Adrenal Beta-Arrestin 1 Mediates Physiological Production of Aldosterone In Vivo

Aldosterone is one of the various hormones with detrimental functions for the failing heart, whose circulating levels are elevated in chronic heart failure (HF). Aldosterone is produced and secreted by the adrenal cortex in response to angiotensin II (AngII) acting through AngII type 1A receptors (AT_1ARs), which are expressed in the adrenocortical zona glomerulosa cells. The AT_1ARs are G-protein coupled receptors (GPCRs) that have recently been shown to also signal through G-protein-independent pathways. The scaffolding actions of b-arrestin-1 and -2 (barr1 and -2), originally discovered as terminators of GPCR signaling following phosphorylation of these receptors by GPCR kinases (GRKs), have a central role in mediating G-protein-independent signal transduction by these receptors. We hypothesized that adrenal barr1 mediates, at least in part, AT_1AR signaling to aldosterone production/secretion in vivo. To test this hypothesis, we used in vivo adenoviral-mediated gene transfer to the adrenal gland of normal young male Sprague-Dawley rats to induce overexpression of barr1, GRK2, or the GRK2 inhibitor peptide bARKct. One week post-infection, the animals were sacrificed, in vivo transgene overexpression was assessed by Western blotting and plasma aldosterone levels were determined by ELISA. In addition, we measured plasma aldosterone levels in chronic HF rats, and transfected their adrenals with a dominant negative (DN) mutant to inhibit adrenal barr1 action in vivo in these rats. All three transgene proteins were robustly overexpressed specifically in the adrenal glands of these rats. Adrenal-targeted barr1 overexpression (Adbarr1) led to a significant increase in plasma aldosterone levels in the infected rats (536 ± 50 pg/ml) compared to control rats that received adenovirus encoding for Green Fluorescent Protein (AdGFP) in their adrenals (235 ± 40 pg/ml, p < 0.01, n = 5). Adrenal GRK2 overexpression caused a small but significant increase in plasma aldosterone levels, whereas bARKct overexpression was without effect compared to control AdGFP rats. Studies are currently being done in the HF model trying to determine if aldosterone levels in HF can be manipulated through alterations of barr function. Adrenal barr1 appears to mediate physiological and pathological (increased in HF) aldosterone turnover in vivo, and appears to do so independently of the actions of its co-factor in receptor desensitization, GRK2.