AT2 receptor non-peptide agonist C21 promotes natriuresis in obese Zucker rats

Abstract

Previously, we demonstrated that angiotensin II type 2 (AT(2)) receptors have a role in natriuresis in obese Zucker rats (OZR). In the present study, we investigated the role of a novel, non-peptide agonist, C21, in natriuresis via AT(2) receptor activation in OZR. Infusion of C21 (1 and 5 μg kg(-1) min(-1)) into rats under anesthesia caused a dose-dependent increase in urine flow (UF) and urinary Na volume (U(Na)V). These effects of C21 were blocked by pre-infusion of the AT(2) receptor antagonist, PD123319, (50 μg kg(-1) min(-1)), suggesting involvement of the AT(2) receptor. Infusion of C21 (5 μg kg(-1) min(-1)) significantly increased the fractional excretion of sodium without changing the glomerular filtration rate or blood pressure, suggesting a tubular effect. Similarly, C21 infusion increased the fractional excretion of lithium, suggesting a proximal tubular effect. Furthermore, we tested the effect of C21 on natriuresis after blocking two main, distal-nephron Na transporters, the epithelial Na channels (ENaC), with amiloride (AM, 3 mg kg(-1) body wt), and the NaCl cotransporters (NCC), with bendroflumethiazide (BFTZ, 7 mg kg(-1) body wt). Infusion of AM + BFTZ caused significant increases in both diuresis and natriuresis, which were further increased by infusion of C21 (5 μg kg(-1) min(-1)). Natriuresis in response to C21 was associated with increases in urinary NO and cGMP levels. The data indicate that the AT(2) receptor agonist, C21, promotes natriuresis via AT(2) receptor activation and that this effect is potentially based in the proximal tubules and linked to the nitric oxide/cyclic guanosine monophosphate pathway. The natriuretic response to C21 may have therapeutic significance by improving kidney function in obesity.

Figure 1

Schematic representation of protocols used in the study. (a) To determine the dose-dependent effect of C21 on natriuresis/diuresis. (b) To determine whether C21 is acting via AT₂ receptor. (c) To determine FE_Li. (d)To determine the effect of C21 after blocking ENaC and NCC.

Figure 2

Effect of C21 (1 and 5 μg kg⁻¹ min⁻¹) on (a) urine flow (UF) and (b) urinary Na volume (U_NaV) in obese Zucker rats. Values are represented as mean±s.e.m.; One-way analysis of variance followed by Neuman–Keuls test, *significantly different from basal, P<0.05; N=5–6 rats.

Figure 3

Effect of C21 (5 μg kg⁻¹ min⁻¹) and PD123319 (50 μg kg⁻¹ min⁻¹) on (a) urine flow (UF) and (b) urinary Na volume (U_NaV) in obese Zucker rats. We have added C21 data from protocol 1 to demonstrate the effect of C21, which is much higher than any basal. Values are represented as mean±s.e.m.; One-way analysis of variance followed by Neuman-Keuls test, *significantly different from basal, P<0.05; N=5–3 rats.

Figure 4

Effect of C21 on (a) fractional excretion of sodium (FE_Na) and (b) fractional excretion of lithium (FE_Li) in obese Zucker rats. Values are represented as mean±s.e.m.; Student’s t-test, *significantly different from basal, P<0.05; N=5–7.

Figure 5

Effect of C21 alone and in combination with amiloride (AM) and bendroflumethiazide (BFTZ) on (a) urine flow (UF) and (b) urinary Na volume (U_NaV) in obese Zucker rats. We have added C21 data from protocol 1 to demonstrate the effect of C21, which is higher than any basal. Values are represented as mean±s.e.m.; One-way analysis of variance followed by Neuman–Keuls test, *significantly different from basal, ^Φsignificantly different from C21 and AM + BFTZ, P<0.05; N=5–10 rats.

Figure 6

Effect of C21 alone and in combination with amiloride (AM) and bendroflumethiazide (BFTZ) on (a) urinary nitrates/nitrites and (b) cGMP in obese Zucker rats. We have added C21 data from protocol 1 here, to demonstrate the effect of C21, which is higher than any basal. Values are represented as mean±s.e.m.; One-way analysis of variance followed by Neuman–Keuls test, *^$significantly different from basal, ^#significantly different from AM + BFTZ, P<0.05; N=7–10 rats.

Figure 7

Effect of C21 alone and in combination with amiloride (AM) and bendroflumethiazide (BFTZ) on (a) mean arterial pressure, (b) heart rate and (c) glomerular filtration rate (GFR) in obese Zucker rats. C21 data on MAP, HR and GFR were taken from protocol 1. Values are represented as mean±s.e.m.; One-way analysis of variance followed by Neuman–Keuls test, P<0.05; N=7–10 rats.