Abstract 1405: The Alpha-1A is the Predominant Alpha-1-Adrenergic Receptor in the Human Heart at the mRNA but not the Protein Level
Background: Alpha-1-adrenergic receptors (ARs) play an adaptive role in the heart and protect against the development of HF. There are three alpha-1-AR subtypes: A, B, and D. Limited prior studies find that the human heart contains primarily the A subtype mRNA. Alpha-1-AR subtype proteins have never been measured in human heart.
Hypothesis: This study tests the hypothesis that all three alpha-1-AR subtypes are present in both the failing (F) and non-failing (NF) human heart.
Methods: F human hearts [n =6, mean EF 26%, mean age 47, 3 males (M), 3 females (Fe)] were obtained after cardioplegia during heart transplant. NF controls were unused donor hearts (n =6, mean EF 58%, mean age 41, 4 M, 2 Fe). Left (LV) and right ventricular (RV) tissue from the mid free wall was flash frozen in liquid nitrogen. mRNA was purified and treated with DNase. Quantitative real time reverse transcription PCR results were normalized to beta-actin and TATA binding protein. Saturation binding used tritiated prazosin and competition binding used 5-methylurapidil, an alpha-1A-specific antagonist.
Results: All three alpha-1-AR subtype mRNAs were identified, and the A subtype was the most abundant in both LV and RV (p <0.05). Subtype mRNA levels did not change significantly in F versus NF, in either LV or RV [see table⇓. Arbitrary Units ±SEM (% of total alpha-1-AR mRNA )]. Mean alpha-1-AR protein density was unchanged in HF by saturation binding (NF: 4.6 ±0.7 fmol/mg vs. F: 3.9 ±0.8 fmol/mg). Competition binding in NF LV tissue (n =6) showed a classic two-site binding curve with 38% high affinity (alpha-1A) binding sites and 62% low affinity (alpha-1B/D) sites.
Conclusions: These results are the first to measure alpha-1-AR subtype proteins in the human heart. At the protein level, alpha-1A is less abundant than the sum of alpha-1B and D, even though alpha-1A mRNA is most abundant. Given that alpha-1-ARs protect against HF in animal models, specific activation of cardiac alpha-1-AR subtypes might be feasible.