Chronic Hypoxia Increases β 1 -Adrenergic Receptor mRNA and Density but Not Signaling in Neonatal Rat Cardiac Myocytes

Background It is well recognized that the β-adrenergic receptor–adenylylcyclase system is altered during myocardial ischemia/hypoxia. However, there are no data regarding either regulation of β-adrenergic receptors, particularly at the mRNA level, or adenylylcyclase activity in isolated cardiac myocytes exposed to chronic hypoxia. Methods and Results In a chronic hypoxia model in which neonatal rat ventricular myocytes were exposed to a 1% O 2 environment for 72 hours, we investigated (1) β 1 -mRNA and receptor expression and adenylylcyclase activity and (2) β 1 -mRNA and receptor downregulation and adenylylcyclase desensitization induced by prolonged norepinephrine incubation. We found that hypoxia for 72 hours increased myocardial membrane β 1 -adrenergic receptor density by 44%. This increase was not associated with a corresponding decrease in cytosolic β 1 -adrenergic receptors. RNase protection assays demonstrated that hypoxia increased the steady-state levels of β 1 -mRNA by 109%. Adenylylcyclase activity stimulated by isoproterenol, sodium fluoride, guanyl-5′-imidodiphosphate, and forskolin in hypoxic membranes was not altered compared with normoxic controls. Hypoxia for 72 hours also did not affect norepinephrine-induced β 1 -mRNA and receptor downregulation and adenylylcyclase desensitization in response to isoproterenol, guanyl-5′-imidodiphosphate, or forskolin. Conclusions In neonatal rat cardiac myocytes, chronic hypoxia (1) increases β 1 -mRNA and receptor expression but does not alter adenylylcyclase activity stimulated at either the receptor or the postreceptor level and (2) does not affect agonist-induced β 1 -mRNA and receptor downregulation and desensitization of the adenylylcyclase response.