Chronic hypoxia differentially regulates alpha 1-adrenergic receptor subtype mRNAs and inhibits alpha 1-adrenergic receptor-stimulated cardiac hypertrophy and signaling.

Abstract

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 alpha 1-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 alpha 1-AR-mediated signaling and alpha 1-AR changes at both the protein and mRNA levels. 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 alpha 1-AR-transduced phosphatidylinositol hydrolysis, as indicated by a reduction in alpha 1-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 alpha 1-AR-mediated signaling and could result from changes in alpha 1-AR numbers and/or subtype distribution. To address this issue, we determined alpha 1-AR density and subtype distribution by radioligand binding and alpha 1-AR subtype mRNAs, including alpha 1A/D-, alpha 1B-, and alpha 1C-ARs, by RNase protection assays. We found that chronic hypoxia differentially regulated both the pharmacologically defined alpha 1-AR subtypes and the mRNAs for the alpha 1-AR subtypes. Thus, hypoxia for 72 hours coordinately downregulated both the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level. During normoxia, NE increased the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level, but hypoxia for 72 hours prevented these NE-mediated changes. Chronic hypoxia (1) inhibits alpha 1-AR-mediated hypertrophy of cardiac myocytes and alpha 1-AR-transduced phosphatidylinositol hydrolysis and (2) downregulates both the pharmacologically defined alpha 1A-AR density and the alpha 1C-AR mRNA level.