Abstract
An interaction between angiotensin II (Ang II) and ghrelin has been established in many tissues relevant to cardiovascular control, but nothing is known about their relationship within the kidney. Intrarenal ghrelin receptors (GRs) localize to the collecting duct (CD) where they couple to an adenylyl cyclase second messenger system to increase cAMP and ENaC-dependent Na+ reabsorption. Ang II also stimulates the activity of ENaC in the CD (independent of aldosterone), via actions at AT1Rs. The following studies seek to determine whether CD GRs are an important mechanism of Ang II-induced antinatriuresis. Uninephrectomized Sprague-Dawley rats received 3 cumulative 1h renal interstitial (RI) infusions of vehicle 5% dextrose in water (D5W, N=8), Ang II (2 ng/kg/min, N=8), Ang II + D-LYS-GHRP-6, a highly selective GR antagonist (D-LYS, 2, 4, 6 μg/min, N=8) or D-LYS alone (N=8). Urine Na+ excretion rate (UNaV) was measured each hour and compared to baseline, during which only vehicle was infused. RI fluid was collected each hour for cAMP determinations. RI Ang II induced a significant antinatriuresis (UNaV was reduced by 34% at 1h, P<0.01; by 46% at 2h, P<0.001; and by 56% at 3h, P<0.001 from baseline). Ang II-induced antinatriuresis was accompanied by a significant increase in RI cAMP levels from a baseline value of 2.97±0.56 pmol/mL to 10.9±2.2, 13.4±2.2, and 15.3±2.7 pmol/mL after 1h, 2h, and 3h respectively (all P<0.01). However, each of these effects of RI Ang II infusion was abolished by concurrent GR blockade with D-LYS. These data suggest that intact intrarenal GR activity is necessary for Ang II-induced Na+ reabsorption in vivo. Furthermore, since cAMP fails to increase in response to Ang II when GRs are blocked, (and GRs are known to signal via cAMP in the kidney), these data strongly suggest that one of the mechanisms of Ang II-induced Na+ reabsorption in the kidney is via GR-induced increases in cAMP.
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