Absorption of the cerebrospinal fluid and intracranial compliance in an amphibian, Rana pipiens.

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Abstract
The absorption of the (C.S.F.) and the intracranial compliance in an amphibian, R. pipiens, using infusions into the CSF system through glass micropipettes. Resistance to absorption of the CSF was estimated by the constant rate infusion technique. Mean absorption resistance for infusions of artificial CSF into the lateral ventricles and into the cerebral subarachnoid space were 15.48 and 16.52 mmH20 min/.mu.l, respectively. This difference was not significant and the pores in the posterior tela situated in the roof of the 4th ventricle do not offer any resistance to the flow of CSF out of the ventricles. The resistance to drainage of the CSF in this amphibian is higher than that found for mammals. Mean resting CSF 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 .mu.l 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 < 0.01) suggesting that the outflow sites can become partially blocked by particulate matter. During a continuous 3 h infusion of artificial CSF containing [14C]dextran or [125I]-labeled human serum albumin (RISA) into the lateral ventricles, the mean percentage uptake 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 CSF 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. At least some CSF 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 injection of artificial CSF into 1 lateral ventricle. Compliance was estimated to be 0.11, 0.10 and 0.09 .mu.l/mmH2O for injection rates of 12.74, 16.62 and 25.10 .mu.l/min, respectively. The difference between these values is not significant. For injection volumes over 5 .mu.l the CSF system may behave elastically.