Sympathoexcitation in ANG II-salt hypertension involves reduced SK channel function in the hypothalamic paraventricular nucleus

Abstract

Hypertension (HTN) resulting from subcutaneous infusion of ANG II and dietary high salt (HS) intake involves sympathoexcitation. Recently, we reported reduced small-conductance Ca(2+)-activated K(+) (SK) current and increased excitability of presympathetic neurons in the paraventricular nucleus (PVN) in ANG II-salt HTN. Here, we hypothesized that ANG II-salt HTN would be accompanied by altered PVN SK channel activity, which may contribute to sympathoexcitation in vivo. In anesthetized rats with normal salt (NS) intake, bilateral PVN microinjection of apamin (12.5 pmol/50 nl each), the SK channel blocker, remarkably elevated splanchnic sympathetic nerve activity (SSNA), renal sympathetic nerve activity (RSNA), and mean arterial pressure (MAP). In contrast, rats with ANG II-salt HTN demonstrated significantly attenuated SSNA, RSNA, and MAP (P < 0.05) responses to PVN-injected apamin compared with NS control rats. Next, we sought to examine the individual contributions of HS and subcutaneous infusion of ANG II on PVN SK channel function. SSNA, RSNA, and MAP responses to PVN-injected apamin in rats with HS alone were significantly attenuated compared with NS-fed rats. In contrast, sympathetic nerve activity responses to PVN-injected apamin in ANG II-treated rats were slightly attenuated with SSNA, demonstrating no statistical difference compared with NS-fed rats, whereas MAP responses to PVN-injected apamin were similar to NS-fed rats. Finally, Western blot analysis showed no statistical difference in SK1-SK3 expression in the PVN between NS and ANG II-salt HTN. We conclude that reduced SK channel function in the PVN is involved in the sympathoexcitation associated with ANG II-salt HTN. Dietary HS may play a dominant role in reducing SK channel function, thus contributing to sympathoexcitation in ANG II-salt HTN.

High Salt Intake Augments Excitability of PVN Neurons in Rats: Role of the Endoplasmic Reticulum Ca2+ Store

“…Coronal slices through the rostral caudal plane encompassing the PVN were examined to ensure microinjections were confined to the PVN as previously described by our laboratory (Larson et al, 2015a). Figure 6B shows the composite diffusion area of injected dye compiled by overlying brain slices from separate rats to demonstrate the outermost diffusion area.…”

“…Our lab has recently demonstrated that dysfunction of SK channels in the pre-sympathetic PVN-RVLM neurons contributes to the augmented neuronal excitability and sympathoexcitation in AngII-salt HTN using both in vitro and in vivo approach (Chen et al, 2010; Larson et al, 2015a). Furthermore, we showed that SK channel dysfunction is present in normotensive rats fed a 2% HS diet (Larson et al, 2015a). Interestingly, depletion of ER Ca 2+ store via bilateral microinjection of TG, an inhibitor of the ER Ca 2+ ATPase, significantly increased SSNA, RSNA, and MAP.…”

“…Rats were prepared for recording of splanchnic (SSNA) and renal (RSNA) SNA according to protocols previously described (Gui et al, 2012; Larson et al, 2015a). Briefly, a left flank incision was made and a left renal nerve and a postganglionic splanchnic nerve were separated from surrounding tissue.…”

“…PVN microinjections were performed as previously described (Gui et al, 2012; Larson et al, 2015a). Animals were placed in a stereotaxic head frame and the skull was leveled between bregma and lambda.…”

“…Furthermore, alterations of intrinsic membrane properties that influence PVN neuronal excitability also play an important role in neurogenic HTN. Our lab (Larson et al, 2015a), and others (Pachuau et al, 2014) have recently demonstrated that dysfunction of small conductance Ca 2+ -activated K + (SK) channels contribute to neurogenic HTN. Interestingly, we further demonstrated SK channel dysfunction in rats fed a 2% high salt (HS) diet even in the absence of hypertension.…”