REGULATION OF PH IN RAT-BRAIN SYNAPTOSOMES .1. ROLE OF SODIUM, BICARBONATE, AND POTASSIUM

1. We investigated the regulation of intracellular pH (pH(i)) in rat b rain isolated nerve terminals (synaptosomes), using fluorescence pH in dicators and time-resolved fluorescence spectroscopy. 2. The resting p H(i) was not significantly affected by the presence or absence of HCO3 -. Removal of external Na+, in the absence or presence of HCO3- caused a rapid acidification of pH(i). The recovery from acid loads was prim arily due to the activity of the Na+/H+ exchanger, confirming the rele vance of this transport system in synaptosomes. 3. Our data revealed t hat in synaptosomes the activity of the Na+/H+ exchanger was not regul ated by either protein kinase C or kinase A. In contrast, Ca2+ played an important role in the regulation of Na+/H+ exchanger. This was supp orted by the observation that 4Br-A23187 induced a Na+-dependent alkal inization of the resting pH(i) and greatly enhanced the initial rate a nd the degree of the recovery from acid loads. 4. In most eukaryotic c ells, HCO3--based transport mechanisms play an important role in pH(i) regulation. In synaptosomes, however, HCO3- transport is not signific antly involved in pH(i) regulation, because the presence or absence of HCO3- does not affect resting pH(i) nor the rate of pH(i) recovery to acid loads. Further studies to address the role of Cl- and HCO3- in p H(i) regulation in synaptosomes are discussed in the companion paper. 5. Increasing the concentration of K-o(+) also resulted in a rise of s teady-state pi-Ii by a processes that is Ca2+ and HCO3- independent. T his alkalinization could be due to either K+/H+ exchanger activity, K-induced depolarization, reduction of Delta mu(H+), or a direct reduct ion of Delta mu(K+). Calculated H+ driving forces suggest that the red uction in the inwardly directed H+ leak is sufficient to explain this K+-induced alkalinization because it changes the Delta mu(H+) by virtu e of setting the membrane potential difference (E(m)) to the K+ equili brium potential(E(K+)).