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High Salt Intake Augments Excitability of PVN Neurons in Rats: Role of the Endoplasmic Reticulum Ca 2+ Store

View Article: PubMed Central - PubMed

ABSTRACT

High salt (HS) intake sensitizes central autonomic circuitry leading to sympathoexcitation. However, its underlying mechanisms are not fully understood. We hypothesized that inhibition of PVN endoplasmic reticulum (ER) Ca2+ store function would augment PVN neuronal excitability and sympathetic nerve activity (SNA). We further hypothesized that a 2% (NaCl) HS diet for 5 weeks would reduce ER Ca2+ store function and increase excitability of PVN neurons with axon projections to the rostral ventrolateral medulla (PVN-RVLM) identified by retrograde label. PVN microinjection of the ER Ca2+ ATPase inhibitor thapsigargin (TG) increased SNA and mean arterial pressure (MAP) in a dose-dependent manner in rats with a normal salt (NS) diet (0.4%NaCl). In contrast, sympathoexcitatory responses to PVN TG were significantly (p < 0.05) blunted in HS treated rats compared to NS treatment. In whole cell current-clamp recordings from PVN-RVLM neurons, graded current injections evoked graded increases in spike frequency. Maximum discharge was significantly augmented (p < 0.05) by HS diet compared to NS group. Bath application of TG (0.5 μM) increased excitability of PVN-RVLM neurons in NS (p < 0.05), yet had no significant effect in HS rats. Our data indicate that HS intake augments excitability of PVN-RVLM neurons. Inhibition of the ER Ca2+-ATPase and depletion of Ca2+ store likely plays a role in increasing PVN neuronal excitability, which may underlie the mechanisms of sympathoexcitation in rats with chronic HS intake.

No MeSH data available.


(A) Representative traces showing HR, SSNA, RSNA, and ABP responses to bilateral PVN microinjection of TG (0.75 nmol) in a rat on a 0.4% normal salt (NS) diet (left), and a 2% high salt (HS) (right). Bilateral PVN microinjection (100 nl each) of TG (arrowheads) markedly increased HR, SSNA, RSNA and ABP in a NS diet rat, whereas responses were attenuated in a rat fed a HS diet. (B) Summary data showing peak changes in SSNA, RSNA, MAP, and HR after bilateral PVN microinjection of TG (0.75 nmol) in normal salt (NS, n = 6) and high salt (HS, n = 6) rats. Note that SSNA and RSNA responses to PVN TG were significantly attenuated in HS rats compared to NS. *P < 0.05 HS vs. NS (unpaired student t-test). Avg, average; ∫, integrated.
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Figure 3: (A) Representative traces showing HR, SSNA, RSNA, and ABP responses to bilateral PVN microinjection of TG (0.75 nmol) in a rat on a 0.4% normal salt (NS) diet (left), and a 2% high salt (HS) (right). Bilateral PVN microinjection (100 nl each) of TG (arrowheads) markedly increased HR, SSNA, RSNA and ABP in a NS diet rat, whereas responses were attenuated in a rat fed a HS diet. (B) Summary data showing peak changes in SSNA, RSNA, MAP, and HR after bilateral PVN microinjection of TG (0.75 nmol) in normal salt (NS, n = 6) and high salt (HS, n = 6) rats. Note that SSNA and RSNA responses to PVN TG were significantly attenuated in HS rats compared to NS. *P < 0.05 HS vs. NS (unpaired student t-test). Avg, average; ∫, integrated.

Mentions: HS diet is known to increase the excitability of brainstem autonomic circuitry (Adams et al., 2007, 2009), but little is known regarding the effects of HS diet on PVN neuronal excitability. Therefore, we sought to determine whether HS diet influences sympathoexcitatory responses to PVN microinjection of TG. There was no significant difference in baseline MAP between NS and HS treatment groups (Table 1). Figure 3A, left, demonstrates representative raw tracings before, and after bilateral microinjection of TG from the NS treatment group. Figure 3A, right, demonstrates a representative response to bilateral PVN microinjection of TG (0.75 nmol, 100 nl) from the HS treatment group with a significantly attenuated response compared to NS. Microinjection of 0.75 nmol TG was utilized for comparison, as it was the minimum dose to elicit a maximum response in control rats (Figures 1, 2). Maximum increases in SSNA (33 ± 6%; P < 0.0001) and RSNA (26 ± 5%; P = 0.0002), were significantly attenuated compared to NS as demonstrated by summary data in Figure 3A. The average latency from microinjection of TG to the maximum response was not significantly different (P = 0.336) between NS (40 ± 5 min) and HS (35 ± 3 min) treatment groups. There were no significant differences in MAP (7 ± 2 mmHg; P = 0.208) and HR (5 ± 3 bpm; p = 0.223) between NS and HS treatment groups (Figure 3B).


High Salt Intake Augments Excitability of PVN Neurons in Rats: Role of the Endoplasmic Reticulum Ca 2+ Store
(A) Representative traces showing HR, SSNA, RSNA, and ABP responses to bilateral PVN microinjection of TG (0.75 nmol) in a rat on a 0.4% normal salt (NS) diet (left), and a 2% high salt (HS) (right). Bilateral PVN microinjection (100 nl each) of TG (arrowheads) markedly increased HR, SSNA, RSNA and ABP in a NS diet rat, whereas responses were attenuated in a rat fed a HS diet. (B) Summary data showing peak changes in SSNA, RSNA, MAP, and HR after bilateral PVN microinjection of TG (0.75 nmol) in normal salt (NS, n = 6) and high salt (HS, n = 6) rats. Note that SSNA and RSNA responses to PVN TG were significantly attenuated in HS rats compared to NS. *P < 0.05 HS vs. NS (unpaired student t-test). Avg, average; ∫, integrated.
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Figure 3: (A) Representative traces showing HR, SSNA, RSNA, and ABP responses to bilateral PVN microinjection of TG (0.75 nmol) in a rat on a 0.4% normal salt (NS) diet (left), and a 2% high salt (HS) (right). Bilateral PVN microinjection (100 nl each) of TG (arrowheads) markedly increased HR, SSNA, RSNA and ABP in a NS diet rat, whereas responses were attenuated in a rat fed a HS diet. (B) Summary data showing peak changes in SSNA, RSNA, MAP, and HR after bilateral PVN microinjection of TG (0.75 nmol) in normal salt (NS, n = 6) and high salt (HS, n = 6) rats. Note that SSNA and RSNA responses to PVN TG were significantly attenuated in HS rats compared to NS. *P < 0.05 HS vs. NS (unpaired student t-test). Avg, average; ∫, integrated.
Mentions: HS diet is known to increase the excitability of brainstem autonomic circuitry (Adams et al., 2007, 2009), but little is known regarding the effects of HS diet on PVN neuronal excitability. Therefore, we sought to determine whether HS diet influences sympathoexcitatory responses to PVN microinjection of TG. There was no significant difference in baseline MAP between NS and HS treatment groups (Table 1). Figure 3A, left, demonstrates representative raw tracings before, and after bilateral microinjection of TG from the NS treatment group. Figure 3A, right, demonstrates a representative response to bilateral PVN microinjection of TG (0.75 nmol, 100 nl) from the HS treatment group with a significantly attenuated response compared to NS. Microinjection of 0.75 nmol TG was utilized for comparison, as it was the minimum dose to elicit a maximum response in control rats (Figures 1, 2). Maximum increases in SSNA (33 ± 6%; P < 0.0001) and RSNA (26 ± 5%; P = 0.0002), were significantly attenuated compared to NS as demonstrated by summary data in Figure 3A. The average latency from microinjection of TG to the maximum response was not significantly different (P = 0.336) between NS (40 ± 5 min) and HS (35 ± 3 min) treatment groups. There were no significant differences in MAP (7 ± 2 mmHg; P = 0.208) and HR (5 ± 3 bpm; p = 0.223) between NS and HS treatment groups (Figure 3B).

View Article: PubMed Central - PubMed

ABSTRACT

High salt (HS) intake sensitizes central autonomic circuitry leading to sympathoexcitation. However, its underlying mechanisms are not fully understood. We hypothesized that inhibition of PVN endoplasmic reticulum (ER) Ca2+ store function would augment PVN neuronal excitability and sympathetic nerve activity (SNA). We further hypothesized that a 2% (NaCl) HS diet for 5 weeks would reduce ER Ca2+ store function and increase excitability of PVN neurons with axon projections to the rostral ventrolateral medulla (PVN-RVLM) identified by retrograde label. PVN microinjection of the ER Ca2+ ATPase inhibitor thapsigargin (TG) increased SNA and mean arterial pressure (MAP) in a dose-dependent manner in rats with a normal salt (NS) diet (0.4%NaCl). In contrast, sympathoexcitatory responses to PVN TG were significantly (p &lt; 0.05) blunted in HS treated rats compared to NS treatment. In whole cell current-clamp recordings from PVN-RVLM neurons, graded current injections evoked graded increases in spike frequency. Maximum discharge was significantly augmented (p &lt; 0.05) by HS diet compared to NS group. Bath application of TG (0.5 &mu;M) increased excitability of PVN-RVLM neurons in NS (p &lt; 0.05), yet had no significant effect in HS rats. Our data indicate that HS intake augments excitability of PVN-RVLM neurons. Inhibition of the ER Ca2+-ATPase and depletion of Ca2+ store likely plays a role in increasing PVN neuronal excitability, which may underlie the mechanisms of sympathoexcitation in rats with chronic HS intake.

No MeSH data available.