Chronic hypoxia differentially regulates α₁-adrenergic receptor subtype mRNAs and inhibits α₁-adrenergic receptor-stimulated cardiac hypertrophy and signaling

Background: After myocardial ischemia and/or infarction, surviving cardiac myocytes in and around the injured zone develop hypertrophy to compensate for the loss of contractile units due to myocyte injury and death. One of the factors that may be involved in the development of hypertrophy after ischemic injury is norepinephrine (NE), an agent that induces hypertrophy of cardiac myocytes through the α₁-adrenergic receptor (AR). It is not known, however, whether hypoxia, a major component of ischemia, has any direct effect on NE- stimulated hypertrophy. Therefore, we sought to determine whether chronic hypoxia could alter NE-stimulated hypertrophy and if so, whether this alteration was related to α₁-AR-mediated signaling and α₁-AR changes at both the protein and mRNA levels. Methods and Results: We developed a model of chronic hypoxia in cultured neonatal rat cardiac myocytes in which myocytes were exposed to 1% oxygen for 72 hours. Initially, we observed that chronic hypoxia inhibited NE-stimulated hypertrophy, as reflected by decreases in both new protein synthesis and total protein content during chronic hypoxia. Then we found that chronic hypoxia also inhibited α₁-AR- transduced phosphatidylinositol hydrolysis, as indicated by a reduction in α₁-AR-stimulated inositol phosphate production in hypoxic cells. These observations suggested that the inhibition of NE-stimulated hypertrophy seen during chronic hypoxia was due to impairment of α₁-AR-mediated signaling and could result from changes in α₁-AR numbers and/or subtype distribution. To address this issue, we determined α₁-AR density and subtype distribution by radioligand binding and α₁-AR subtype mRNAs, including α(1A/D)-, α(1B)-, and α(1C)-ARs, by RNase protection assays. We found that chronic hypoxia differentially regulated both the pharmacologically defined α₁-AR subtypes and the mRNAs for the α₁-AR subtypes. Thus, hypoxia for 72 hours coordinately downregulated both the pharmacologically defined α(1A)-AR density and the α(1C)-AR mRNA level. During normoxia, NE increased the pharmacologically defined α(1A)-AR density and the α(1C)-AR mRNA level, but hypoxia for 72 hours prevented these NE-mediated changes. Conclusions: Chronic hypoxia (1) inhibits α₁-AR-mediated hypertrophy of cardiac myocytes and α₁-AR-transduced phosphatidylinositol hydrolysis and (2) downregulates both the pharmacologically defined α(1A)-AR density and the α(1C)-AR mRNA levels.