Reversal of impaired myocardial beta-adrenergic receptor signaling by continuous-flow left ventricular assist device support

Abstract

Background: Myocardial beta-adrenergic receptor (beta-AR) signaling is severely impaired in chronic heart failure (HF). This study was conducted to determine if left ventricular (LV) beta-AR signaling could be restored after continuous-flow LV assist device (LVAD) support.

Methods: Twelve patients received LVADs as a bridge to transplant. Paired LV biopsy specimens were obtained at the time of LVAD implant (HF group) and transplant (LVAD group). The mean duration of LVAD support was 152 +/- 34 days. Myocardial beta-AR signaling was assessed by measuring adenylyl cyclase (AC) activity, total beta-AR density (B(max)), and G protein-coupled receptor kinase-2 (GRK2) expression and activity. LV specimens from 8 non-failing hearts (NF) were used as controls.

Results: Basal and isoproterenol-stimulated AC activity was significantly lower in HF vs NF, indicative of beta-AR uncoupling. Continuous-flow LVAD support restored basal and isoproterenol-stimulated AC activity to levels similar to NF. B(max) was decreased in HF vs NF and increased to nearly normal in the LVAD group. GRK2 expression was increased 2.6-fold in HF vs NF and was similar to NF after LVAD support. GRK2 activity was 3.2-fold greater in HF vs NF and decreased to NF levels in the LVAD group.

Conclusions: Myocardial beta-AR signaling can be restored to nearly normal after continuous-flow LVAD support. This is similar to previous data for volume-displacement pulsatile LVADs. Decreased GRK2 activity is an important mechanism and indicates that normalization of the neurohormonal milieu associated with HF is similar between continuous-flow and pulsatile LVADs. This may have important implications for myocardial recovery.

Figure 1. Total myocardial β-adrenergic receptor density (B_max)

NF, non-failing controls (n=8); HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). * P<0.05 versus NF; ^# P<0.01 versus HF and P>0.05 versus NF.

Figure 2. Myocardial sarcolemmal membrane adenylyl cyclase activity

NF, non-failing controls (n=8); HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). * P<0.01 versus NF; ^# P<0.05 versus HF and P>0.05 versus NF. All experiments performed in triplicate.

Figure 3. Left ventricular protein expression of G protein-coupled receptor kinase-2 (GRK2) with a representative Western blot

NF, non-failing controls (n=8); HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). * P<0.05 versus NF; ^# P<0.01 versus HF and P>0.05 versus NF.

Figure 4. GRK2 mRNA expression measured by semi-quantitative real-time reverse transcription polymerase chain reaction

NF, non-failing controls (n=8); HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). * P<0.05 versus NF; ^# P<0.02 versus HF and P>0.05 versus NF. All experiments performed in triplicate.

Figure 5. GRK2 activity in left ventricular tissue preparations measured by rhodopsin phosphorylation. A representative autoradiogram of phospho-incorporation into Rho following gel electrophoresis

NF, non-failing controls (n=8); HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). * P<0.05 versus NF; ^# P<0.01 versus HF and P>0.05 versus NF. Rho, rhodopsin.

Figure 6. Lymphocyte GRK2 protein expression in paired blood samples

HF, heart failure group, pre-LVAD (n=12); LVAD, post-LVAD implant (n=12). *P<0.04 versus HF.