Background-Intracellular sodium concentration ([Naϩ ]

No abstract

Rationale Hyperamylinemia is common in patients with obesity and insulin resistance, coincides with hyperinsulinemia, and results in amyloid deposition. Amylin amyloids are generally considered a pancreatic disorder in type-2 diabetes. However, elevated circulating levels of amylin may also lead to amylin accumulation and proteotoxicity in peripheral organs, including the heart. Objective To test whether amylin accumulates in the heart of obese and type-2 diabetic patients and to uncover the effects of amylin accumulation on cardiac morphology and function. Methods and Results We compared amylin deposition in failing and non-failing hearts from lean, obese, and type-2 diabetic humans using immunohistochemistry and western blots. We found significant accumulation of large amylin oligomers, fibrils and plaques in failing hearts from obese and diabetic patients, but not in normal hearts and failing hearts from lean, non-diabetic humans. Small amylin oligomers were even elevated in non-failing hearts from overweight/obese patients suggesting an early state of accumulation. Using a rat model of hyperamylinemia transgenic for human amylin, we observed that amylin oligomers attach to the sarcolemma, leading to myocyte Ca2+ dysregulation, pathological myocyte remodeling, and diastolic dysfunction, starting from pre-diabetes. In contrast, pre-diabetic rats expressing the same level of wild-type rat amylin, a non-amyloidogenic isoform, exhibited normal heart structure and function. Conclusions Hyperamylinemia promotes amylin deposition in the heart causing alterations of cardiac myocyte structure and function. We propose that detection and disruption of cardiac amylin buildup may be both a predictor of heart dysfunction and a novel therapeutic strategy in diabetic cardiomyopathy.

Abstract-Cardiac sympathetic stimulation activates ␤-adrenergic (␤-AR) receptors and protein kinase A (PKA) phosphorylation of proteins involved in myocyte Ca regulation. The Na/K-ATPase (NKA) is essential in regulating intracellular [Na] ([Na] i ), which in turn affects [Ca] i via Na/Ca exchange. However, how PKA modifies NKA function is unknown. Phospholemman (PLM), a member of the FXYD family of proteins that interact with NKA in various tissues, is a major PKA substrate in heart. Here we tested the hypothesis that PLM phosphorylation is responsible for the PKA effects on cardiac NKA function using wild-type (WT) and PLM knockout (PLM-KO) mice. We measured NKA-mediated [Na] i decline and current (I Pump ) to assess ␤-AR effects on NKA function in isolated myocytes. In WT myocytes, 1 mol/L isoproterenol (ISO) increased PLM phosphorylation and stimulated NKA activity mainly by increasing its affinity for internal Na (K m decreased from 18.8Ϯ1.4 to 13.6Ϯ1.5 mmol/L), with no significant effect on the maximum pump rate. This led to a significant decrease in resting [Na] i (from 12.5Ϯ1.8 to 10.5Ϯ1.4 mmol/L). In PLM-KO mice under control conditions K m (14.2Ϯ1.5 mmol/L) was lower than in WT, but comparable to that for WT in the presence of ISO. Furthermore, ISO had no significant effect on NKA function in PLM-KO mice. ATPase activity in sarcolemmal vesicles also showed a lower K m (Na) in PLM-KO versus WT (12.9Ϯ0.9 versus 16.2Ϯ1.5). Thus, PLM inhibits NKA activity by decreasing its [Na] i affinity, and this inhibitory effect is relieved by PKA activation. We conclude that PLM modulates the NKA function in a manner similar to the way phospholamban affects the related SR Ca-ATPase (inhibition of transport substrate affinity, that is relieved by phosphorylation). Key Words: Na pump Ⅲ phospholemman Ⅲ signal transduction Ⅲ ion channels A ctivation of the sympathetic nervous system and cardiac ␤-adrenergic (␤-AR) receptors causes cAMP formation and activation of protein kinase A (PKA). In cardiac myocytes, PKA phosphorylates several targets with key roles in the control of excitation-contraction coupling (ECC), including L-type Ca 2ϩ channels, phospholamban (PLB) and troponin-I, as well as other sarcolemmal proteins such as voltage-gated Na and K channels and phospholemman (PLM).During sympathetic activation, the larger Ca influx via more frequent and larger Ca current must be balanced by enhanced Ca extrusion via the Na/Ca exchange (NCX) that is driven by larger Ca transients. This increases Na influx at each beat, along with more frequent and larger Na current, which increases intracellular [Na] ([Na] i ). To limit the rise in [Na] i , the greater Na influx must be compensated for by an enhanced Na extrusion via the Na/K pump (NKA). Indeed, many early studies indicated stimulation of the Na-pump by ␤-AR activation. 1-3 However, there is controversy at present because some recent studies in single myocytes using NKA pump current (I Pump ) found either stimulation, 4 -6 inhibition, 7,8 or no change 9 in I Pump on ␤-...