Arachidonic acid (AA) exerts multiple physiological and pathophysiological effects in the brain. By continuously measuring the intracellular pH (pHi) and Ca2+ levels ([Ca2+]i) in primary cultured rat cerebellar granule cells, we have found, for the first time, that 20 min treatment with 10 μm AA resulted in marked increases in Ca2+ and H+ levels in both the cytosol and nucleus.
A much higher concentration (40 mm) of another weak acid, propionic acid, was needed to induce a similar change in pHi. The [Ca2+]i increase was probably caused by AA‐induced activation of Ni2+‐sensitive cationic channels, but did not involve NMDA channels or the Na+‐Ca2+ exchanger.
AA‐induced acidosis occurs by a different mechanism involving predominantly the passive diffusion of the un‐ionized form of AA, rather than a protein carrier, as proposed by Kamp & Hamilton for fatty acids (FAs) in artificial phospholipid bilayers (the ‘flip‐flop’ model). The following results, which are similar to those observed in lipid bilayers, support this conclusion: (1) FAs containing a ‐COOH group (AA, linoleic acid, α‐linolenic acid, and docosahexaenoic acid) induced intracellular acidosis, whereas a FA with a ‐COOCH3 group (AA methyl ester) had little effect on pHi, (2) a FA amine, tetradecylamine, induced intracellular alkalosis, and (3) the AA‐/FA‐induced pHi changes were reversed by bovine serum albumin.
Further evidence in support of a passive diffusion model, rather than a membrane protein carrier, is that: (1) there was a linear relationship between the initial rate of acid flux and the concentration of AA (2‐100 μm), (2) acidosis was not inhibited by 4,4′‐diisothiocyanatostilbene‐2,2′‐disulphonic acid, a potent inhibitor of the plasma membrane FA carrier protein, and (3) the involvement of most known H+‐related membrane carriers and H+ conductance has been ruled out.
Since AA can be released under both physiological and pathophysiological conditions, the possible significance of the AA‐evoked increases in H+ and Ca2+ in both the cytoplasm and nucleoplasm is discussed.