Abstract 818: Modulation Of Catecholamine Secretion By In Vivo Gene Transfer Of G-protein Coupled Receptor Kinase (GRK) 2 Or A GRK2 Inhibitor Peptide In The Adrenal Gland
We recently reported that adrenal G-protein coupled Receptor Kinase (GRK) 2 up-regulation causes enhanced secretion of the catecholamines (CA‘s) epinephrine (Epi) and norepinephrine (NEpi) in chronic heart failure by desensitizing the α2-adrenergic receptors (ARs) of chromaffin cells (ChrCs), the autoreceptors responsible for autocrine feedback inhibition of CA release from these cells [ Lymperopoulos et al., Nat. Med. 2007; 13: 315–323]. In the present study, we sought to investigate whether modulation of adrenal GRK2 levels/activity regulates physiological CA secretion in vivo in rats. For this purpose, we devised two different in vivo adenoviral gene delivery methods to the adrenal medulla: direct injection in the suprarenal gland and retrograde delivery via the suprarenal veins, of GRK2 or of its inhibitor peptide βARKct, and we performed in vivo gene delivery in normal young Sprague-Dawley rats. We found that both delivery approaches are equally effective at inducing a robust (>80% of the whole organ) and organ-restricted transgene expression in the whole adrenal gland, both in the cortical and medullary regions. Additionally, rats with AdenoGRK2 (AdGRK2)-infected adrenals exhibit enhanced plasma CA levels (235 ± 10 pg/ml for Epi, 333 ± 30 pg/ml for NEpi, n = 5) compared to control rats (adrenals injected with Adeno-Green Fluorescent Protein, AdGFP, or saline, 128 ± 6 pg/ml for Epi, 230 ± 10 pg/ml for NEpi, n = 5), whereas plasma CA levels in rats with AdβARKct-infected adrenals are significantly lower than control rats (30 ± 8 pg/ml for Epi, 67 ± 11 pg/ml for NEpi, n35). Finally, in ChrCs isolated from these infected adrenals, α2-AR stimulation with the specific α2-AR agonist UK14304 of AdGRK2-infected ChrCs failed to inhibit CA secretion in response to nicotine, which activates the nicotinic cholinergic receptors present in ChrCs, the physiological stimulus for CA secretion from these cells. In ChrCs derived from AdβARKct-infected adrenals in vivo however, the ability of UK14304 to inhibit nicotine-induced CA secretion was completely restored. These results suggest that adrenal GRK2 activity is a critical regulator of CA secretion in vivo, leading to increased CA secretion and plasma levels.