Chronic hypoxia increases beta 1-adrenergic receptor mRNA and density but not signaling in neonatal rat cardiac myocytes.

BACKGROUND

It is well recognized that the beta-adrenergic receptor-adenylylcyclase system is altered during myocardial ischemia/hypoxia. However, there are no data regarding either regulation of beta-adrenergic receptors, particularly at the mRNA level, or adenylylcyclase activity in isolated cardiac myocytes exposed to chronic hypoxia.

RESULTS

In a chronic hypoxia model in which neonatal rat ventricular myocytes were exposed to a 1% O2 environment for 72 hours, we investigated (1) beta 1-mRNA and receptor expression and adenylylcyclase activity and (2) beta 1-mRNA and receptor downregulation and adenylylcyclase desensitization induced by prolonged norepinephrine incubation. We found that hypoxia for 72 hours increased myocardial membrane beta 1-adrenergic receptor density by 44%. This increase was not associated with a corresponding decrease in cytosolic beta 1-adrenergic receptors. RNase protection assays demonstrated that hypoxia increased the steady-state levels of beta 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 beta 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 beta 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 beta 1-mRNA and receptor downregulation and desensitization of the adenylylcyclase response.