PREDOMINANT EXPRESSION OF BETA-1-ADRENERGIC RECEPTOR IN THE THICK ASCENDING LIMB OF RAT-KIDNEY - ABSOLUTE MESSENGER-RNA QUANTITATION BY REVERSE TRANSCRIPTION AND POLYMERASE CHAIN-REACTION

Beta1- and beta2-adrenergic receptor (beta-ARs) expression in the thick ascending limb of rat kidney was studied at the level of mRNA and receptor coupling to adenylyl cyclase. Absolute quantitation of beta1- and beta2-AR mRNAs in microdissected nephron segments was performed with an assay based on reverse transcription and polymerase chain reaction, using in vitro transcribed mutant RNAs as internal standards. In the cortical thick ascending limb (CTAL), the number of mRNA molecules/mm of tubular length was 2,806+/-328 (n = 12) for beta1-AR and 159+/-26 for beta2-AR (P < 0.01 ). Lower levels were obtained in the medullary thick ascending, beta1-AR mRNA still being predominant. The pharmacological properties of beta-ARS was also studied in the CTAL. Cyclic AMP accumulation was stimulated by beta-agonist with a rank order of potency of isoproterenol > norepinephrine > epinephrine. This observation, and the higher efficiency of a beta1 than of a beta2, antagonist to inhibit isoproterenol-induced cAMP accumulation, establish the typical beta1-AR sensitivity of the CTAL. No detectable contribution of atypical or beta3-ARs to adenylyl cyclase stimulation could be found. In conclusion, this study, which shows markedly different levels of beta1- and beta2-AR mRNAS in the CTAL, provides a molecular basis for the predominant expression of the beta1 receptor subtype in this nephron segment.