Effect of hypercapnia and cerebral perfusion pressure on cerebrospinal fluid production in cat

Brain ventricles of anesthetized cats were perfused with an artificial cerebrospinal fluid (CSF) containing inulin (or [14C]dextran) and 3H-labeled sucrose while each animal respired in turn either room air or an 8-11% CO2-in-air gas mixture. Perfusion inflow (Vi) and outflow (Vo) rates and concentrations of the test molecules were measured to calculate steady-state CSF production (Vf), CSF absorption (Va), and ependymal sucrose permeability (Ksuc). During respiratory acidosis Vf varied inversely as a function of normocapnic Vf, Ksuc increased, and Va was the same as during normocapnia. Vf increased with cerebral perfusion pressure (CPP) during normocapnia but was inversely related to it during hypercapnia. When a normocapnic animal's CPP is high in the range 70-105 Torr, its Vf will also be high, but it will increase its Vf little or not at all during hypercapnia. In the same range, if its CPP is low, its Vf will also be low, but its Vf will increase predictably fourfold or more when it breathes CO2. CPP is an influential determinant of Vf at any level of acid-base balance, possibly due to variations in blood flow at CSF production sites.