Experiments have been carried out to investigate the absorption of the cerebrospinal fluid (c.s.f.) and the intracranial compliance in an amphibian, Rana pipiens, using infusions into the c.s.f. system through glass micropipettes. Resistance to absorption of the c.s.f. was estimated by the constant rate infusion technique. Mean absorption resistance for infusions of artificial c.s.f. into the lateral ventricles and into the cerebral subarachnoid space were 15.48 and 16.52 mmH2O min microliter-1 respectively. This difference was not significant and it is concluded that the pores in the posterior tela situated in the roof of the fourth ventricle do not offer any resistance to the flow of c.s.f. out of the ventricles. The resistance to drainage of the c.s.f. in this amphibian is higher than that found for mammals. Mean resting c.s.f. pressure, estimated from the intercept of the regression line with the pressure axis at zero infusion rate was 18.0 mmH2O. Absorption resistance was measured from the cerebral subarachnoid space before and after injection of 4 microliter Indian ink solution. There was a 3-fold increase in resistance following ink injection. Two-way analysis of variance showed the difference to be significant (P less than 0.01) suggesting that the outflow sites can become partially blocked by particulate matter. During a continuous 3 h infusion of artificial c.s.f. containing [14C]dextran or [125I]-labelled human serum albumin (RISA) into the lateral ventricles, the mean percentage uptakes into the systemic circulation after the first 0.5 h of a 3 h period were 74.1 and 61.9% respectively. The difference is not significant. The rapid and high uptake into blood suggests there is a direct communication between c.s.f. and blood in amphibians. During continuous infusion of RISA into the lateral ventricles, simultaneous blood samples were taken from the femoral artery and the internal dorsal vertebral vein. Radioactivity was found to be 13.2% higher in venous samples. This suggests that at least some c.s.f. drainage takes place directly into the spinal venous system. Intracranial compliance was investigated by recording the peak pressure in response to a series of bolus injections of artificial c.s.f. into one lateral ventricle. Compliance was estimated to be 0.11, 0.10 and 0.09 microliter mmH2O-1 for injection rates of 12.74, 16.62 and 25.10 microliters min-1 respectively. The difference between these values is not significant. The results suggest that for injection volumes over 5 microliters the c.s.f. system behaves elastically.
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