Upregulation of β 1-adrenergic receptors in ovariectomized rat hearts

Abstract

Changes in cardiac myofilament Ca 2+ activation have been demonstrated in ovariectomized rats. The underlying mechanisms responsible for these changes, however, are unknown. Accordingly, we measured both density and binding affinity of cardiac β 1-adrenergic receptors in sarcolemmal preparations from 10-week ovariectomized rats, pair-fed ovariectomized rats, and sham-operated control rats. Receptor protein content was also measured by immunoblotting. Deprivation of ovarian sex hormones for 10 weeks induced a significant upregulation of β 1-adrenergic receptors without affecting binding affinity. The same magnitude of receptor upregulation was also detected in pair-fed ovariectomized hearts. To determine which hormone is responsible for the observed increase in β 1-adrenergic receptor density, various sex hormone supplemental regimens were administered to ovariectomized rats. Subcutaneous injection of estrogen (5 μg/rat), progesterone (1 mg/rat), or estrogen plus progesterone three times a week all effectively prevented the upregulation of the β 1-adrenoceptors. Western blot analyses using polyclonal antibody of β 1-adrenergic receptors revealed the same pattern of changes in the protein content of the receptors in these various groups of experimental hearts as those obtained from the receptor binding assay. These results suggest a possible direct suppressive effect of ovarian sex hormones on the expression of cardiac β 1-adrenergic receptors.