A MODULATION OF GLUTAMATE-INDUCED PHOSPHOINOSITIDE BREAKDOWN BY INTRACELLULAR PH CHANGES

The influence of intracellular pH (pHi) changes on the formation of in ositol phosphate metabolites (IPs) produced by glutamatergic stimulati on was studied in 8-day-old rat brain synaptoneurosomes. For this purp ose pHi was measured using 2',7'-bis-(2-carboxyl)-5,6-carboxyfluoresce in (BCECF) fluorimetric assay in parallel with the basal and receptor- mediated formations of inositol monophosphate (IP1) and inositol bisph osphate (IP2). We found that glutamate (1 mM), which induces a transie nt acidification (Delta pH = -0.05), produces an identical accumulatio n of IP1 and IP2. K+ (30 mM), which provokes an alkalinization of the internal medium (Delta pH = +0.22), mainly leads to the formation of I P1 metabolites. Paired combinations of glutamate with 1, 5 and 10 mM N H4+ finally result in an alkalinization of the intrasynaptoneurosomal medium. These combinations produce a strong decrease of the IP2 level concomitant with an increase of the IP1 formation, compared to the lev els of IP1 and IP2 evoked by glutamate alone. The total amount of IPs (IP1 + IP2) produced by these combinations is not different from that obtained with glutamate alone. Paired combinations of carbachol with N H4+ produce an identical alkalinization to that produced by NH4+ alone . These combinations produce an increased IP1 accumulation, while the IP2 formation is slightly decreased. When the internal medium is acidi fied by dimishing the external concentration of Na+, the ratio IP1/IP2 produced after metabotropic glutamate receptor (mGluR) activation is shifted to lower values, while it is not affected for the muscarinic s timulation. These data suggest that the mGluR-associated pathway in sy naptoneurosomes is sensitive to pHi shifts, while the muscarinic recep tor-associated pathway is less altered when pHi is manipulated. It may be proposed that pH-sensitive inositol phosphate dephosphorylating sy stems, i.e. phosphatases, are associated with mGluRs in this preparati on. Copyright (C) 1996 Elsevier Science Ltd.