Increased dietary sodium alters neural control of blood pressure during intravenous ANG II infusion

Microglia and brain angiotensin type 1 receptors are involved in desensitising baroreflex by intracerebroventricular hypertonic saline in male Sprague-Dawley rats

High salt diet alters cardiovascular control by increasing concentration of sodium ions (Na⁺) in cerebrospinal fluid (CSF) and is a risk factor for hypertension. Hypernatremic conditions activate microglia and upregulate renin-angiotensin system in the brain. Thus, we checked if chronic elevation of CSF Na⁺ affects neural control of circulatory system via microglia and brain angiotensin type 1 receptors (AT1Rs). Normotensive adult male Sprague-Dawley rats received two-week intracerebroventricular (ICV) infusion of either isoosmotic saline (0.9% NaCl); hyperosmotic saline (5% NaCl); 5% NaCl with minocycline - inhibitor of microglia; 5% NaCl with losartan - AT1R blocker. Fluid intake, urine output, and urinary Na⁺ excretion were measured before and during ICV infusions. At the end of ICV infusions, blood pressure and heart rate were recorded in awake rats at rest, in response to acute air jet stressor, during pharmacological evaluation of baroreflex, and after autonomic ganglia blockade. CSF and blood were collected for evaluation of Na⁺ concentration. Baroreflex was blunted in rats ICV infused with 5% NaCl. ICV treatment with losartan or minocycline prevented decrease in baroreflex sensitivity. Hemodynamic parameters at rest, in response to acute stressor and autonomic ganglia blockade were similar in all groups. Neither treatment affected water intake, urine output and urinary Na⁺ excretion. ICV infusion of 5% NaCl resulted in higher concentration of Na⁺ in CSF than in control group (0.9% NaCl) and in plasma. Our results indicate that chronic ICV infusion of hyperosmotic saline blunts baroreflex in normotensive rats and this desensitization is mediated by microglia and AT1Rs.

Why does the choice of dialysate sodium concentration remain controversial?

The choice of the ideal dialysate sodium concentration remains controversial. Most dialysis centers have a standard dialysate concentration. In theory, choosing a dialysate sodium concentration lower than serum sodium should result in an additional loss of sodium by diffusion with a reduction in the prevalence of hypertension and interdialytic weight gains (IDWGs) on one hand, but with potential increased risk of intradialytic hypotension and cramps on the other hand, and the opposite effects may accompany the choice of dialysate sodium concentrations greater than serum concentration. Although most studies have reported a reduction in IDWG with lower dialysate sodium concentrations, the effects on blood pressure control, and adverse intradialytic events have been variable. Different outcomes between studies may be partially explained by patient selection, with differences in dietary sodium intake, urinary sodium losses, and sodium stores in the body. In addition, multicenter trials potentially introduce additional confounders, including differences in local quality control of delivered dialysate sodium concentration and sodium measurements. Although there may be advantages for lower dialysate sodium concentration, observational studies have reported a survival advantage for higher dialysate sodium concentrations for those patients with lower serum sodium concentrations pre-dialysis. As there is no current consensus for a universal dialysate sodium concentration, attention has turned to considering an individualized approach to choosing a dialysate sodium concentration.

Revisiting the dialysate sodium prescription as a tool for better blood pressure and interdialytic weight gain management in hemodialysis patients

Hypertension and chronic volume overload are complications often seen in hemodialysis patients. Current hemodialysis practices adopt a standard dialysate sodium prescription that is typically higher than the plasma sodium concentration of most patients. As a general rule, hemodialysis patients have stable predialysis plasma sodium concentrations, and each patient has a fixed "osmolar set point." Hypertonic dialysate sodium prescriptions, including sodium modeling, predispose to positive sodium balance and lead to higher blood pressure and increased interdialytic weight gain. Conversely, lowering or individualizing dialysate sodium reduces thirst, interdialytic weight gain, and blood pressure in non-hypotension prone dialysis patients. Optimization of the dialysate sodium prescription is an important step in achieving sodium balance and improving blood pressure control in hypertensive hemodialysis patients.

Increased dietary sodium alters Fos expression in the lamina terminalis during intravenous angiotensin II infusion

These studies examined the effects of increased dietary sodium on expression of Fos, the protein product of c-fos, in forebrain structures in the rat following intravenous infusion with angiotensin II (AngII). Animals were provided with either tap water (Tap) or isotonic saline solution (Iso) as their sole drinking fluid for 3-5 weeks prior to testing. Rats were then implanted with catheters in a femoral artery and vein. The following day, the conscious, unrestrained animals received iv infusion of either isotonic saline (Veh), AngII, or phenylephrine (Phen) for 2 h. Blood pressure and heart rate were monitored continuously throughout the procedure. Brains were subsequently processed for evaluation of Fos-like immunoreactivity (Fos-Li IR) in the organum vasculosum of the lamina terminalis (OVLT), the subfornical organ (SFO), and the median preoptic nucleus (MnPO). Fos-Li IR was significantly increased in the SFO and OVLT of animals consuming both Tap and Iso following AngII, but not Phen, compared to Veh infusions. Furthermore, Fos-Li IR in the MnPO was increased following AngII infusion in rats consuming a high sodium diet, but not in animals drinking Tap. These data suggest that increased dietary sodium sensitizes the MnPO neurons to excitatory input from brain areas responding to circulating AngII.