Cloning of the rat α_1C-adrenergic receptor from cardiac myocytes: α_1C, α_1B, and α_1D mRNAs are present in cardiac myocytes but not in cardiac fibroblasts

α₁-Adrenergic receptor (AR) activation in cardiac muscle has several different physiological effects that might be mediated through different α₁-AR subtypes. Two α₁-AR subtypes have been cloned from the rat, the α_1B and the α_1D; both are present in adult rat heart. A third subtype, the α_1C, cloned from the cow and human, was reported to be absent in the rat. However, we recently found α_1C mRNA in adult rat heart by using a partial α_1C cDNA. Thus, all three cloned α₁-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 α_1C-AR was cloned from cultured neonatal cardiac myocytes. α_1C mRNA transcripts of 3, 9.5, and 11 kb were present in adult rat heart by Northern blot analysis. α_1B-, α_1C-, and α_1D-subtype mRNAs were each present in isolated adult and neonatal cardiac myocytes by RNase protection assay. In addition, cultured neonatal cardiac myocytes expressed the three α₁-AR subtype mRNAs. In contrast, none of the α₁-AR mRNAs was detected in cultured neonatal cardiac fibroblasts. In addition, α₁-ARs were absent in fibroblasts by [³H]prazosin binding and norepinephrine-stimulated [³H]inositol phosphate production. The absence of α₁-ARs in cardiac fibroblasts differs from β-adrenergic and angiotensin II receptors, which are present in both cardiac fibroblasts and cardiac myocytes. Three α₁-AR subtypes in cardiac myocytes will need to be considered in future studies of the physiological effects of α₁-AR activation in cardiac muscle.