Abstract 43: Non-Canonical AT1R/b-Arrestin2 Activation In Brain As A Regulator Of Fluid Homeostasis And Blood Pressure

The brain renin angiotensin system (RAS) is known for its role in cardiovascular and metabolic regulation. Angiotensin II (Ang II) is the major active product of the RAS, exerting most of its physiological effects through the angiotensin type-1 receptor (AT 1 R). Canonical or G-protein-mediated signaling of the AT 1 R within the brain has long been known to induce a dipsogenic and pressor response upon Ang II stimulation. Non-canonical or β-Arrestin mediated signaling is thought to counterbalance the detrimental effects of canonical signaling. However, the non-canonical AT 1 R/β-Arrestin pathway within the brain is understudied. Therefore, it is hypothesized that β-Arrestin activation contributes to fluid homeostasis and blood pressure (BP) regulation. Global β-Arrestin1 ( Arrb 1) and β-Arrestin2 ( Arrb 2) knockout (KO) mice were employed to evaluate drinking behavior and BP with and without deoxycorticosterone acetate (DOCA). Age- and sex-matched C57BL/6J mice served as controls. Mice were subjected to the two-bottle choice paradigm, in which the animals were presented with two bottles, one containing water and one containing 0.15M saline. In the absence of DOCA, mice lacking β-Arrestin2 had increased saline intake when compared to β-Arrestin1-KO and wildtype (WT=2.2±0.2 and Arrb 1-KO=2±0.4 vs Arrb 2-KO=5±0.7 mL/day; p<0.001; n=13, 11 and 9, respectively). This resulted in a saline preference, which means mice preferred saline over water by more than 50% by volume. In the presence of DOCA, mice lacking β-Arrestin2 had increased saline intake when compared to β-Arrestin1-KO and wildtype (WT=10.6±1.2 and Arrb 1-KO=6.5±0.8 vs Arrb 2-KO=16.6±2 mL/day; p<0.001; n=13, 11 and 9, respectively). However, these mice did not develop a saline preference. Preliminarily, β-Arrestin2-KO mice exhibited higher BP when compared to WT at baseline (WT=108±5 vs Arrb 2-KO=124±6 mmHg; n=2), which was exacerbated in response to DOCA (WT=122±6 vs Arrb 2-KO=140±5 mmHg; n=2). These findings suggest that β-Arrestin2 might counterbalance effects of canonical activation of the AT 1 R through G proteins. Overall, β-Arrestin2 appears to protect against cardiovascular diseases since the genetic ablation of β-Arrestin2 resulted in an increase in saline intake and exacerbated BP.

Effects of aging on mineralocorticoid-induced salt appetite in rats

This work examined the effects of age on salt appetite measured in the form of daily saline (i.e., 0.3 M NaCl) drinking in response to administration of deoxycorticosterone acetate (DOCA; 5 mg/kg body wt) using young (4 mo), “middle-aged” adult (12 mo), and old (30 mo) male Brown Norway rats. Water and sodium intakes, excretions, and balances were determined daily. The salt appetite response was age dependent with “middle-aged” rats ingesting the most saline solution followed in order by young and then old rats. While old rats drank the least saline solution, the amounts of saline ingested still were copious and comprise an unambiguous demonstration of salt appetite in old rats. Middle-aged rats had the highest saline preference ratios of the groups under baseline conditions and throughout testing consistent with an increased avidity for sodium taste. There were age differences in renal handling of water and sodium that were consistent with a renal contribution to the greater saline intakes by middle-aged rats. There was evidence of impaired renal function in old rats, but this did not account for the reduced saline intakes of the oldest rats.

Sodium chloride preference of genetically hypertensive and normotensive rats

Preference for 0.9% saline was examined, using two-bottle preference tests over 6-7 days, in the spontaneously hypertensive rat (SHR) and normotensive Wistar Kyoto rat (WK) of the Okamoto strain, the genetically hypertensive (GHR) and normotensive rat (NT) of the Smirk strain, and the Sprague-Dawley (SD), Dark Agouti (DA), and hooded Long-Evans or Brattleboro (BB) rat. Only the SHR exhibited a sustained and marked preference for 0.9% saline on each test day. The WK, GHR, NT, and SD preferred saline in the first 24-48 h of testing but thereafter showed neither a preference for, nor aversion to, saline. The BB showed neither a preference for, nor aversion to, saline in the first 24 h of testing and thereafter showed a significant aversion to saline on each test day. Saline preference was further examined in both the SHR and WK offered a choice of water and 0.9%, 2.0, or 2.7% saline. While preference for saline decreased in both SHR and WK with increasing saline concentration, the SHR maintained a significantly greater preference for saline and greater total sodium intake than the WK at each concentration. Hydralazine (5 mg . kg-1 . day-1, po) administered to SHR, while they were offered a choice of water and 0.9% saline, significantly lowered blood pressure over a 4-day period but failed to alter their saline preference significantly. We conclude that of the seven strains of rats examined only the SHR exhibited a preference for saline in extended two-bottle preference tests. Furthermore this preference for saline appears to be maintained independently of the blood pressure of the SHR.