Pharmacological and molecular characterisation of beta-adrenoceptors in adult rat diaphragm muscle.

Using pharmacological and molecular approaches to investigate beta-adrenoceptor (beta-AR) subtype expression in adult rat diaphragm, we found that adenylyl cyclase (AC) was potently stimulated by the beta2-AR-selective agonist fenoterol, weakly stimulated by the beta1-AR-selective agonist prenalterol and unaffected by the beta3-AR agonist CGP12177. AC activity in response to a submaximal isoproterenol concentration was potently inhibited by the beta2-AR-selective antagonist ICI118551, whereas the beta1-AR-selective antagonist CGP20712A was effective only in very high concentrations. (-)-[125I]-cyanopindolol ([125I]-CYP) saturation binding experiments indicated a single affinity component (dissociation constant (Kd) = 22 +/- 2 pM) for beta-AR sites (maximal beta -AR density (Bmax) = 14 +/- 2 fmol/ mg). Eadie-Hofstee analysis of [125I]-CYP displacement curves by beta1-, beta2- or beta3-AR-selective ligands allowed to characterise a homogeneous population of beta2-AR sites. Finally, reverse transcriptase-polymerase chain reaction analysis of beta-AR subtype mRNAs identified beta2-AR transcripts but no beta1- and beta3-AR mRNAs. Our results demonstrate that beta2-AR is the only beta-AR subtype expressed in the diaphragm.