The Role of Adrenergic Receptor Signaling in Embryonic Ventricular Cell Proliferation and Differentiation
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In the adult heart, cardiomyocytes (CM) that die in response to aging or pathological insults are replaced by scar tissue. Transplantation of embryonic cardiac progenitor cells (CPC) was shown to increase the contractile function of a failing heart. Previous studies demonstrated that a non-selective beta-adrenergic receptor (β-AR) agonist isoproterenol (ISO) can decrease proliferation of CPCs in vitro and reduce graft size after intracardiac cell transplantation. A β1-AR antagonist (Metoprolol) abrogated the anti-proliferative effects of ISO and increased graft size. Carvedilol, a commonly used heart failure medication is known to block both alpha-adrenergic receptor (α-AR) and β-AR subtypes. There is no information available on the expression profiles of different α1-AR subtypes during cardiac ontogeny and whether these receptors play any role in proliferation and differentiation of embryonic ventricular cells. It is hypothesized that expression of α1-AR subtypes is differentially regulated during embryonic heart development and α1-AR signaling plays an important role in ventricular cell proliferation and differentiation. Total RNA samples isolated from different developmental stages of embryonic ventricles were processed for quantitative RT-PCR analysis using α1-AR subtype specific primers. These experiments revealed that α1B or α1D gene expression levels were significantly higher than that of α1A at several stages of cardiac development. Subcellular localization of α1-AR subtypes in embryonic ventricular cells revealed the presence of α1A, α1B, and α1D subtypes in the nucleus as well as the cytoplasm. Embryonic ventricular cultures treated with carvedilol in the presence or absence of ISO did not show any changes in cell size compared to control cultures. Additionally, cells treated with carvedilol and prazosin, resulted in no change in proliferation and cell size parameters in embryonic ventricular cells. However, treatment of embryonic ventricular cells with isoproterenol and carvedilol led to a significant decrease in the relative gene expression of a cardiac transcription factor, Hand2. However, these drug treatments did not affect the relative percentages of CPCs and differentiated cardiomyocytes in embryonic ventricular cell cultures. Therefore, these results suggest that it may be safe to use non-selective adrenergic receptor blockers with cell transplantation studies.