Carotid bodies contribute to sympathoexcitation induced by acute salt overload

Abstract

New findings: What is the central question of this study? Does carotid body input contribute to the hyperosmotic responses? What is the main finding and its importance? The response to NaCl overload is sympathorespiratory excitation. Eliminating the carotid body input reduced sympathoexcitation but did not affect the increase in phrenic burst frequency, whereas eliminating the hypothalamus prevented the tachypnoea and sympathoexcitation. We conclude that the carotid body inputs are essential for the full expression of the sympathetic activity during acute NaCl overload, whereas the tachypnoea depends on hypothalamic mechanisms.

Abstract: Acute salt excess activates central osmoreceptors, which trigger an increase in sympathetic and respiratory activity. The carotid bodies also respond to hyperosmolality of the extracellular compartment, but their contribution to the sympathoexcitatory and ventilatory responses to NaCl overload remains unknown. To evaluate their contribution to acute NaCl overload, we recorded thoracic sympathetic (tSNA), phrenic (PNA) and carotid sinus nerve activities in decorticate in situ preparations of male Holtzman rats (60-100 g) while delivering intra-arterial infusions of hyperosmotic NaCl (0.17, 0.3, 0.7, 1.5 and 2.0 mol l^-1 ; 200 μl infusion over 25-30 s, with a 10 min time interval between solutions) or mannitol (0.3, 0.5, 1.0, 2.7 and 3.8 mol l^-1 ) progressively. The cumulative infusions of hyperosmotic NaCl increased the perfusate osmolality to 341 ± 5 mosmol (kg water)^-1 and elicited an immediate increase in PNA and tSNA (n = 6, P < 0.05) in sham-denervated rats. Carotid body removal attenuated sympathoexcitation (n = 5, P < 0.05) but did not affect the tachypnoeic response. A precollicular transection disconnecting the hypothalamus abolished the sympathoexcitatory and tachypnoeic responses to NaCl overload (n = 6, P < 0.05). Equi-osmolar infusions of mannitol did not alter the PNA and tSNA in sham-denervated rats (n = 5). Sodium chloride infusions increased carotid sinus nerve activity (n = 10, P < 0.05), whereas mannitol produced negligible changes (n = 5). The results indicate that carotid bodies are activated by acute NaCl overload, but not by mannitol. We conclude that the carotid bodies contribute to the increased sympathetic activity during acute NaCl overload, whereas the ventilatory response is mainly mediated by hypothalamic mechanisms.

Keywords: carotid sinus nerve; hyperosmotic NaCl; sympathetic nerve activity.

FIGURE 1

(a) Recordings showing the typical changes in the phrenic (PNA) and thoracic sympathetic (tSNA) nerve activities during 301 (left) and 341 mosmol (kg water)⁻¹ (right) produced by infusion of Ringer solution and 2 mol l⁻¹ NaCl, respectively, in sham-denervated rats. The black lines above the recordings indicate the period of Ringer and hyperosmotic NaCl infusions. The changes in the phrenic nerve frequency (PNA Freq) produced by 341 mosmol (kg water)⁻¹ are highlighted in light grey, whereas the prolongation of expiratory time and sympathetic response are highlighted in dark grey. (b) Changes in PNA Freq and tSNA induced by 301 mosmol (kg water)⁻¹ (Ringer solution) and 302, 303, 306, 322 and 341 mosmol (kg water)⁻¹ (0.17, 0.3, 0.7, 1.5 and 2.0 mol l⁻¹ NaCl, respectively; n = 6). One-way repeated-measures ANOVA followed by Newman–Keuls test. Results in (b) are represented as means ± SD. *P < 0.05

FIGURE 2

(a) Typical recordings showing the respiratory–sympathetic coupling during 301 (left) and 341 mosmol (kg water)⁻¹ (right) during the infusion of Ringer solution and 2 mol l⁻¹ NaCl, respectively, in sham-denervated rats. The arrow indicates the additional bursts in the thoracic sympathetic nerve activity (tSNA) produced by 341 mosmol (kg water)⁻¹ during the postinspiratory (PI) phase. (b) Values of tSNA in the late-expiratory (LE), inspiratory (IP) and PI phases during 301 (Ringer solution) and 341 mosmol (kg water)⁻¹ (2 mol l⁻¹ NaCl; n = 8). The levels of tSNA were calculated as mean values and expressed in absolute units (microvolts). Student’s paired t test. Results in (b) are represented as means ± SD. *P < 0.05

FIGURE 3

(a) Recordings showing the typical responses in the phrenic (PNA) and thoracic sympathetic (tSNA) nerve activities during 341 mosmol (kg water)⁻¹ produced by 2 mol l⁻¹ NaCl infusion in sham-denervated rats (left) and rats after carotid body removal (CBX; right). The black lines above the recordings indicate the period of the infusions. (b) Changes in the phrenic nerve frequency (PNA Freq) and tSNA during 301 (Ringer solution) and 341 mosmol (kg water)⁻¹ (2 mol l⁻¹ NaCl) in sham-denervated (n = 6) and CBX rats (n = 5). Note that CBX reduced the increase in the tSNA in response to NaCl infusion but did not affect the tachypnoea. Two-way ANOVA followed by Newman–Keuls test. Results in (b) are represented as means ± SD. *P < 0.05

FIGURE 4

(a) Recordings showing the typical responses in the phrenic (PNA) and thoracic sympathetic (tSNA) nerve activities during 341 mosmol (kg water)⁻¹ produced by 2 mol l⁻¹ NaCl overload in sham (left) and decerebrate rats (right). The black lines above the recordings indicate the period of the infusions. Photomicrograph of a sagittal brain section taken from a decerebrate rat showing the precollicular transection (arrow). (b) Changes in the phrenic burst frequency (PNA Freq) and tSNA during 301 (Ringer solution) and 341 mosmol (kg water)⁻¹ (2 mol l⁻¹ NaCl) in sham (n = 6) and decerebrate rats (n = 6). Note that hypothalamic disconnection at the precollicular level completely abolished the NaCl-induced sympathoexcitation and tachypnoea. Data for hyperosmotic NaCl overload were obtained using a group of decorticate rats and another group of decerebrate rats. In decorticate in situ preparation all telencephalic and thalamus structure is removed. Only hypothalamus and conections with hindbrain is preserved. In decerebrate in situ preparation all structures rostral to anterior colliculus is removed. Two-way ANOVA followed by Newman–Keuls test. Results in (b) are represented as means ± SD. *P < 0.05

FIGURE 5

(a) Recordings showing the typical changes in the phrenic (PNA) and thoracic sympathetic (tSNA) nerve activities in sham-denervated rats that received intra-arterial infusion of 2 mol l⁻¹ NaCl (left) or 3.8 mol l⁻¹ mannitol (right). The black lines above the recordings indicate the period of hyperosmotic NaCl or mannitol infusions. The changes produced by NaCl overload are highlighted in grey. (b) Changes in phrenic nerve frequency (PNA Freq) and tSNA produced by intra-arterial infusion of Ringer solution and mannitol (0.3, 0.5, 1, 2.7 and 3.8 mol l⁻¹; n = 5). (c) Comparison between the changes in PNA Freq and tSNA induced by intra-arterial infusions of 2 mol l⁻¹ NaCl or 3.8 mol l⁻¹ mannitol. Note that hyperosmotic NaCl overload increased tSNA and PNA Freq, whereas mannitol did not modify the sympathetic and respiratory activities. A group of sham-denervated rats received NaCl intra-arterially, whereas another group of rats received mannitol. Friedman repeated-measures ANOVA on ranks (panel b, mannitol effects on PNA Freq), one-way repeated-measures ANOVA (panel b, mannitol effects on tSNA) or two-way ANOVA (panel c, NaCl versus mannitol on PNA Freq and tSNA) followed by Newman–Keuls test. Results in (b) and (c) are represented as means ± SD. *P < 0.05

FIGURE 6

(a) Recordings showing the typical changes in the phrenic (PNA) and carotid sinus (CSNA) nerve activities induced by intra-arterial infusions of potassium cyanide (KCN; left), 2 mol l⁻¹ NaCl (middle) or 3.8 mol l⁻¹ mannitol (right) in sham-denervated rats. The arrow above the recordings indicates the infusion of KCN, and black lines indicate the period of hyperosmotic NaCl or mannitol infusions. (b) Changes in CSNA produced by intra-arterial infusions of Ringer solution, NaCl (0.17, 0.3, 0.7, 1.5 and 2 mol l⁻¹; n = 10) or mannitol (0.3, 0.5, 1, 2.7 and 3.8 mol l⁻¹; n = 5). (c) Comparison between the changes in CSNA produced by KCN, 2 mol l⁻¹ NaCl or 3.8 mol l⁻¹ mannitol. Note that hyperosmotic NaCl overload increased CSNA, whereas mannitol produced negligible changes. Friedman repeated-measures ANOVA on ranks (panel b, NaCl effects on CSNA), one-way repeated-measures ANOVA (panel b, mannitol effects on CSNA) or two-way ANOVA (panel c, NaCl versus mannitol on CSNA) followed by Newman–Keuls test. Results in (b) and (c) are represented as means ± SD. *P < 0.05