Electrical properties and structure of the frog arachnoid membrane

We have used an in vitro preparation of the frog arachnoid membrane to study the role of this membrane in the maintenance of the “blood-cerebrospinal fluid (csf) barrier.” Electron microscopy showed that the membrane was made up of 10–15 layers of flat epithelial cells joined together by numerous cell junctions. The electrical resistance of the preparation was about 2000 ohms cm². The steady-state transmural potential difference (pd) ranged up to 45 mV, csf positive, and this was eliminated by either the addition of ouabain to the csf, or by replacing the NaCl with TEA Cl. The pd across the membrane increased when bicarbonate was added to the external bathing solutions. We conclude that this pd is due to the active transport of sodium from the subural fluid to the csf. In some preparations the transmural pd was reversed, i.e., csf negative, and this was also abolished by the addition of ouabain to the csf, or by replacing chloride with isethionate. We conclude that this pd is related to active chloride transport. These, and other experiments, lead us to the conclusion that the arachnoid membrane is involved in the production of the cerebrospinal fluid and the maintenance of the blood-cerebrospinal fluid barrier.     

Serotonin-induced chloride secretion in hen colon. Possible second messengers

1. Serotonin, 100 microM, induces a peak increase in short circuit current of about 150 microA/cm2 and in cord conductance of about 7 mS/cm2 and a more prolonged increase of 30 microA/cm2 and 1.4 mS/cm2 which lasts more than 30 min in hen colon. 2. The peak increase in short circuit current and cord conductance is due to a concomitant Cl- secretion. 3. The second messenger, which mediates Cl- secretion, increases in short circuit current and cord conductance, is cyclic AMP as theophylline, 0.5 mM, increases the response in short circuit current to 1 microM serotonin from 38 +/- 5 to 78 +/- 8 microA/cm2 and in g from 1.1 +/- 0.4 to 2.0 +/- 0.3 mS/cm2. 4. Theophylline, 0.5 mM, also sensitizes the hen colon to cyclic AMP yielding an EC50 of 0.24 +/- 0.03 mM in the presence of theophylline compared with an EC50 of 2.3 +/- 0.2 mM in the absence of theophylline. 5. Manipulations of other putative second messenger systems, such as the prostaglandins/leucotrienes, the phosphoinositides and external Ca2+ or calmodulin-sensitive enzymes, did not influence the serotonin response in short circuit current and cord conductance, thus ruling out their importance as intracellular mediators.