Alpha 2-adrenoceptors in homogenates of rat nasal mucosa

Evidence that Drug Binding to Non-Adrenergic [3H]-Idazoxan Binding Sites (I-Receptors) Occurs to Interacting or Interconvertible Affinity Forms of the Receptor

Characterization of [3H]idazoxan binding to guinea pig kidney membranes showed that approximately 90% bound to nonadrenergic I-receptors and approximately 10% to alpha 2-adrenoceptors. I-Receptors could be studied separately by including 3 microM rauwolscine to the assay. During these conditions 22 different compounds out of 29 (including imidazoline and guanidinium compounds) generated biphasic or shallow competion curves (Hill coefficients down to 0.57), for which computer modelling suggested that drugs bound to two sites with different affinities. However, the proportion of sites varied considerably depending on which drug was used as competitor; the variation being from approximately 50/50% to approximately 8/92% and analysis of variance clearly indicated that the variation of proportions of sites could be attributed to an effect induced by the drugs which indicated that the sites were dynamically formed or modulated by the presence of the drug. A few drugs (guanabenz, (-)-medetomidine, phentolamine, clemastin, prazosin and idazoxan itself) yielded steep uniphasic curves (Hill coefficients near unity) which were resolved only into one site fits. One drug (detomidine) yielded supersteep competition curves (Hill coefficient 1.29), the data being reminiscent of positive cooperativity. In another set of experiments attempts were made to block one of the affinity forms of the I-receptor with histamine, a compound which had grossly different affinities for the two I-receptor sites, and competition curves were then obtained using other drugs which also seemed to distinguish between the two sites. Computer analysis of experimentally obtained as well as simulate,d computer generated, competition curves clearly showed that the kidney I-receptors did not exist in two independent non-interacting forms. Instead the data suggested that the I-receptor was composed of interacting or interconverting receptor entities with different affinities for drugs. Monovalent cations such as Cs+ or NH4+ were found to be powerful inhibitors of [3H]idazoxan binding to the kidney I-receptors, the ions decreasing both the apparent Bmax and affinity of the ligand for the receptor. The cations also decreased the affinities of UK-14,304 for both the high and low affinity forms of the I-receptor, but the apparent proportions of sites were not at all affected by the ions. Moreover, ligand binding to kidney I-receptor were not at all affected by non-hydrolyzable guanine nucleotides. It is suggested that the binding data reflects functional properties of the I-receptor protein.

Rat spinal cord alpha 2-adrenoceptors are of the alpha 2A-subtype: comparison with alpha 2A- and alpha 2B-adrenoceptors in rat spleen, cerebral cortex and kidney using 3H-RX821002 ligand binding

Binding of the alpha 2-adrenoceptor antagonist radioligand 3H-RX821002 was investigated in membranes from rat spinal cord, spleen, cerebral cortex and kidney. The ligand was found to bind to saturable binding sites with apparent uniform affinities within each tissue. Seven compounds, some of which have previously been reported to be selective for either alpha 2A- or alpha 2B-adrenoceptors, were used in competition with 3H-RX821002. By using computer modelling, competition curves generated for three of these compounds (ARC 239, prazosin and oxymetazoline) could be resolved into two site fits in the kidney, Kd's of the drugs being compatible with the notion that these sites corresponded to alpha 2A- and alpha 2B-adrenoceptors. Moreover, rauwolscine and yohimbine were found to be about 14 and 9-fold selective for alpha 2B-adrenoceptors in the kidney. In all other tissues studied drug competition curves were uniphasic and computer modelled into one site fits, drug Kd's being well correlated to those for the alpha 2A-adrenoceptor. In rat spinal cord 26 further drugs, which showed wide variation in structure, were evaluated in competition with 3H-RX821002. Of these compounds, competion curves of the agonists UK-14,304, (-) and (+) adrenaline were modelled into two site fits whereas those of the remaining compounds could be modelled only into one site fits. Since the high affinity site for UK-14,304, (-) and (+) adrenaline was eliminated when EDTA, Gpp(NH)p and 140 mM NaCl was present in the assay the heterogeniety observed in spinal cord was considered to be due to formation of high and low affinity conformations of the alpha 2-adrenoceptor for agonists. It is concluded that 3H-RX821002 is useful to label both alpha 2A- and alpha 2B-adrenoceptors in the rat. Moreover, the binding sites labelled by 3H-RX821002 in the spinal cord appear to consist of a single population of alpha 2-adrenoceptors of the alpha 2A-type.

Delineation of rat kidney alpha 2A- and alpha 2B-adrenoceptors with [3H]RX821002 radioligand binding: computer modelling reveals that guanfacine is an alpha 2A-selective compound

We developed a method for the simultaneous determination of drug affinity constants for rat alpha 2A- and alpha 2B-adrenoceptor subtypes by using [3H]RX821002 radioligand binding in the kidney. Three competition curves were obtained for each drug: one for the test compound in the absence of ARC 239 (a drug found to have 108-fold higher affinity for alpha 2B- than for alpha 2A-adrenoceptors), one in the presence of ARC 239, and one for ARC 239. It is possible to determine the Kds of a tested drug for both alpha 2A- and alpha 2B-adrenoceptors by simultaneous computer modelling because of the increased constraint in the calculations given by the inclusion of ARC 239 into the assay. Using this approach, we found guanfacine and oxymetazoline to be highly alpha 2A-selective. The most alpha 2B-selective were ARC 239, prazosin and corynanthine. A number of other drugs, for example UK-14,304, rilmenidine and clonidine, were non-selective or showed minor selectivity for alpha 2A- or alpha 2B-adrenoceptors. Moreover, using Monte Carlo simulations, we showed that the three-curve method gives more accurate estimates of drug binding constants for assays when two receptor sites are present than methods analysing only one competition curve.