Angiotensin and osmoreceptor inputs to the area postrema: Role in long- term control of fluid homeostasis and arterial pressure

1. The role of the area postrema (AP) in the long-term control of body fluid homeostasis and arterial pressure under conditions of increased dietary salt intake is reviewed. A model is proposed in which sympathetic nerve activity is suppressed when dietary salt is increased. It is hypothesized that the AP acts as an essential integrative site in the hind-brain for this response. 2. An essential component of the hypothesis is that basal levels of circulating angiotensin II support arterial pressure in animals consuming a normal salt diet by acting on the AP to drive sympathetic nerve activity. This hypothesis is supported by the observation that the long-term hypotensive response to losartan, the AT₁ receptor antagonist, is attenuated in AP-lesioned (APx) rats. 3. The role of hepatoportal sodium receptors in signalling the AP about changes in dietary salt intake is discussed. Intragastric hypertonic saline infusion increases portal venous, but not systemic plasma, osmolality and increases Fos-like immunoreactivity in the AP, nucleus tractus solitarius and the supraoptic, paraventricular and lateral parabrachial nuclei. Other studies have shown that stimulation of these receptors decreases renal sympathetic nerve activity. 4. The hypothesis that the AP is critical in long-term control of arterial pressure and body fluid homeostasis under conditions of altered dietary salt intake was studied. The responses of arterial pressure and sodium and water balance to changes in dietary salt intake were measured in intact and APx rats. Contrary to the hypothesis, APx rats did not exhibit impaired regulation of arterial pressure or water balance. However, APx rats did demonstrate an impaired ability to excrete sodium when salt intake was elevated. 5. Based on these observations, it is concluded that the AP is important in the control of sodium balance, but not arterial pressure, when dietary salt intake is altered.