Abstract 18032: Increased Late Sodium Current During Early Heart Failure Development Induced by Chronic Sympathetic Activation
Chronic sympathetic activation promotes myocardial hypertrophy and heart failure. In mice overexpressing the human β1-adrenoreceptor (β1-OE), cytosolic [Na+]i increases during early stages of cardiac remodeling (8-12 weeks), thereby slowing relaxation and increasing diastolic [Ca2+]i due to increased Na+/Ca2+ exchanger (NCX) reverse mode activity. Here we investigate whether reduced Na+ efflux via Na+/K+ ATPase (NKA) regulation, or enhanced Na+ influx (voltage-dependent Na+ current or TRPC channel regulation) may account for increased [Na+]i in young β1-OE mice.
In β1-OE mice (8-12 weeks) and their wild-type (WT) littermates protein expression of NKA and phospholemman (PLM) was determined by Western blot in ventricular homogenates or isolated ventricular myocytes (TRPC 1, 3, 6) and normalized to GAPDH. NKA current (Ipump, whole cell voltage clamp) was measured as the difference in outward current after rapid solution switch to 0 K+ external solution. The persistent component of the Na+ current, INa,late, was measured as the current sensitive to 30 μm Tetrodotoxin (TTX).
In β1-OE, NKA α1 subunit expression was unchanged, whereas the α2 subunit was reduced (0.87±0.09 in β1-OE, n=5 vs. 1.24±0.08 in WT, n=4; p<0.05; mean±s.e.). Phosphorylated PLM was increased (1.3±0.05 in β1-OE, n=5 vs. 0.748±0.06 in WT, n=4; p<0.001), and total PLM was reduced in β1-OE mice (0.71±0.08 in β1-OE, n=5 vs. 1.23±0.18 in WT, n=4;p<0.05). TRPC 1, 3 and 6 expression was unaltered. Maximal Ipump density (100 mM [Na+]i in the pipette) was unchanged. With 10 mM [Na+]i, Ipump was significantly increased, indicating a higher Na+ affinity of NKA in β1-OE myocytes. INa,late was increased in β1-OE myocytes: at the end of pulse amplitude was -0.149 ± 0.034 pA/pF in β1-OE, n=10 vs. -0.063± 0.018 pA/pF in WT, n=9; p<0.05; integrated current was 138 ± 17 pC/pF in β1-OE, n=10 vs 85 ± 17 pC/pF in WT, n =9; p<0.05.
In conclusion, in β1-OE mice, increased INa,late contributes to elevated [Na+]i during chronic sympathetic activation. Higher Na+ affinity of NKA due to less inhibition by phospholemman only partially compensates increased Na+ influx at this early stage of heart failure development.
- © 2013 by American Heart Association, Inc.