(�)[125Iodo]cyanopindolol, a new ligand for ?-adrenoceptors: Identification and quantitation of subclasses of ?-adrenoceptors in guinea pig

Abstract

(+/-)[125Iodo] cyanopindolol (ICYP) is a radioligand which binds with an extraordinarily high affinity and specificity to beta-adrenoceptors. In contrast to (+/-) [125Ido]-hydroxybenzylpindolol (IHYP), the new ligand has neither affinity to alpha-nor to 5-HT-receptors. The dissociation constants of ICYP for beta- adrenoceptors in various tissues range from 27 to 40 pM, thereby exceeding the affinity of IHYP by a factor of approximately 3. ICYP does not discriminate between beta 1- and beta 2-adrenoceptors. Therefore, the densities of the two receptor subtypes can be determined from competition curves of ICYP by drugs previously found to show in vitro selectivity for beta 1-adrenoceptors. The guinea pig left ventricle contains only beta 1-adrenoceptors, whereas in a lung tissue, the ratio of beta 1-to beta 2-adrenoceptors is 1 to 4. The calculated affinities of five beta 1-selective antagonists for beta 1-adrenoceptors were nearly identical in the ventricle and the lung. Kinetic studies of ICYP binding to guinea pig lung membranes indicated that the dissociation reaction consists of two components, a fast process (t 1/2 = 9 min) and a slower process (t 1/2 = 8.8 h). A mathematical treatment revealed two possibilities of interpretation: 1. Two forms of the receptor exist which are interconvertible. 2. The (+)- and (-)- enantiomers of ICYP dissociate with different rate constants. The low dissociation constant of ICYP in combination with its high specific radioactivity (2175 Ci mmole -1) allows binding studies to be carried out with small protein and ligand concentrations, e.g. 3 microgram protein per assay in guinea pig lung membranes.

α1‐Adrenoceptor antagonist properties of CGP 12177A and other β‐adrenoceptor ligands: evidence against β3‐ or atypical β‐adrenoceptors in rat aorta

“…Later, a third b-adrenoceptor was cloned and the pharmacological properties of this b 3 -adrenoceptor were shown to correspond to those previously attributed to 'atypical' b-adrenoceptors (Emorine et al, 1989). b 3 -Adrenoceptors are pharmacologically characterised by (i) low affinity of classical b-adrenoceptor antagonists, (ii) activation by selective agonists, such as BRL 37344 (Arch et al, 1984), (iii) activation by 'nonconventional partial agonists' (potent b 1 -/b 2 -adrenoceptor antagonists with b 3 -agonist activity at higher concentrations (Kaumann, 1989) such as cyanopindolol (Engel et al, 1981) and CGP 12177A (Mohell & Dicker, 1989)) and (iv) blockade by selective b 3 -adrenoceptor antagonists such as SR 59230A (Manara et al, 1996). A further atypical b-adrenoceptor, designated the putative b 4 -adrenoceptor, was also characterised in the heart (Kaumann, 1989;Kaumann & Molenaar, 1996;Malinowska & Schlicker, 1996) and in adipose tissue (Galitzky et al, 1997), which shared properties (i) and (iii) above but not (ii) and (iv).…”

Tracheal Relaxing Effects and β2-Selectivity of TA-2005, a Newly Developed Bronchodilating Agent, in Isolated Guinea Pig Tissues

“…In the lung membrane preparation, the ex periment was conducted in the presence of an appropriate concentration (10-7 M) of the highly 81-selective antagonist bisoprolol to suppress X31-adrenoceptors which occupy 20% of the total /3-adrenoceptors (11,12). All the experiments were performed both in the pres ence and absence of 0.2 mM guanosine 5' triphosphate (GTP).…”

“…Though the guinea pig ventricle con tains a homogeneous population of R 1-adre noceptor, 20% of the /3-adrenoceptors are of the /31-subtype in lung membranes (11,12,21). Thus, we prepared a relatively pure ,82 adrenoceptor preparation by including highly selective /31-antagonist bisoprolol (10-7 M) in the lung membrane to suppress ,81-adre noceptors.…”

Mechanisms for the emergence of catecholamine‐sensitive adenylate cyclase and β‐adrenergic receptors in cultured hepatocytes

“…,&Adrenoceptor binding was assessed by incubating washed particles with ( +)- ['251]cyanopindolol [29]. The cells were lysed as described above, the lysates were suspended in 50 mM Tris (pH 7.5), 30 mM KCl, 4 mM MgCL, and particles were prepared by centrifuging at 40000 x g for 20 min, washing and respinning.…”