Pharmacologically distinct phenotypes of α_1B‐adrenoceptors: variation in binding and functional affinities for antagonists

Abstract

BACKGROUND AND PURPOSEThe pharmacological properties of particular receptors have recently been suggested to vary under different conditions. We compared the pharmacological properties of the α1B-adrenoceptor subtype in various tissue preparations and under various conditions. EXPERIMENTAL APPROACH[ 3 H]-prazosin binding to α1B-adrenoceptors in rat liver (segments, dispersed hepatocytes and homogenates) was assessed and the pharmacological profiles were compared with the functional and binding profiles in rat carotid artery and recombinant α1B-adrenoceptors. KEY RESULTSIn association and saturation-binding experiments with rat liver, binding affinity for [ 3 H]-prazosin varied significantly between preparations (KD value approximately ten times higher in segments than in homogenates). The binding profile for various drugs in liver segments also deviated from the representative α1B-adrenoceptor profile observed in liver homogenates and recombinant receptors. L-765,314 and ALS-77, selective antagonists of α1B-adrenoceptors, showed high binding and antagonist affinities in liver homogenates and recombinant α1B-adrenoceptors. However, binding affinities for both ligands in the segments of rat liver and carotid artery were 10 times lower, and the antagonist potencies in α1B-adrenoceptor-mediated contractions of carotid artery were more than 100 times lower than the representative α1B-adrenoceptor profile. CONCLUSIONS AND IMPLICATIONSIn contrast to the consistent profile of recombinant α1B-adrenoceptors, the pharmacological profile of native α1B-adrenoceptors of rat liver and carotid artery varied markedly under various receptor environments, showing significantly different binding properties between intact tissues and homogenates, and dissociation between functional and binding affinities. In addition to conventional 'subtype' characterization, 'phenotype' pharmacology must be considered in native receptor evaluations in vivo and in future pharmacotherapy.

The role of α1- and α2-adrenoceptor subtypes in the vasopressor responses induced by dihydroergotamine in ritanserin-pretreated pithed rats

“…(systemic) administration of compounds and the additional role of pharmacokinetic factors (which cannot be completely ruled out in pithed rats). Consistent with these views, other studies performed in vivo with these compounds have also shown limited selectivity [ 31 ]. Notwithstanding, the pithed rat model is predictive of (cardio)vascular side effects [ 11 , 12 ] and provides information that cannot be obtained from in vitro studies [ 32 ].…”

Regulation of synaptic acetylcholine concentrations by acetylcholine transport in rat striatal cholinergic transmission

“…, ; Yoshiki et al . ). Briefly, the rat striatum was carefully cut into small pieces at 4°C, using a razor and fine ophthalmic scissors under a stereoscopic microscope.…”

“…At the end of the incubation with 50 nM [ 3 H]ACh for 20 min or 50 nM [ 3 H]choline for 10 min, 3 mL ice‐cold incubation medium was added and the segments were washed for 15–20 s. Finally, the segments were dissolved in 0.3 M NaOH, and the radioactivity and protein content were measured (Yoshiki et al . ). Nonspecific tissue accumulation of both radioligands was determined after incubation in the presence of 3 mM tetraethylammonium (TEA) for [ 3 H]ACh uptake or 3 mM hemicholinium‐3 (HC‐3) for [ 3 H]choline uptake (Muramatsu et al .…”

“…This solution was oxygenated with a mixture of 95% O 2 and 5% CO 2 and maintained at 0°C (Muramatsu et al 2015). Then the striatal segments were prepared according to a previously described method (Muramatsu et al 2005(Muramatsu et al , 2016Yoshiki et al 2014). Briefly, the rat striatum was carefully cut into small pieces at 4°C, using a razor and fine ophthalmic scissors under a stereoscopic microscope.…”

Label-Free Kinetics: Exploiting Functional Hemi-Equilibrium to Derive Rate Constants for Muscarinic Receptor Antagonists

“…A number of "antagonists" have been shown to display hitherto unappreciated agonistic effects, for example, propranolol and carvedilol at the b 2 -adrenoceptor (Baker et al, 2003a;Wisler et al, 2007) and atropine at the muscarinic M 3 receptor (Stewart et al, 2010). Furthermore, there is no a priori reason why antagonists may not display differential affinities dependent on the pathway/effector system being monitored (Kenakin, 2014); propranolol has a 5-fold higher affinity to inhibit adrenaline-stimulated cAMP accumulation compared with CRE-SPAP transcription at the b 2 -adrenoceptor (Baker et al, 2003b), and even larger differences have been observed for a 1B -adrenoceptor antagonists (Yoshiki et al, 2014). The reason for multiple antagonist affinity states remains largely unknown, but it is possible that the residence time of ligands may contribute to differential signal pathway activation/ inhibition.…”