Stimulation of Ca2+-Regulated Olfactory Phospholipase C by Amino Acids

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L-Amino acids are potent olfactory stimuli for Atlantic salmon. A plasma membrane fraction, previously shown to be rich in amino acid binding sites, was prepared from olfactory rosettes of Atlantic salmon (Salmo salar) and utilized to investigate the role of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis in olfactory signal transduction. A cocktail of l-amino acids (Ser, Glu, Lys, and Gly) stimulated PIP2 hydrolysis by phospholipase C (PLC) in a dose-dependent manner with half-maximal stimulation when all amino acids were present at approximately 1 µM. Stimulation of PIP2 hydrolysis by amino acids required GTPγS, which alone had no effect on PLC activity. Unlike GTPγS, AlF4− and Ca2+ stimulated PIP2 breakdown. Preincubation with 1 mM GDPβS eliminated the effect of amino acids and AlF4− on PIP2 hydrolysis, suggesting the involvement of G protein regulation. The lack of stimulation by GTPγS alone suggested that there was negligible exchange of GTPγS for GDP in the absence of odorant. There were no significant effects of amino acids on either adenylate cyclase or guanylate cyclase activities in the membrane preparation under these conditions. The effect of the amino acid cocktail was maximal at 1–10 nM free Ca2+. At or above 100 nM free Ca2+, no effect of amino acids on PIP2 hydrolysis was found. However, between 100 nM and 100 µM, Ca2+ directly stimulated PLC activity in a dose-dependent manner. This stimulation by Ca2+ appeared to be G protein independent because it did not require GTPγS and was not inhibited by GDPβS. Thus, low Ca2+ appears to sensitize olfactory PLC to G protein dependent stimulation by amino acids, whereas direct activation of PLC by elevated Ca2+ may contribute to amplification in olfactory signal transduction. © 1993, American Chemical Society. All rights reserved.

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