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 flui d homeostasis and arterial pressure under conditions of increased dietary s alt intake is reviewed. A model is proposed in which sympathetic nerve acti vity 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 res ponse. 2. An essential component of the hypothesis is that basal levels of circula ting angiotensin II support arterial pressure in animals consuming a normal salt diet by acting on the AP to drive sympathetic nerve activity. This hy pothesis is supported by the observation that the long-term hypotensive res ponse to losartan, the AT(1) receptor antagonist, is attenuated in AP-lesio ned (APx) rats. 3. The role of hepatoportal sodium receptors in signalling the AP about cha nges in dietary salt intake is discussed. Intragastric hypertonic saline in fusion increases portal venous, but not systemic plasma, osmolality and inc reases Fos-like immnnoreactivity in the AP, nucleus tractus solitarius and the supraoptic, paraventricular and lateral parabrachial nuclei. Other stud ies have shown that stimulation of these receptors decreases renal sympathe tic 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 sal t intake was studied. The responses of arterial pressure and sodium and wat er balance to changes in dietary salt intake were measured in intact and AP x rats. Contrary to the hypothesis, APx rats did not exhibit impaired regul ation of arterial pressure or water balance, However, APx rats did demonstr ate 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 sal t intake is altered.