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 permit 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 β-adrenergic receptor (β-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. β-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 (~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 (~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.