Abstract 138: High Salt Intake Augments Excitability of Pre-sympathetic PVN Neurons Through Dysfunction of the Endoplasmic Reticulum Ca2+ ATPase

Abstract

High salt (HS) intake sensitizes pre-sympathetic neurons in the hypothalamic paraventricular nucleus (PVN) leading to augmented neuronal excitability. Recently, we reported that dysfunction of Ca 2+ dependent K + channels in the PVN contributes to HS intake induced sympathoexcitation. The endoplasmic reticulum (ER) acts as a Ca 2+ store and plays an important role in regulating intracellular Ca 2+ homeostasis. The ER Ca 2+ ATPase is responsible for maintaining the high level of ER Ca 2+ and loss of function would deplete the Ca 2+ store contributing to the reduced activity of Ca 2+ dependent K + channels. We hypothesized that a 2% (NaCl) HS diet for 5 weeks would reduce function of the ER Ca 2+ ATPase and augment excitability of PVN neurons with axon projections to the rostral ventrolateral medulla (PVN-RVLM) identified by retrograde label. In whole cell current-clamp recordings from PVN-RVLM neurons, graded current injections evoked graded increases in spike frequency. Maximum discharge was evoked by +200 pA injections and averaged 22±2 Hz (n=6) in normal salt (NS) control and was significantly augmented (p<0.05) by HS diet 34±5 Hz (n=8). Bath application of thapsigargin (TG) (0.5 μM), the ER Ca 2+ ATPase inhibitor, augmented excitability of PVN-RVLM neurons in NS (32±4 Hz, n=5, p<0.05), yet had no significant effect in HS rats (32±6 Hz, n=6). ER Ca 2+ ATPase function was assessed in whole animal preparations by bilateral PVN microinjection of TG in anesthetized rats. PVN microinjection of TG (0.15, 0.3 0.75 and 1.5 nmol/100nl) increased sympathetic nerve activity (SNA) and mean arterial pressure (MAP) in a dose-dependent manner in NS rats. Maximum increases in splanchnic SNA (SSNA), renal SNA (RSNA) and MAP elicited by PVN TG (0.75 nmol/100nl; n=5) were 93±7%, 75±7%, and 11±2mmHg, respectively. In contrast, sympathoexcitatory responses to PVN TG (0.75 nmol/100nl; n=5) were attenuated in HS treated rats (SSNA 41±8%, RSNA 22±5%, p<0.05 vs. NS) while MAP responses demonstrated no significant difference (+8±2 mmHg, p>0.05 vs NS). Our data indicate that a HS diet reduces ER Ca 2+ ATPase activity and augments excitability of PVN-RVLM neurons in vitro. Altered ER Ca 2+ homeostasis may contribute to sympathoexcitation through loss of Ca 2+ dependent K + channel activity in the PVN.