STIMULATION OF CA2-REGULATED OLFACTORY PHOSPHOLIPASE-C BY AMINO-ACIDS()

L-Amino acids are potent olfactory stimuli for Atlantic salmon. A plas ma membrane fraction, previously shown to be rich in amino acid bindin g sites, was prepared from olfactory rosettes of Atlantic salmon (Salm o 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 hy drolysis by phospholipase C (PLC) in a dose-dependent manner with half -maximal stimulation when all amino acids were present at approximatel y 1 muM. Stimulation of PIP2 hydrolysis by amino acids required GTPgam maS, which alone had no effect on PLC activity. Unlike GTPgammaS; AlF4 - and Ca2+ stimulated PIP2 breakdown. Preincubation with 1 mM GDPbetaS eliminated the effect of amino acids and AlF4- on PIP2 hydrolysis, su ggesting the involvement of G protein regulation. The lack of stimulat ion by GTPgammaS alone suggested that there was negligible exchange of GTPgammaS for GDP in the absence of odorant. There were no significan t effects of amino acids on either adenylate cyclase or guanylate cycl ase activities in the membrane preparation under these conditions. The effect of the amino acid cocktail was maximal at 1-10 nM free Ca2+. A t or above 100 nM free Ca2+, no effect of amino acids on PIP2 hydrolys is was found. However, between 100 nM and 100 muM, Ca2+ directly stimu lated PLC activity in a dose-dependent manner. This stimulation by Ca2 + appeared to be G protein independent because it did not require GTPg ammaS and was not inhibited by GDPbetaS. Thus, low Ca2+ appears to sen sitize 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.