Cloning of the Rat o ! 1 c-Adrenergic Receptor From Cardiac Myocytes £ y 1 C al 1 B , and aED mRNAs Are Present in Cardiac Myocytes but Not in Cardiac Fibroblasts

a,-Adrenergic receptor (AR) activation in cardiac muscle has several different physiological effects that might be mediated through different a,-AR subtypes. Two a,-AR subtypes have been cloned from the rat, the alB and the alD; both are present in adult rat heart. A third subtype, the a,c, cloned from the cow and human, was reported to be absent in the rat. However, we recently found alc mRNA in adult rat heart by using a partial alc cDNA. Thus, all three cloned a,-AR subtypes are present in the heart, but it is unknown whether each is expressed in cardiac myocytes or in cardiac fibroblasts. In the present study, the full-length rat alc-AR was cloned from cultured neonatal cardiac myocytes. alc mRNA transcripts of 3, 9.5, and 11 kb were present in adult rat heart by Northern blot analysis. alB-, alc-, and alD-subtype mRNAs were each present in isolated adult and neonatal cardiac I n cardiac muscle, a1-adrenergic receptor (AR) activation has several different physiological effects, including control of energy production,' increased and decreased inotropy and chronotropy,2-4 preconditioning against ischemic injury,5 reduction of myocardial stunning,6 and induction of cardiac myocyte hypertrophy and gene transcription.7-9 In addition, in the coronary arteries, a,-AR activation produces vasoconstriction.10"11 a,-ARs belong to the superfamily of G proteincoupled receptors with seven transmembrane domains. Like the other receptors for catecholamines ,-AR, a2-AR, and dopaminergica,-ARs constitute a multigene family. Three a,-AR subtypes have been cloned: the aiB from the hamster,'2 dog,13 rat,14-16 and human17'18; the aD from the rat'5'19 and human18; and the alc from the cow20 and human.18'2' In earlier studies, the mRNAs for two of these a,-AR subtypes were found in adult rat heart, the a1B1522 and the a1D.'5 Recently, we have shown that alc mRNA is also present in adult rat heart,23 contrary to prior reports.15,20,24 Thus, the mRNAs for all three cloned a,-ARs-the alB, the alD, and the ailc-are expressed in adult rat cardiac muscle. Two a,-AR subtypes had been defined by radioligand binding studies of native receptors in a variety of rat Received May 25, 1994; accepted July 29, 1994. From the Division of Cardiology and Research Service, Veterans Affairs Medical Center, San Francisco, and the Cardiovascular Research Institute and Department of Medicine, University of California, San Francisco. Correspondence to Paul C. Simpson, MD, VAMC 111-C-8, 4150 Clement St, San Francisco, CA 94121. myocytes by RNase protection assay. In addition, cultured neonatal cardiac myocytes expressed the three a,-AR subtype mRNAs. In contrast, none of the a,-AR mRNAs was detected in cultured neonatal cardiac fibroblasts. In addition, a,-ARs were absent in fibroblasts by ['Hiprazosin binding and norepinephrine-stimulated ['H]inositol phosphate production. The absence of a,-ARs in cardiac fibroblasts differs from 18-adrenergic and angiotensin II receptors, which are present in both cardiac fibroblasts and cardiac myocytes. Three a,-AR subtypes in cardiac myocytes will need to be considered in future studies of the physiological effects of a,-AR activation in cardiac muscle. (Circ Res. 1994;75:796-802.)