Comparison between the pressor response and the rise in plasma catecholamines induced by acutely elevated intracranial pressure.

The influence of an abrupt elevation of intracranial pressure (ICP) on arterial blood pressure and on plasma catecholamines was studied and compared both in anaesthetized rats and cats. In rats, the elevation of ICP to 160 mm Hg caused an immediate rise in mean arterial blood pressure (MAP) and heart rate. Plasma noradrenaline and adrenaline also increased, though much slower than the circulatory changes. The duration of the pressor response was reduced after either reserpinization or adrenalectomy and virtually abolished after a combination of both pretreatments. The rise in plasma noradrenaline and adrenaline was virtually abolished in rats pretreated with reserpine or subjected to bilateral adrenalectomy. Acute elevation of ICP up to 160 mm Hg also caused pressor and chronotropic responses in cats, which were reduced by preceding reserpinization and adrenalectomy and abolished by a combination of both pretreatments. The rise in plasma noradrenaline was similar to that in rats, also with respect to its development, which proved to be much slower than that of the circulatory changes. The rise in plasma adrenaline was much less pronounced in cats. Pretreatment with reserpine or adrenalectomy virtually abolished the ICP-induced rise in plasma noradrenaline. Reserpine pretreatment had relatively little influence on the rise in plasma adrenaline but bilateral adrenalectomy virtually abolished it. In venous blood directly obtained from the adrenal gland the rise in plasma noradrenaline ICP elevation caused a rapid increase in plasma catecholamines, which could be reduced by pretreatment with reserpine. Our experiments confirm the hypothesis that the elevation of ICP causes an increase in sympathetic tone. The determination of plasma catecholamines, however, is not an adequate method to study rapidly occurring changes in sympathetic tone.