A study was made of the interrelationship of serotonin, cAMP, and calcium ions in the regulation of regeneration of cilia by Tetrahymena pyriformis. All these compounds stimulated the regeneration, whereas a blocker of serotonin synthesis, p-chlorophenylalanine, and a calcium chelator, EGTA, inhibited the process. This inhibition could be overcome by the addition of any of the stimulatory compounds. cAMP was also found to be inhibitory at high concentrations. The intracellular concentration of this nucleotide was found to increase during the regeneration, and this increase occurred precociously in the presence of serotonin. It was concluded that serotonin may regulate ciliary regeneration by a mechanism involving cAMP And calcium ions, but that the causal relationships among these compounds still need to be established.

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