β -adrenergic receptor activation plays an important role in the depressed myocardial contractility via both elevated levels of cellular free Zn and reactive nitrogen species.

Role of β -AR dysregulation, as either cardio-conserving or cardio-disrupting mediator, remains unknown yet. Therefore, we examined the molecular mechanism of β -AR activation in depressed myocardial contractility using a specific agonist CL316243 or using β -AR overexpressed cardiomyocytes. Since it has been previously shown a possible correlation between increased cellular free Zn ([Zn ] ) and depressed cardiac contractility, we first demonstrated a relation between β -AR activation and increased [Zn ] , parallel to the significant depolarization in mitochondrial membrane potential in rat ventricular cardiomyocytes. Furthermore, the increased [Zn ] induced a significant increase in messenger RNA (mRNA) level of β -AR in cardiomyocytes. Either β -AR activation or its overexpression could increase cellular reactive oxygen species (ROS) and reactive nitrogen species (RNS) levels, in line with significant changes in nitric oxide (NO)-pathway, including increases in the ratios of pNOS3/NOS3 and pGSK-3β/GSK-3β, and PKG expression level in cardiomyocytes. Although β -AR activation induced depression in both Na - and Ca -currents, the prolonged action potential (AP) seems to be associated with a marked depression in K -currents. The β -AR activation caused a negative inotropic effect on the mechanical activity of the heart, through affecting the cellular Ca -handling, including its effect on Ca -leakage from sarcoplasmic reticulum (SR). Our cellular level data with β -AR agonism were supported with the data on high [Zn ] and β -AR protein-level in metabolic syndrome (MetS)-rat heart. Overall, our present data can emphasize the important deleterious effect of β -AR activation in cardiac remodeling under pathological condition, at least, through a cross-link between β -AR activation, NO-signaling, and [Zn ] pathways. Moreover, it is interesting to note that the recovery in ER-stress markers with β -AR agonism in hyperglycemic cardiomyocytes is favored. Therefore, how long and to which level the β -AR agonism would be friend or become foe remains to be mystery, yet.