Dietary sodium modulates the interaction between efferent and afferent renal nerve activity by altering activation of α2-adrenoceptors on renal sensory nerves

Abstract

Activation of efferent renal sympathetic nerve activity (ERSNA) increases afferent renal nerve activity (ARNA), which then reflexively decreases ERSNA via activation of the renorenal reflexes to maintain low ERSNA. The ERSNA-ARNA interaction is mediated by norepinephrine (NE) that increases and decreases ARNA by activation of renal α(1)-and α(2)-adrenoceptors (AR), respectively. The ERSNA-induced increases in ARNA are suppressed during a low-sodium (2,470 ± 770% s) and enhanced during a high-sodium diet (5,670 ± 1,260% s). We examined the role of α(2)-AR in modulating the responsiveness of renal sensory nerves during low- and high-sodium diets. Immunohistochemical analysis suggested the presence of α(2A)-AR and α(2C)-AR subtypes on renal sensory nerves. During the low-sodium diet, renal pelvic administration of the α(2)-AR antagonist rauwolscine or the AT1 receptor antagonist losartan alone failed to alter the ARNA responses to reflex increases in ERSNA. Likewise, renal pelvic release of substance P produced by 250 pM NE (from 8.0 ± 1.3 to 8.5 ± 1.6 pg/min) was not affected by rauwolscine or losartan alone. However, rauwolscine+losartan enhanced the ARNA responses to reflex increases in ERSNA (4,680 ± 1,240%·s), and renal pelvic release of substance P by 250 pM NE, from 8.3 ± 0.6 to 14.2 ± 0.8 pg/min. During a high-sodium diet, rauwolscine had no effect on the ARNA response to reflex increases in ERSNA or renal pelvic release of substance P produced by NE. Losartan was not examined because of low endogenous ANG II levels in renal pelvic tissue during a high-sodium diet. Increased activation of α(2)-AR contributes to the reduced interaction between ERSNA and ARNA during low-sodium intake, whereas no/minimal activation of α(2)-AR contributes to the enhanced ERSNA-ARNA interaction under conditions of high sodium intake.

Fig. 1.

In vivo low-salt diet. Effects of renal pelvic administration of vehicle (open bar) and losartan, 0.44 mM, (hatched bar) (A), rauwolscine, 0.1 μM, (hatched bar) (B), or losartan+rauwolscine (cross-hatched bar) (C) on the increases in ERSNA and ARNA produced by placing the rat's tail in 47°C water. *P < 0.05, **P < 0.01 vs. baseline. ‡P < 0.01, ARNA responses to thermal cutaneous stimulation in the absence and presence of renal pelvic perfusion with losartan+rauwolscine. ΔERSNA, efferent renal sympathetic nerve activity response; ΔARNA, afferent renal nerve activity response; AUC, area under the curve of RNA vs. time.

Fig. 2.

In vivo high-salt diet: Effects of renal pelvic administration of vehicle (open bar) and rauwolscine (hatched bar) on the increases in ERSNA and ARNA produced by placing the rat's tail in 47°C water. **P < 0.01 vs. baseline.

Fig. 3.

In vivo low-sodium diet: Effects of renal pelvic administration of vehicle (open bar) and losartan (hatched bar), rauwolscine (hatched bar), or losartan+rauwolscine (cross-hatched bar) on the ARNA responses to renal pelvic administration of 10 pM NE. Losartan or rauwolscine (random order) preceded the administration of rauwolscine+losartan. *P < 0.05, **P < 0.01 vs. baseline, †P < 0.05, ‡P < 0.01, ARNA responses to NE during renal pelvic administration of rauwolscine+losartan vs. during vehicle administration. NE, norepinephrine.

Fig. 4.

In vitro isolated renal pelvises, low-sodium diet. A: effects of 1,250 pM NE on the release of substance P and PGE₂ in the presence of vehicle (solid line) or 0.1 μM rauwolscine (dashed line) in the bath. B: effects of 250 pM NE on the release of substance P and PGE₂ in the presence of rauwolscine (dashed line) or rauwolscine+losartan (dotted line) in the bath.**P < 0.01, vs. CNT and REC; ‡P < 0.01 NE-induced substance P and PGE₂ release in the presence of vehicle vs. rauwolscine (A) and rauwolscine vs. rauwolscine+losartan (B). CNT, control; REC, recovery.

Fig. 5.

In vitro isolated renal pelvises, high-sodium diet: effects of 2 pM NE on the release of substance P and PGE₂ in the presence of vehicle (solid line) or 0.1 μM rauwolscine (dashed line) in the bath.

Fig. 6.

Immunofluorescence double-labeling of renal tissue with antibodies against α_2A-adrenoceptors (AR), α_2C-AR (green) and calcitonin gene-related peptide (CGRP; red) shows α_2A-AR-immunoreactive (ir) fibers (A) and α_2C-AR-ir fibers (E) close or on CGRP-ir sensory nerve fibers (B, C, and F, G, respectively) (colocalization yellow, arrows) in the renal pelvic wall. Higher magnification showed α_2A-AR-ir (green, D) and α_2C-AR-ir (green, H) on CGRP-ir fibers (red), colocalization yellow (arrows).

Fig. 7.

Immunofluorescence double-labeling of renal tissue with antibodies against α_2C-AR (green) and NE-t (red) shows α_2C-AR-ir fibers (A) close or on NE-t-ir fibers (B, C) (colocalization yellow, arrows) in the renal pelvic wall. Higher magnification showed α_2C-AR-ir (green, D) on NE-t-ir fibers (red), colocalization yellow (arrows) on a vessel in renal cortical tissue.

Fig. 8.

A: immunofluorescence labeling of renal tissue shows α_2A-AR-ir (a) and α_2C-AR-ir (c) fibers in the renal pelvic wall (arrows). The α_2A-AR and α_2C-AR labeling is blocked by adsorption with the appropriate peptide (b and d). B: α_2A-AR antibody labeled CHO cells transfected to express α_2A-AR (a) but not cells transfected to express α_2C-AR (b) or WT cells (c). The α_2C-AR antibody labeled CHO cells transfected to express α_2C-AR (d) but not cells transfected to express α_2A-AR (e) or WT cells (f).

Fig. 9.

Graphs showing conclusion/hypothesis: Decreased activation of α₂-AR contributes to enhanced NE-mediated activation of renal sensory nerves in high-sodium dietary conditions (left). In low-sodium dietary conditions, increased activation of α₂-AR together with activation of AT1 receptors contribute to the reduced responsiveness of the renal sensory nerves to NE (right). Modulation of the interaction between ERSNA and ARNA is an appropriate physiological response to changes in dietary sodium intake in the overall goal of maintaining water and sodium balance.