α-Adrenoceptor-effector coupling: affinity states or heterogeneity of the α2-adrenoceptor?

117

3 The affinities of 8-OH-DPAT, RU 24969 and methysergide determined against [3H]-idazoxan binding to the cortex and hippocampus correlate significantly with the binding site displaying low affinity for prazosin and previously designated a2A. In contrast, a poor correlation exists for the high affinity site for prazosin designated a2B* 4 [3H]-idazoxan, in the presence of 3Mm yohimbine, labels a site that displays high affinity towards cirazoline, naphazoline and guanabenz, but low affinity towards clonidine, p-aminoclonidine, adrenaline, noradrenaline and the a2-adrenoceptor antagonists yohimbine, rauwolscine, WY 26703 and BDF 6143.5 The results of this study indicate that [3H]-yohimbine labels two sites; the a2A-and a2B-adrenoceptors whereas [3H]-idazoxan labels an a2-adrenoceptor with a profile consistent with the CX2A-adrenoceptor subtype. In addition, [3H]-idazoxan labels an imidazoline binding site in the rat cortex that is pharmacologically distinct from a2-adrenoceptors. The low affinity of clonidine and p-aminoclonidine indicates that the imidazoline-like binding site in rat cortex is different from the site labelled by [3H]-clonidine and [3H]-p-aminoclonidine in human, rat and bovine brain stem, providing evidence of potential heterogeneity within this class of binding sites.

Section: Introductionmentioning

confidence: 99%

“…'Selective tissues', such as human platelet OX2A) (Nahorski et al, 1985), neonatal rat lung (a2B) (Latifpour et al, 1982) and selective cell lines HT29 (xe2A) and NG108-15 (@2B) have also been described (Bylund et al, 1988) which may be useful in further studies of the proposed subtypes.…”

Section: Introductionmentioning

confidence: 99%

α₂‐Adrenoceptor subtypes and imidazoline‐like binding sites in the rat brain

Brown

MacKinnon

McGrath

et al. 1990

117

“…Although prazosin is clearly an o,-adrenoceptor antagonist, it has also been shown to display a greater selectivity for the L.2b-adrenoceptor subtype over other LX2-subtypes (Uhlen & Wikberg, 1991a). Based on the low affinity of prazosin for the 22a-subtype and high affinity for the 12b-subtype, the initial distinction between these two subtypes was discerned (Bylund 1985;Nahorski et al, 1985). Thus, we hypothesize that the free water response to clonidine was mediated by the 12b-adrenoceptor subtype.…”

Section: Discussionmentioning

confidence: 99%

Clonidine‐induced increase in osmolar clearance and free water clearance via activation of two distinct α₂‐adrenoceptor sites

Intengan

Smyth

1996

1 Clonidine, an a2-adrenoceptor agonist, will increase urine flow rate in the anaesthetized rat by increasing both free water and osmolar clearance. In the present study, we investigated whether these effects of clonidine were mediated at two sites which could be distinguished pharmacologically in uninephrectomized male Sprague-Dawley rats. 2 Clonidine (1.0 nmol kg-' min-1) infused into the renal artery increased osmolar and free water clearance. Following pretreatment with prazosin (0.15 mg kg-', i.v.), an antagonist with reported selectivity for the a2b-adrenoceptor subtype, the increase in free water but not osmolar clearance was decreased. Pretreatment with the opioid receptor antagonist, naltrexone (3.0 mg kg-', i.v.) attenuated the increase in osmolar but not free water clearance. This disparate antagonism of clonidine by prazosin and naltrexone was consistent with two distinct sites. 3 We submit the hypothesis that the au-and a2b-adrenoceptor subtypes mediated the clonidine-induced osmolar and free water clearance. The blockade in free water clearance by prazosin indicated a possible role of the a2b-adrenoceptor subtype whereas the au,-adrenoceptor subtype was considered as the site mediating the clonidine-induced increase in osmolar clearance. UK-14,304 (1.0 nmol kg-' min-'), a mixed a2-adrenoceptor/imidazoline receptor agonist with selectivity for the oeu-subtype, increased only osmolar clearance. This increase was blocked by naltrexone but not prazosin pretreatment. The imidazoline receptor was not involved, as naltrexone failed to alter the moxonidine (3.0 nmol kg-' -min-') induced increase in osmolar clearance. These data suggested to us that the aux-/a2b-subtype hypothesis should be investigated more closely in future studies. 4 These findings indicate that the increase in osmolar and free water clearance following clonidine can be distinguished pharmacologically indicating that two sites were involved. Furthermore, we propose the hypothesis that the acx-adrenoceptor subtype mediated osmolar clearance whereas the 12b-subtype mediated free water clearance. The prazosin-sensitive increase in free water clearance following clonidine suggested a possible role for the x2b-subtype. The naltrexone-sensitive increase in osmolar clearance following clonidine and UK-14,304 (but not moxonidine) suggested a possible role of the au-subtype.Clearly, this postulate requires further study.

“…The M2A subtype has low affinity for prazosin and high affinity for oxymetazoline whereas the a2B-subtype has higher affinity for prazosin and lower affinity for oxymetazoline (Cheung et al, 1982;Bylund, 1985;Nahorski et al, 1985;Petrash & Bylund, 1986). The a2-adrenoceptor on human platelets is considered as a pure population of the x2A-adrenoceptor subtype (Cheung et al, 1982) whereas neonatal rat lung contains a homogeneous population of the E2B-adrenoceptor subtype (Latifpour et al, 1982).…”

Section: Introductionmentioning

confidence: 99%

Sodium modulation of ³H‐agonist and ³H‐antagonist binding to α₂‐adrenoceptor subtypes

MacKinnon¹,

Spedding²,

Brown³

1993

1 The a2-adrenoceptors on human platelets and neonatal rat lung were characterized with the agonist and antagonist ligands [3H]-adrenaline and [3H]-RS-15385-197 respectively. A correlation of affinities for 3H-antagonist binding showed the receptors to be of the M2A-(platelet) and x25-(neonatal rat lung) adrenoceptor subtypes, whereas a correlation of affinities for 3H-agonist binding showed the receptors to have similar characteristics (r = 0.88). 2 NaCI (100 mM) had no effect on [3H]- RS-15385-197 13 fold. The affinity of the catecholamines, adrenaline and noradrenaline was significantly decreased, whereas the imidazolines, oxymetazoline and UK-14,304 were much less affected. The affinity of prazosin and imiloxan for [3H]-adrenaline binding was significantly increased in the presence of 10 and 100 mM NaCl. Conversely, the affinity of adrenaline and noradrenaline was decreased in the presence of NaCl, and there was no change in the affinity of the imidazoline agonists. 4 In the human platelet, NaCl had no effect on the affinity of prazosin for [3H]-RS-15385-197 binding but the affinity of imiloxan was significantly increased. NaCl significantly decreased the affinity of the catecholamines adrenaline and noradrenaline, whereas the affinity of 304 and oxymatazoline was much less affected. Competition experiments with [3H]-adrenaline in the presence of NaCl in platelets were difficult to characterize as there was no specific binding under these conditions. 5 The results show that both the M2A-and a2B-adrenoceptor subtypes are allosterically regulated by Na+, but only the a2B-subtype showed a significant increase in density. Interestingly, there is a differential regulation of imidazoline (unchanged) and catecholamine (decreased affinity) agonist interactions with these subtypes. Na+ may therefore critically affect receptor subtype selectivity of drugs. The implications for receptor subclassification are discussed.