Regulation of G-protein coupled receptor kinases GRK2 and GRK5 in heart transplantation candidates supported by ventricular assist devices

Aims: In endstage heart-failure beta-adrenergic signal-transduction is blunted due to receptor-downregulation, upregulation of Gi-alpha-protein and receptor-modification by G-protein coupled receptor kinases (GRK). We showed upregulation of beta1-adrenergic receptors (b1AR) in cardiac membrane preparations during ventricular assist device (VAD) support. This is regarded as the most important mechanism for restoration of the b1AR response after VAD-support. However, stimulation of adenylyl-cyclase activity (ACYC) by catecholamines appeared not to be directly correlated to b1AR regulation in VAD-patients. Thus, intracellular regulators are important for restoration of the bAR response. Thus, we analysed the expression of GRK2-mRNA and GRK5 on mRNA and protein level in VAD patients.

Methods: We analysed the mRNA of GRK2 and GRK5 by realtime RT-PCR using GAPDH as a reference in 24 paired left ventricular samples (LV) of VAD-patients and compared the data to LV of 14 non-failing donor hearts (NF) and 17 electively transplanted failing hearts (eHTx) without previous mechanical circulatory support. In addition we analysed the protein of GRK5 by western blotting in LV of 11 patients from VAD-implantation, 7 of VAD-transplantation and 5 NF.

Results: GRK5-mRNA was significantly downregulated (p<0.005) during VAD-support, whereas GRK2-mRNA was not regulated. Surprisingly, GRK2-mRNA was signficantly higher in NF versus VAD-implantation samples (p<0.005). GRK5-protein was not regulated during VAD-support.

Conclusion: Mechanical unloading does not lead to a significant regulation of GRK2 mRNA, whereas the strong downregulation of GRK5 mRNA was independent of its protein. It appears, that GRK5-regulation plays a minor role for coupling of the upregulated b1AR and ACYC after VAD-support.