Age‐ and sex‐specific differences in activation of the cardiac adrenergic cascade

Abstract

Heart failure (HF) is a costly and devastating public health problem, which differentially impacts male and female patients of all ages across the life‐course. The adrenergic system is dysregulated with HF and is the target of front‐line therapeutics. However, the majority of the work aimed at elucidating the adrenergic signaling response has been conducted in adult males. Consequently, mechanisms of adrenergic signaling that differ by sex and age remain unclear. Here, we set out to identify mechanistic differences in the adrenergic pathway in response to isoproterenol (ISO) stimulation in juvenile (4 weeks), adult (4‐6 months) and aged (18 months) C57Bl6 mice of both sexes. ISO was delivered by mini‐osmotic pump (30 mg/kg/day) for one week. We assessed changes in β‐adrenergic receptor (β‐AR) expression, protein kinase activity, and myofilament protein phosphorylation and function. β1‐AR and β2‐AR protein expression were significantly lower in response to ISO in juvenile males with no difference in expression in juvenile females. In contrast, ISO stimulation in aged males significantly upregulated β1‐AR and β2‐AR protein expression without changes in expression in aged females. Analysis of sarcomeric proteins indicated that phosphorylation of troponin varied in an age‐ and sex‐dependent manner. Phosphorylation of the inhibitory subunit of troponin (TnI) at Ser23/24 was upregulated in response to ISO in females of all ages with no differences in males. These changes in phosphorylation decreased myofilament Ca2+ sensitivity in female hearts in response to ISO. Together, this work suggests that male hearts responded robustly to ISO stimulation as evident by activation throughout the adrenergic cascade while female hearts demonstrated activation of downstream mechanisms at the level of the sarcomere. Ongoing experiments will characterize calcium handling and myofilament mechanics to mechanistically link upstream signals to downstream changes in contractility. These data demonstrate that age and sex are significant biological factors which contribute to adrenergic‐mediated changes in cardiac performance. Understanding these differences is critical for successful therapeutic intervention within the adrenergic signaling cascade for male and female patients of all ages.