Abstract
1. 1. Serotonin, 100 μM, induces a peak increase in short circuit current of about 150 μA/cm 2 and in cord conductance of about 7 mS/cm 2 and a more prolonged increase of 30 μA/cm 2 and 1.4 mS/cm 2 which lasts more than 30 min in hen colon. 2. 2. The peak increase in short circuit current and cord conductance is due to a concomitant Cl − secretion. 3. 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 μM serotonin from 38 ± 5 to 78 ± 8 μA/cm 2 and in g from 1.1 ± 0.4 to 2.0 ± 0.3 mS/cm 2. 4. 4. Theophylline, 0.5 mM, also sensitizes the hen colon to cyclic AMP yielding an ec 50 of 0.24 ± 0.03 mM in the presence of theophylline compared with an ec 50 of 2.3 ± 0.2 mM in the absence of theophylline. 5. 5. Manipulations of other putative second messenger systems, such as the prostaglandins/leucotrienes, the phosphoinositides and external Ca 2+ 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.
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