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Classical and atypical binding sites for β‐adrenoceptor ligands and activation of adenylyl cyclase in bovine skeletal muscle and adipose tissue membranes
Author(s) -
Sillence M.N.,
Matthews M.L.
Publication year - 1994
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1994.tb14818.x
Subject(s) - adipose tissue , skeletal muscle , medicine , endocrinology , iodocyanopindolol , second messenger system , ligand (biochemistry) , isoprenaline , radioligand , chemistry , population , biology , receptor , stimulation , intrinsic activity , agonist , environmental health
1 The radioligand [ 125 I]‐iodocyanopindolol ([ 125 I]‐ICYP) was used under standard ligand binding conditions, to detect β 1 and β 2 ‐adrenoceptors in membrane preparations from bovine skeletal muscle and adipose tissue. High concentrations of [ 125 I]‐ICYP were also used, to identify an ‘atypical’ binding site in skeletal muscle. Finally, adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) production was measured in the same membrane preparations, to determine the relationship between the β‐adrenoceptor sub‐types present and the production of this second‐messenger. 2 According to the results of radioligand binding studies, both skeletal muscle and adipose tissue membranes have β 2 ‐adrenoceptors, characterized by a high affinity for the β 2 ‐selective antagonist, ICI 118551 (p K 8.3 and 8.6 respectively); and a low affinity for the β 1 ‐selective antagonist CGP 20712A (p K 5.2 in both tissues). Antagonism of (–)‐isoprenaline‐stimulated cyclic AMP production by low concentrations of ICI 118551, yielded pseudo pA 2 values in muscle and adipose tissue of 7.6 and 8.7 respectively, confirming that β 2 ‐adrenoceptors in these tissues are linked to the production of the second‐messenger. 3 Although β 1 ‐adrenoceptors could not be detected in either skeletal muscle or adipose tissue membranes by use of ligand binding techniques, high pseudo pA 2 values were obtained (8.0 and 8.2 respectively), when CGP 20712A was used to block the stimulation of cyclic AMP production by (–)‐isoprenaline. This finding is consistent with the presence in both tissues of a population of β 1 ‐adrenoceptors which is small, but efficiently coupled to the second‐messenger. 4 In addition to identifying standard β 1 ‐ and β 2 ‐adrenoceptors, it was also established that skeletal muscle membranes have an ‘atypical’ binding site which has a relatively low affinity for [ 125 I]‐ICYP (p K 8.84), but which exists in abundance. At high concentrations of radioligand, the ‘latypical’ site accounted for 89% of the total [ 125 I]‐ICYP binding sites present. 5 The results of second‐messenger studies do not support the hypothesis that skeletal muscle or adipose tissue membranes contain functional β 3 ‐adrenoceptors: based on the failure of a β 3 ‐adrenoceptor‐selective agonist (BRL 37344) to stimulate cyclic AMP production, the absence of a triphasic response to (–)‐isoprenaline, and the observation that cyclic AMP production was not resistant to blockade by either ICI 118551 or CGP 20712A. 6 It is concluded that data from radioligand binding studies do not accurately reflect the contribution made by β 1 ‐ and β 2 ‐adrenoceptors to cyclic AMP production in bovine skeletal muscle and adipose tissue membranes. Furthermore, the ‘atypical’ [ 125 I]‐ICYP binding site identified in bovine skeletal muscle does not represent a functional bovine β 3 ‐adrenoceptor.