INOSITOL 1,4,5-TRISPHOSPHATE SIGNALS THE MOTILE RESPONSE OF FISH CHROMATOPHORES .1. AGGREGATION OF PIGMENT IN THE TILAPIA MELANOPHORE

After permeabilization of the cell membrane by the treatment with a surfactant, Brij 58, inositol 1,4,5-trisphosphate (IP3) was found to be effective in causing aggregation of melanosomes within melanophores in primary cultures of the tilapia, Oreochromis niloticus. Inositol 1-monophosphate (IP) and 1,4-bisphosphate (IP2) did not have such an effect. Phentolamine, an alpha-adrenolytic agent, and prazosin, a more specific alpha1-blocker, did not interfere with the action of IP3, even though they were effective in antagonizing the aggregation of pigment in response to norepinephrine. It appears, therefore, that IP3 may function as an intracellular physiological mediator in the signal transduction of pigment-aggregating stimuli, including at least alpha1-adrenergic stimuli, possibly via the release of Ca2+ from the intracellular compartments in which they are stored.