enhancement of selectivity for a particular subtype or by design of a specific profile of affinity for the individual subtypes.