Down regulation of the L-type Ca2+ channel, GRK2, and phosphorylated phospholamban:: protective mechanisms for the denervated failing heart

We previously found that a canine model of selective surgical ventricular denervation (VD), which does not pen-nit increased sympathetic tone during the pathogenesis of heart failure (HF), tolerated the development of HF better than controls. To investigate the cellular mechanisms, we examined cellular contraction and L-type Ca2+ channel currents (I-Ca) and their responses to beta-adrenergic receptor (beta-AR) stimulation in left ventricular myocytes from 1) control, 2) VD, 3) HF induced by rapid pacing, and 4) HF induced in VD (VD-HF) dogs. The magnitude of myocyte contraction and rate of relaxation in VD were similar to control but were depressed in both HF and VD-HF. These changes were associated with reduced protein expression of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) and protein kinase A phosphorylated phospholamban (PLB), which was reduced in HF, but essentially abolished in VD-HF. beta-AR kinase (GRK2) was increased in HF but reduced in VD-HF. Basal I-Ca density did not differ among control, VD, and HF groups, but VD-HF myocytes showed a markedly reduced I-Ca density (similar to 40%). Compared to controls, the sensitivity of I-Ca to isoproterenol (ISO), was significantly higher in VD, but reduced in HF. While I-Ca responses to ISO in VD-HF were maintained at control levels, the amplitude of the ISO-stimulated I-Ca was significantly smaller (similar to 50%) compared with HF myocytes. The relative decrease in Ca2+ influx due to downregulation of I-Ca density may contribute to the cardioprotective effects in VD-HF hearts by preventing Ca2+ overload during the development of HF. These findings, in combination with the virtual abolition of phosphorylated PLB in VD-HF and the decrease in GRK2, may explain, in part, why VD dogs tolerate the development of HF better than control dogs. (c) 2006 Elsevier Ltd. All rights reserved.