PHARMACOLOGICAL AND MOLECULAR CHARACTERIZATION OF BETA-ADRENOCEPTORS IN ADULT-RAT DIAPHRAGM MUSCLE

Using pharmacological and molecular approaches to investigate beta-adr enoceptor (beta-AR) subtype expression in adult rat diaphragm, we foun d that adenylyl cyclase (AC) was potently stimulated by the beta(2)-AR -selective agonist fenoterol, weakly stimulated by the beta(1)-AR-sele ctive agonist prenalterol and unaffected by the beta(3)-AR agonist CGP 12177. AC activity in response to a submaximal isoproterenol concentra tion was potently inhibited by the beta(2)-AR-selective antagonist ICI 118551, whereas the beta(1)-AR-selective antagonist CGP20712A was effe ctive only in very high concentrations. (-)-[I-125]-cyanopindolol ([I- 125]-CYP) saturation binding experiments indicated a single affinity c omponent (dissociation constant (K-d) = 22 +/- 2 pM) for beta-AR sites (maximal beta-AR density (B-max) = 14 +/- 2 fmol/mg). Eadie-Hofstee a nalysis of [I-125]-CYP displacement curves by beta(1)-, beta(2)- or be ta(3)-AR-selective ligands allowed to characterise a homogenous popula tion of beta(2)-AR sites. Finally, reverse transcriptase-polymerase ch ain reaction analysis of beta-AR subtype mRNAs identified beta(2)-AR t ranscripts but no beta- and beta(3)-AR mRNAs. Our results demonstrate that beta(2)-AR is the only beta-AR subtype expressed in the diaphragm . (C) 1998 Elsevier Science B.V. All rights reserved.