Circulatory control and the supercontrollers.

NEUROHUMORAL SYSTEMS AS SUPERCONTROLLERS

The brain and closely linked hormone systems play a crucial part in short- and long-term cardiovascular control and have many features of adaptive control systems. The cardiovascular control system is a multivariate system, while changes in environmental conditions often result in alterations in system parameters and other non-linearities, in contrast to the fixed parameters of linear control systems. In blood pressure control these features are exemplified by diurnal circadian fluctuations, alterations in lifestyle and psychosocial stress. Because the neurohumoral controllers are involved in virtually all aspects of homeostasis, they can be regarded as supercontrollers. THE CIRCULATORY SYSTEM AND THE BRAIN: Analysis in conscious animals of the effects of circulatory disturbances suggests that the central nervous system integrates information from multiple sources of afferents. Integration of the information associated with most reflex and behavioural disturbances is mediated by many neuron groups at different levels of the neuraxis, including suprapontine brain regions. The disturbances considered include baroreflexes in intact animals, some central actions of alpha-methyldopa and reflex responses to hypoxia and haemorrhage. The operations involve the brain in comparisons of the relative magnitude of different inputs, while the occurrences of non-linear changes in baroreflex properties signify alterations in the parameters of the controller. NEUROHUMORAL MECHANISMS AND CARDIOVASCULAR DEVELOPMENT: Neurohumoral mechanisms also play a key role in cardiovascular development. Increased sympathetic activity early in life causes hypertension in spontaneously hypertensive rats (SHR) and accounts for the differences in blood pressure and structural variables from corresponding values in Wistar-Kyoto (WKY) rats. In contrast, the renin-angiotensin system affects early cardiovascular development in the same way in each strain, so that it is unlikely to be a cause of hypertension in SHR. However, after drug withdrawal following treatment of young rats with the angiotensin converting enzyme inhibitor enalapril, there were between-strain differences in late cardiovascular development. Late development is relatively small in SHR compared to WKY rats, which contributes to the long-term attenuation of hypertension in SHR and to the normalization of blood pressure in WKY rats.